genus. Its colony forms a globe the size of a pin's head, floating on the
surface of ponds, drains, or even puddles or water-barrels freely open to
the light. It has what may be called a skeleton of gelatinous matter,[139]
condensed towards the surface into a denser layer in which the minute cells
are scattered. These have each an eye-spot, a contractile vacuole, and two
flagella, by the combined action of which the colony is propelled. Delicate
boundary lines in the colonial wall mark out the proper investment of each
cell. The cells give off delicate plasmic threads which meet those of their
neighbours, and form a bond between them. In that half of the hemisphere
which is posterior in swimming, a few (five to eight) larger cells
("macrogonidia" of older writers) are evenly distributed, protruding as
they increase in size into the central jelly. These as they grow segment to
form a new colony.
{127}[Illustration: FIG. 44.—_Volvox globator._ A, entire colony, enclosing
several daughter-colonies; B, the same during sexual maturity; C, four
zooids in optical section; D^1-D^5, development of parthenogonidium; E,
ripe spermogonium; F, sperm; G, ovum; H, oosperm. _a_, Parthenogonidia;
_fl_, flagellum; _ov_, ovum; _ovy_, ovaries; _pg_, pigment spot; _sp_,
sperms; _Spy_, spermogonia dividing to form sperms. (From Parker's
_Biology_, after Cohn and Kirchner.)]
The divisions are only in two planes at right angles, so that the young
colony is at first a plate, but as the cells multiply the plate bends up
(as in the gastrulation of the double cellular plate of the Nematode
_Cucullanus_, Vol. II. p. 136), and finally forms a hollow sphere bounded
by a single layer of cells: the site of the original orifice may be traced
even in the adult as a blank space larger than exists elsewhere. Among the
cells of the young colony some cease to divide, but continue to grow at an
early period, and these are destined to become in turn the mothers
("parthenogonidia") of a new colony; they begin segmenting before the
colony of which they are cells is freed. The young colonies are ultimately
liberated by the rupture of the sphere as small-sized spheres, which
henceforth only grow by enlargement of the sphere as a whole, and the wider
separation of the vegetative cells. Thus the vegetative cells soon cease to
grow; all the supply of food material due to their living activities goes
to the nourishment of the parthenogonidia, or the young colonies, as
{128}the case may be. These vegetative cells have therefore surrendered the
power of fission elsewhere inherent in the Protist cell. Moreover, when the
sphere ruptures for the liberation of the young colonies, it sinks and is
doomed to death, whether because its light-loving cells are submerged in
the ooze of the bottom, or because they have no further capacity for life.
When conjugation is about to take place, it is the cells that otherwise
would be parthenogonidia that either act as oospheres or divide as
"spermogonia" to form a flat brood of minute yellow male cells ("sperms").
These resemble vegetative cells, in the possession of an eye-spot and two
contractile vacuoles, but differ in the enormously enlarged nucleus which
determines a beaked process in front. After one of these has fused with the
female cell ("oosphere") the product ("oosperm") encysts, passes into a
stage of profound rest, and finally gives rise to a new colony. The
oospheres and sperm-broods may arise in the same colony or in distinct
ones, according to the species.
Before we consider the bearings of the syngamic processes of _Volvox_, we
will study those presented by its nearer allies, which have the same
habitat, but are much more minute. Three of these are well known,
_Stephanosphaera_, _Pandorina_, and _Eudorina_, all of which have spherical
colonies of from eight to thirty-two cells embedded at the surface of a
sphere, and no differentiation into vegetative cells and parthenogonidia
(or reproductive cells).
_Stephanosphaera_ has its eight cells spindle shaped, and lying along
equidistant meridians of its sphere; in vegetative reproduction each of
these breaks up in its place to form a young colony, and the eight
daughter-colonies are then freed. In conjugation, each cell of the colony
breaks up into broods of 4, 8, 16, or 32 small gametes, which swim about
within the general envelope, and pair and fuse two and two: this is
"isogamous," "endogamous" conjugation. In _Pandorina_ (Fig. 45) the cells
are rounded, and are from 16 to 32 in each colony. The vegetative
reproduction in this, as in _Eudorina_, is essentially the same as in
_Stephanosphaera_. In conjugation the cells are set free, and are of three
sizes in different colonies, small (S), medium (M), and large (L). The
following fusions may occur: S × S, S × M, S × L, M × M, M × L. Thus the
large are always female, as it were, the medium may play the part of male
to the large, female to the small; the small are males to the medium and to
the large. The medium {129}and small are capable, each with its like, of
equal, undifferentiated conjugation; so that we have a differentiation of
sex far other than that of ordinary, binary sex. _Eudorina_, however, has
attained to "binary sex," for the female cells are the ordinary vegetative
cells, at most a little enlarged, and the male cells are formed by ordinary
cells producing a large flat colony of sixty-four minute males or sperms.
In some cases four cells at the apex of a colony are spermogonia, producing
each a brood of sperms, while the rest are the oospheres. The transition to
_Volvox_ must have arisen through the sterilisation of the majority of
cells of a colony for the better nutrition of the few that are destined
alone for reproduction.
[Illustration: FIG. 45.—_Pandorina morum._ A, entire colony; B, asexual
reproduction, each zooid dividing into a daughter-colony; C, liberation of
gametes; D-F, three stages in conjugation of gametes; G, zygote; H-K,
development of zygote into a new colony. (From Parker's _Biology_, after
Goebel.)]
_Volvox_, as we have seen, has attained a specialisation entirely
comparable to that of a Metazoon, where the segmentation of the fertilised
ovum results in two classes of cells: those destined {130}to form tissues,
and condemned to ultimate death with the body as a whole, and those that
ultimately give rise to the reproductive cells, ova, and sperms. But this
is a mere parallelism, not indicating any sort of relationship: the
oospores of the Volvocaceae show that tendency to an encysted state, in
which fission takes place, that is so characteristic of Algae, and these
again show the way to Cryptogams of a higher status. Thus, _Volvox_,
despite the fact that in its free life and cellular differentiation it is
the most animal of all known Flagellates, is yet, with the rest of the
Volvocaceae, inseparable from the Vegetable Kingdom, and is placed here
only because of the impossibility of cleaving the Flagellates into two.
The Dinoflagellata (Figs. 46, 47) are often of exceptionally large
dimensions in this class, attaining a maximum diameter of 150 µ (1/160")
and even 375 µ (1/67") in _Pyrocystis noctiluca_. The special character of
the group is the presence of two flagella; the one, filiform, arises in a
longitudinal groove, and extending its whole length projects behind the
animal, and is the conspicuous organ of motion: the other, band-like,
arises also in the longitudinal groove, but extends along a somewhat spiral
transverse groove,[140] and never protrudes from it in life, executing
undulating movements that simulate those of a girdle of cilia, or a
continuous undulating membrane (Fig. 46). This appearance led to the old
name "Cilioflagellata," which had of course to be abandoned when Klebs
discovered the true structure.[141] There is a distinct cellulose membrane,
sometimes silicified, to the ectoplasm, only interrupted by a bare space in
the longitudinal groove, whence the flagella take origin. This cuticle is
usually hard, sculptured, and divided into plates of definite form,
bevelled and overlapping at their junction; occasionally the cell has been
seen to moult them.
A large vacuolar space, traversed by plasmic strings, separates the
peripheral cytoplasm from the central, within which is the large nucleus.
There are in most species one or more chromatophores, coloured by a
yellowish or brownish pigment, which is a mixture of lipochromes, distinct
from diatomin. In a few species the presence of these is not constant, and
these species {131}show variability as to their nutrition, which is
sometimes holozoic. Under these conditions the cell can take in
food-particles as bulky as the eggs of Rotifers and Copepods, by the
protrusion of a pseudopod at the junction of the two grooves. As in most
coloured forms an eye-spot is often present, a cup-shaped aggregation of
pigment, with a lenticular refractive body in its hollow. A contractile
vacuole, here termed a "pusule," occurs in many species, communicating with
the longitudinal groove by a canal. Nematocysts (see p. 246 f.) are present
in _Polykrikos_, trichocysts (see p. 142) in several genera.
[Illustration: FIG. 46.—_Peridinium divergens._ _a_, Flagellum of
longitudinal groove; _b_, flagellum of transverse groove; _cr.v_,
contractile vacuole surrounded by formative vacuoles; _n_, nucleus. (After
Schütt.)]
Division is usually oblique, dividing the body into two dissimilar halves,
each of which has to undergo a peculiar growth to reconstitute the missing
portion, and complete the shell. The incomplete separation of the young
cells leads to the formation of chains, notably in _Ceratium_ and
_Polykrikos_, the latter dividing transversely and occurring in chains of
as many as eight. The process of division may take place when the cell is
active, or in a cyst, as in _Pyrocystis_ (Fig. 47). Again, encystment may
precede multiple fission, resulting in the formation of a brood of minute
swarmers. It has been suggested that these are capable of playing the part
of gametes, and conjugating in pairs.[142]
The Dinoflagellates are for the most part pelagic in habit, floating at the
surface, and when abundant tinge the water of fresh-water lakes or even
ponds red or brown. _Peridinium_ (Fig. 46) and _Ceratium_ (the latter
remarkable for the horn-like backward prolongations of the lower end) are
common genera both in the sea {132}and fresh-waters. _Gymnodinium
pulvisculus_ is sometimes parasitic in _Appendicularia_ (Vol. VII. p. 68).
_Polykrikos_[143] has four transverse grooves, each with its flagellum,
besides the terminal one. Many of the marine species are phosphorescent,
and play a large part in the luminosity of the sea, and some give it a red
colour.
Several fossil forms have been described. _Peridinium_ is certainly found
fossil in the firestone of Delitzet, belonging to the Cretaceous. A full
monograph of the group under the name "Peridiniales" was published by
Schütt.[144]
[Illustration: FIG. 47.—_Pyrocystis fusiformis_, Murray. × 100. From the
surface in the Guinea Current. (From Wyville Thomson.)]
The Cystoflagellates contain only two genera,[145] _Noctiluca_, common at
the surface of tranquil seas, to which, as its name implies, it gives
phosphorescence, and _Leptodiscus_, found by R. Hertwig in the
Mediterranean. _Noctiluca_ is enormous for a Flagellate, for with the form
of a miniature melon it measures about 1 mm. (1/25") or more in diameter.
In the depression is the "oral cleft," from one end of which rises, by a
broad base, a large coarse flagellum, as long as the body or longer and
transversely striated. In front of the base of the flagellum are two
lip-like {133}prominences, of which one, a little firmer than the other,
and transversely ridged, is called the tooth; at the junction of the two is
a second, minute, flagellum, usually called the cilium. Behind these the
oral groove has an oval space, the proper mouth; behind this, again, the
oral groove is continued for some way, with a distinct rod-like ridge in
its furrow. The whole body, including the big flagellum, is coated by a
strong cuticular pellicle, except at the oblong mouth, and the lips and rod
are mere thickenings of this. The cytoplasm has a reticulate arrangement:
the mouth opens into a central aggregate, from which strands diverge
branching as they recede to the periphery, where they pass into a
continuous lining for the cuticular wall, liquid filling the interspaces.
The whole arrangement is not unlike that found in many plant-cells, but the
only other Protists in which it occurs are the Ciliata _Trachelius_ (Fig.
56, p. 153) and _Loxodes_. The central mass contains the large nucleus.
_Noctiluca_ is an animal feeder, and expels its excreta through the mouth.
The large flagellum is remarkable for the transverse striation of its
plasma, especially on the ventral side. The cuticle may be moulted as in
the Dinoflagellates. As a prelude to fission the external differentiations
disappear, the nucleus divides in the plane of the oral groove, and a
meridional constriction parts the two halves, the new external organs being
regenerated. Conjugation occurs also, the two organisms fusing by their
oral region; the locomotive organs and pharynx disappear; the conjoined
cytoplasms unite to form a sphere, and the nuclei fuse to form a zygote or
fertilisation nucleus. This conjugation is followed by sporulation or
brood-formation.[146]
[Illustration: FIG. 48.—_Noctiluca miliaris_, a marine Cystoflagellate.
(From Verworn.)]
{134}The nucleus passes towards the surface, undergoes successive fissions,
and as division goes on the numerous daughter-nuclei occupy little
prominences formed by the upgrowth of the cytoplasm of the upper pole. The
rest of the cytoplasm atrophies, and the hillocks formed by the plasmic
outgrowths around the final daughter-nuclei become separate as so many
zoospores (usually 256 or 512); each of these is oblong with a dorsal
cap-like swelling, from the edge of which arises a flagellum pointing
backwards; parallel to this the cap is prolonged on one side into a style
also extending beyond the opposite pole of the animal.[147] In this state
the zoospore is, to all outward view, a naked Dinoflagellate, whence it
seems that the Cystoflagellates are to be regarded as closely allied to
that group. _Leptodiscus_ is concavo-convex, circular, with the mouth
central on the convex face, 1-flagellate, and attains the enormous size of
1.5 mm. (1/16") in diameter.
The remarkable phosphorescence of _Noctiluca_ is not constant. It glows
with a bluish or greenish light on any agitation, but rarely when
undisturbed. A persistent stimulus causes a continuous, but weak, light.
This light is so weak that several teaspoonsful of the organism, collected
on a filter and spread out, barely enable one to read the figures on a
watch a foot away. As in other marine phosphorescence, no rise of
temperature can be detected. The luminosity resides in minute points,
mostly crowded in the central mass, but scattered all through the
cytoplasm. A slight irritation only produces luminosity at the point
touched, a strong one causes the whole to flash. Any form of irritation,
whether of heat, touch, or agitation, electricity or magnetism, is stated
to induce the glow. By day, it is said, _Noctiluca_, when present in
abundance, may give the sea the appearance of tomato soup.
The earliest account of Noctiluca will be read with interest. Henry Baker
writes in _Employment for the Microscope_:[148]—"A curious Enquirer into
Nature, dwelling at Wells upon the Coast of Norfolk, affirms from his own
Observations that the Sparkling of Sea Water is occasioned by Insects. His
Answer to a Letter wrote to him on that Subject runs thus, 'In the Glass of
Sea Water I send with this are some of the Animalcules which cause the
Sparkling Light in Sea Water; they may be seen by holding {135}the Phial up
against the Light, resembling very small Bladders or Air Bubbles, and are
in all Places of it from Top to Bottom, but mostly towards the Top, where
they assemble when the Water has stood still some Time, unless they have
been killed by keeping them too long in the Phial. Placing one of these
Animalcules before a good Microscope, an exceeding minute Worm may be
discovered, hanging with its Tail fixed to an opake Spot in a Kind of
Bladder, which it has certainly a Power of contracting or distending, and
thereby of being suspended at the Surface, or at any Depth it pleases in
the including Water.'"
"The above-mentioned Phial of Sea Water came safe, and some of the
Animalcules were discovered in it, but they did not emit any Light, as my
Friend says they do, upon the least Motion of the Phial when the Water is
newly taken up. He likewise adds, that at certain Times, if a Stone be
thrown into the Sea, near the Shore, the Water will become luminous as far
as the Motion reacheth: this chiefly happens when the Sea hath been greatly
agitated, or after a Storm." Obviously what Mr. Sparshall, Baker's
correspondent, took for a worm was the large flagellum.
The chief investigators of this group have been Huxley, Cienkowski, Allman,
Bütschli, and G. Pouchet, while Ischikawa and Doflein have elucidated the
conjugation.
{136}CHAPTER VI
PROTOZOA (_CONTINUED_): INFUSORIA (CILIATA AND SUCTORIA)
IV. Infusoria.
_Complex Protozoa, never holophytic save by symbiosis with plant
commensals, never amoeboid, with at some period numerous short cilia, of
definite outline, with a double nuclear apparatus consisting of a large
meganucleus and a small micronucleus (or several),[149] the latter alone
taking part in conjugation (karyogamy), and giving rise after conjugation
to the new nuclear apparatus._
The name Infusoria was formerly applied to the majority of the Protozoa,
and included even the Rotifers. For the word signifies organisms found in
"infusions" of organic materials, including macerations. Such were made
with the most varied ingredients, pepper and hay being perhaps the
favourites. They were left for varying periods exposed to the air, to allow
the organisms to develop therein, and were then examined under the
microscope.[150] With the progress of our knowledge, group after group was
split off from the old assemblage until only the ciliate or flagellate
forms were left. The recognition of the claims of the Flagellates to
independent treatment left the group more natural;[151] while it was
enlarged by the admission of the Acinetans (_Suctoria_), which had for some
time been regarded as a division of the Rhizopoda.
{137}I. CILIATA
_Infusoria, with a mouth, and cilia by which they move and feed; usually
with undulating membranes, membranellae, cirrhi, or some of these._ Genera
about 144: 27 exclusively marine, 50 common to both sea and fresh water, 27
parasitic on or in Metazoa, the rest fresh water. Species about 500.
We divide the Ciliata thus:[152]—
(I.) Mouth habitually closed, opening by retraction of its circular or
slit-like margin; cilia uniform
Order 1. GYMNOSTOMACEAE.
_Lacrymaria_, Ehrb.; _Loxodes_, Ehrb.; _Loxophyllum_, Duj.; _Lionotus_,
Wrez.; _Trachelius_, Schrank; _Amphileptus_, Ehrb.; _Actinobolus_, St.;
_Didinium_, St.; _Scaphiodon_, St; _Dysteria_, Huxl.; _Coleps_, Nitzsch.;
_Dileptus_, Duj.; _Ileonema_, Stokes; _Mesodinium_, St.
(II.) Mouth permanently open, usually equipped with one or more
undulating membranes, receiving food by ciliary action (TRICHOSTOMATA,
Bütschli)
(_a_) Cilia nearly uniform, usually extending over the whole body,
without any special adoral wreath of long cilia or membranellae; mouth
with one or two undulating membranes at its margin or extending into
the short pharynx.
Order 2. ASPIROTRICHACEAE.
_Paramecium_, Hill; _Colpoda_, O. F. Müll.; _Colpidium_, St.;
_Leucophrys_, Ehrb.; _Cyclidium_, Cl. and L.; _Lembadion_, Perty;
_Cinetochilum_, Perty; _Pleuronema_, Duj.; _Ancistrum_, Maup.;
_Glaucoma_, Ehrb.; _Uronema_, Duj.; _Lembus_, Cohn; _Urocentrum_,
Nitzsch; _Icthyophtheirius_, Fouquet.
(_b_) Strong cilia or membranellae forming an adoral wreath, and
bounding a more or less enclosed area, the "peristome," at one point of
which the mouth lies.
(i.) Body more or less equally covered with fine cilia; adoral wreath
an open spiral
Order 3. HETEROTRICHACEAE
_Spirostomum_, Ehrb.; _Bursaria_, O. F. Müll.; _Stentor_, Oken;
_Folliculina_, Lamk.; _Conchophtheirus_, St.; _Balantidium_, Cl. and
L.; _Nyctotherus_, Leidy; _Metopus_, Cl. and L.; _Caenomorpha_,
Perty; _Discomorpha_, Levander; _Blepharisma_, Perty.
(ii.) Body cilia limited in distribution or absent; peristome
anterior, nearly circular, sinistrorse.
Order 4. OLIGOTRICHACEAE.
_Halteria_, Duj.; _Maryna_, Gruber; _Tintinnus_, Schrank;
_Dictyocystis_, Ehrb.; _Strombidium_, Cl. and L. (= _Torquatella_,
Lank.).
(iii.) Peristome extending backwards along the ventral face, which
alone is provided with motile cirrhi, etc.; dorsal cilia fine,
motionless.
Order 5. HYPOTRICHACEAE.
{138}_Stylonychia_, Ehrb.; _Kerona_, O. F. Müll.; _Oxytricha_, Ehrb.;
_Euplotes_, Ehrb.; _Stichotricha_, Perty; _Schizotricha_, Gruber.
(iv.) Body cilia reduced to a posterior girdle, or temporarily or
permanently absent; peristome anterior, nearly circular, edged by the
adoral wreath,[153] bounded by a gutter edged by an elevated rim or
collar.
Order 6. PERITRICHACEAE.
_Lichnophora_, Cl.; _Trichodina_, Ehrb.; _Vorticella_, L.;
_Zoothamnium_, Bory; _Carchesium_, Ehrb.; _Epistylis_, Ehrb.;
_Opercularia_, Lamk.; _Vaginicola_, Lamk.; _Pyxicola_, Kent;
_Cothurnia_, Ehrb.; _Scyphidia_, Lachmann; _Ophrydium_, Bory;
_Spirochona_, St.
The Ciliata have so complex an organisation that, as with the Metazoa, it
is well to begin with the description of a definite type. For this purpose
we select _Stylonychia mytilus_, Ehrb. (Fig. 49), a species common in water
rich in organic matter, and relatively large (1/75" = ⅓ mm.). It is broadly
oval in outline, with the wide end anterior, truncate, and sloping to the
left side behind; the back is convex, thinning greatly in front; the belly
flat. It moves through the water either by continuous swimming or by jerks,
and can either crawl steadily over the surface of a solid or an air surface
such as an air bubble, or advance by springs, which recall those of a
hunting spider. The boundary is everywhere a thin plasmic pellicle, very
tender, and readily undergoing diffluence like the rest of the cell. From
the pellicle pass the cilia, which are organically connected with it,
though they may be traced a little deeper; they are arranged in slanting
longitudinal rows, and are much and variously modified, according to their
place and function. On the edge of the dorsal surface they are fine and
motionless, probably only sensory (_s.h._); except three, which protrude
well over the hinder end (_c.p._), stout, pointed, and frayed out at the
ends, and possibly serving as oars or rudders for the darting movements.
These are distinguished from simple cilia as "cirrhi."
{139}[Illustration: FIG. 49.—Ventral view of _Stylonychia mytilus_. _a.c_,
Abdominal cirrhi; _an_, anus discharging the shell of a Diatom; _c.c_,
caudal cirrhi; _c.p_, dorsal cirrhi; _cv_, contractile vacuole; _e_, part
of its replenishing canal; _f.c_, frontal cirrhi; _f.v_, food vacuoles;
_g_, internal undulating membrane; _l_, lip; _m_, mouth or pharynx; _mc_,
marginal cirrhi; _N_, _N_, lobes of meganucleus; _n_, _n_, micronuclei;
_o_, anterior end; _per_, adoral membranellae; _poc_, preoral cilia;
_p.om_, preoral undulating membrane; _s.h_, sense hairs. (Modified from
Lang.)]
At the right hand of the frontal area there begins, just within the dorsal
edge, a row of strong cilium-like organs (Fig. 49, _per_); these, on
careful examination, prove to be transverse triangular plates, which after
death may fray into cilia.[154] They are the "adoral membranellae." This
row passes to the left blunt angle, and there crosses over the edge of the
body to the ventral aspect, and then curves inwards towards the median
line, which it reaches about half-way back, where it passes into the
pharynx (_m_). It forms the front and left-hand boundary of a wedge-shaped
depression, the "peristomial area," the right-hand boundary being the
"preoral ridge" or lip (_l_), which runs nearly on the median line,
projecting downward and over the depression. This ridge bears on its inner
and upper side a row of fine "preoral cilia" (_poc_) and a wide "preoral
undulating membrane" (_p.om_), which extends horizontally across, below the
peristomial area. The roof of this area bears along its right-hand edge an
"internal undulating membrane" (_g_), and then, as we pass across to the
left, first an "endoral membrane" and then an "endoral" row of cilia. In
some allied genera (not in _Stylonychia_), at the base and on the inner
side of each adoral membranella, is a "paroral" cilium. {140}All these
motile organs, with the exception of the preoral cilia, pass into the
pharynx; but the adoral membranellae soon stop short for want of room.
There are some seventy membranellae in the adoral wreath.
The rest of the ventral surface is marked by longitudinal lines, along
which the remaining appendages are disposed. On either side is a row of
"marginal cirrhi" (_mc._), which, like the membranellae, may fray out into
cilia, but are habitually stiff spine-like, and straight in these rows;
these are the chief swimming organs. Other cirrhi, also arranged along
longitudinal rows, with so many blank spaces that the arrangement has to be
carefully looked for, occur in groups along the ventral surface. On the
right of the peristome are a group which are all curved—the "frontal
cirrhi" (_f.c._). Behind the mouth is a second group—the "abdominal cirrhi"
(_a.c._), also curved hooks; and behind these again the straight spine-like
"caudal" or "anal" cirrhi (_c.c_), which point backwards. These three sets
of ventral cirrhi are the organs by which the animal executes its crawling
and darting movements. Besides the mouth there are two other openings, both
indistinguishable save at the very moment of discharge; the anus (_an_)
which is dorsal, and the pore of the contractile vacuole, which is ventral.
The protoplasm of the body is sharply marked off into a soft, semi-fluid
"endoplasm" or "endosarc," and a firmer "ectoplasm" or "ectosarc." The
former is rich in granules of various kinds, and in food-vacuoles wherein
the food is digested. The mode of ingestion, etc., is described below (p.
145). The ectoplasm is honeycombed with alveoli of definite arrangement,
the majority being radial to the surface or elongated channels running
lengthwise; inside each of these lies a contractile plasmic streak or
myoneme. The contractile vacuole (_cv_) lies in this layer, a little behind
the mouth, and is in connexion with two canals, an anterior (_e_) and a
posterior, from which it is replenished.
The nuclear apparatus lies on the inner boundary of the ectoplasm; it
consists of (1) a large "meganucleus" formed of two ovoid lobes (_N_, _N_),
united by a slender thread; and (2) two minute "micronuclei" (_n_, _n_),
one against either lobe of the meganucleus.
_Stylonychia_ multiplies by transverse fission, the details of which are
considered on pp. 144, 147.
The protoplasm of Ciliata is the most differentiated that we {141}find in
the Protista, and we can speak without exaggeration of the "organs" formed
thereby.
The form of the body, determined by the firm PELLICLE or plasmic membrane,
is fairly constant for each species, though it may be subject to temporary
flexures and contractions. The pellicle varies in rigidity; where the cilia
are abundant it is proportionately delicate, and scarcely differs from the
ectoplasm proper, save for not being alveolate. In the Peritrichaceae it is
especially resistant and proof against decay. In _Coleps_ (Gymnostomaceae)
it is hardened and sculptured into the semblance of plate-armour, and the
prominent points of the plates around the mouth serve as teeth to lacerate
other active Protista, its prey; but, like the rest of the protoplasm, this
disappears by decay soon after the death of the _Coleps_. Where, as in
certain Oligotrichaceae, cilia are absent over part of the body, the
pellicle is hardened; and on the dorsal face and sides of _Dysteria_ it
even assumes the character of a bivalve shell, and forms a tooth-like
armature about the mouth.
From the pellicle protrude the CILIA, each of which is continued inwards by
a slender basal filament to end in a "basal granule" or "blepharoplast."
The body-cilia are fine, and often reversible in action, which is
exceptional in the organic world. They may be modified or combined in
various ways. We have seen that in _Stylonychia_ some are motionless
sensory hairs. The cirrhi and setae sometimes fray out during life, and
often after death, into a brush at the tip, and have a number of
blepharoplasts at their base. The same holds good for the membranellae and
undulating membranes. They are thus comparable to the "vibratile styles" of
Rotifers (Vol. II. p. 202) and the "combs" or "Ctenophoral plates" of the
Ctenophora (p. 412 f.).[155]
{142}[Illustration: FIG. 50.—Ectosarc of Ciliata. _a-f_, from _Stentor
coeruleus_; _g_, _Holophrya discolor_. _a_, Transverse section, showing
cilia, pellicle, canals, and myonemes; _b_, surface view below pellicle,
showing myonemes alternating with blue granular streaks; _c_, more
superficial view, showing rows of cilia adjacent to myonemes; _d_, myoneme,
highly magnified, showing longitudinal and transverse striation; _e_, two
rows of cilia; _f_, _g_, optical sections of ectosarc, showing pellicle,
alveolar layer (_a_), myonemes (_m_), and canals in ectosarc. (From
Calkins, after Metschnikoff, Bütschli, and Johnson.)]
The ECTOSARC has a very complex structure. Like other protoplasm it has a
honeycombed or alveolate structure, but in this case the alveoli are
permanent in their arrangement and position. Rows of these alveoli run
under the surface; and the cilia are given off from their nodal points
where the vertical walls of several unite, and wherein the basal granule or
blepharoplast is contained. Longitudinal threads running along the inner
walls of the alveoli of the superficial layer are differentiated into
muscular fibrils or "myonemes," to which structures so many owe their
marked longitudinal striation on the one hand, and their power of sudden
contraction on the other. The appearance of transverse striation may be
either due to transverse myonemes, or produced by the folds into which the
contraction of longitudinal fibrils habitually wrinkles the pellicles, when
it is fairly dense (Peritrichaceae); circular muscular fibrils, however,
undoubtedly exist in the peristomial collar of this group. Embedded in the
ectosarc are often found TRICHOCYSTS,[156] analogous {143}to the
nematocysts of the Coelenterata (p. 247), and doubtless fulfilling a
similar purpose, offensive and defensive. A trichocyst is an oblong sac (4
µ long in _Paramecium_) at right angles to the surface, which on
irritation, chemical (by tannin, acids, etc.) or mechanical, emits or is
converted into a thread several times the length of the cilia (33 µ), often
barbed at the tip. In the predaceous Gymnostomaceae, such as _Didinium_,
the trichocysts around (or even within) the mouth are of exceptional size,
and are ejected to paralyse, and ultimately to kill, the active Infusoria
on which they feed. In most of the Peritrichaceae they are, when present,
limited to the rim around the peristome, while in the majority of species
of Ciliata they have not been described. Fibrils, possibly nervous,[157]
have been described in the deepest layer of the ectosarc in
Heterotrichaceae.
The innermost layer of the ectosarc is often channelled by a system of
canals,[158] usually inconspicuous, as they discharge continuously into the
CONTRACTILE VACUOLE; but by inducing partial asphyxia (_e.g._ by not
renewing the limited supply of air dissolved in the drop of water on the
slide under the cover-glass), the action of the vacuole is slackened, and
these canals may be more readily demonstrated. The vacuole, after
disappearance, forms anew either by the coalescence of minute formative
vacuoles, or by the enlargement of the severed end of the canal or canals.
The pore of discharge to the surface is visible in several species, even in
the intervals of contraction.[159] The pore is sometimes near that of the
anus, but is only associated with it in Peritrichaceae, where it opens
beside it into the vestibule or first part of the long pharynx, often
through a rounded reservoir (Fig. 60, _r_) or elongated canal.
The ENDOSARC, in most Ciliates well differentiated from the ectosarc, is
very soft; though it is not in constant rotation like that of a Rhizopod,
it is the seat of circulatory movements alternating with long periods of
rest. Thus it is that the food-vacuoles, after describing a more or less
erratic course, come to discharge their undigested products at the one
point, the ANUS. {144}In a few genera (_Didinium_, for instance) the course
from mouth to anus is a direct straight line, and one may almost speak of a
digestive tract. In _Loxodes_ and _Trachelius_ (Fig. 56) the endosarc, as
in the Flagellate _Noctiluca_ (Fig. 48, p. 133), has a central mass into
which the food is taken, and which sends out lobes, which branch as they
approach and join the ectoplasm. The endosarc contains excretory granules,
probably calcium phosphate, droplets of oil or dissolved glycogen, proteid
spherules, paraglycogen grains, etc.
The NUCLEAR APPARATUS lies at the inner boundary of the ectoplasm. The
"meganucleus" may be ovoid, elongated, or composed of two or more rounded
lobes connected by slender bridges (_Stentor_, _Stylonychia_). The
"micronucleus" may be single; but even when the meganucleus is not lobed it
may be accompanied by more than one micronucleus, and when it is lobed
there is at least one micronucleus to each of its lobes.[160] The
meganucleus often presents distinct granules of more deeply staining
material, varying with the state of nutrition; these are especially visible
in the band-like meganuclei of the Peritrichaceae (Figs. 51, 60). At the
approach of fission it is in many cases distinctly fibrillated.[161] But
all other internal differentiation, as well as any constriction, then
disappears; and the ovoid or rounded figure becomes elongated and
hour-glass shaped, and finally constricts into two ovoid
daughter-meganuclei, which, during and after the fission of the cell,
gradually assume the form characteristic of the species. The micronuclei
(each and all when they are multiple) divide by modification of
karyokinesis (or "mitosis") as a prelude to fission: in this process the
chromatin is resolved into threads which divide longitudinally, but the
nuclear wall {145}remains intact. If an Infusorian be divided into small
parts, only such as possess a micronucleus and a fragment of the
meganucleus are capable of survival. We shall see how important a part the
micronuclei play in conjugation, a process in which the old meganuclei are
completely disorganised and broken up and their débris expelled or
digested.
The MOUTH of the Gymnostomaceae is habitually closed, opening only for the
ingestion of the living Protista that form their prey. It usually opens
into a funnel-shaped PHARYNX, strengthened with a circle of firm
longitudinal bars, recalling the mouth of an eel-trap or lobster-pot
("Reusenapparat" of the Germans); and this is sometimes protrusible. In
_Dysteria_ the rods are replaced by a complicated arrangement of jaw- or
tooth-like thickenings, which are not yet adequately described. We have
above noted the strong adoral trichocysts in this group.
In all other Ciliates[162] the "mouth" is a permanent depression lined by a
prolongation of the pellicle, and containing cilia and one or more
undulating membranes, and when adoral membranellae are present, a
continuation of these. In some species, such as _Pleuronema_ (Fig. 57), one
or two large membranes border the mouth right and left. In Peritrichaceae
the first part of the pharynx is distinguished as the "vestibule," since it
receives the openings of the contractile vacuole or its reservoir and the
anus. The pharynx at its lower end (after a course exceptionally long and
devious in the Peritrichaceae; Figs. 51, 60) ends against the soft
endosarc, where the food-particles accumulate into a rounded pellet; this
grows by accretion of fresh material until it passes into the endosarc,
which closes up behind it with a sort of lurch. Around the pellet liquid is
secreted to form the food-vacuole. If the material supplied be coloured and
insoluble, like indigo or carmine, the vacuoles may be traced in a sort of
irregular, discontinuous circulation through the endosarc until their
remains are finally discharged as faeces through the anus. No prettier
sight can be watched under the microscope than that of a colony of the
social Bell-animalcule (_Carchesium_) in coloured water—all producing
food-currents brilliantly shown up by the wild eddies of the pigment
granules, and the vivid blue or crimson colour of {146}the food-vacuoles,
the whole combining to present a most attractive picture. Ehrenberg fancied
that a continuous tube joined up the vacuoles, and interpreted them as so
many stomachs threaded, as it were, along a slender gut; whence he named
the group "Polygastrica."
[Illustration: FIG. 51.—_Carchesium polypinum._ Scheme of the path taken by
the ingested food in digestion and expulsion of the excreta. The food
enters through the pharynx and is transported downward (small circles),
where it is stored in the concavity of the sausage-shaped meganucleus (the
latter is recognised by its containing darker bodies). It remains here for
some time at rest (small crosses). Then it passes upward upon the other
side (dots) and returns to the middle of the cell, where it undergoes
solution. The excreta are removed to the outside, through the vestibule and
cell mouth. The black line with arrows indicates the direction of the path.
(From Verworn, after Greenwood.)]
We owe to Miss Greenwood[163] a full account of the formation and changes
of the food-vacuoles in _Carchesium polypinum_. The vacuole passes steadily
along the endosarc for a certain time after its sudden admission into it,
and then enters on a phase of quiescence. A little later the contents of
the vacuole aggregate together in the centre of the vacuole, where they are
surrounded by a zone of clear liquid; this takes place in the hollow of the
meganucleus, in this species horseshoe-shaped. The vacuole then slowly
passes on towards the peristome, lying deep in the endosarc, and the fluid
peripheral zone is absorbed. {147}For some time no change is shown in the
food-material itself: this is the stage of "storage." Eventually a fresh
zone of liquid, the true digestive vacuole, forms again round the
food-pellet, and this contains a peptic juice, of acid reaction. The
contents, so far as they are capable of being digested, liquefy and
disappear. Ultimately the solid particles in their vacuole reach the anal
area of the vestibule, and pass into it, to be swept away by the overflow
of the food-current. The anus is seated on a transverse ridge about a third
down the tube, the remaining two-thirds being the true pharynx.
FISSION is usually transverse; but is oblique in the conical
Heterotrichaceae, and longitudinal in the Peritrichaceae. It involves the
peristome, of which one of the two sisters receives the greater, the other
the lesser part; each regenerates what is missing. When there are two
contractile vacuoles, as in _Paramecium_, either sister receives one, and
has to form another; where there is a canal or reservoir divided at
fission, an extension of this serves to give rise to a new vacuole in that
sister which does not retain the old one. In some cases the fission is so
unequal as to have the character of budding (_Spirochona_). We have
described above (p. 144) the relations of the nuclear apparatus in fission.
Several of the Ciliata divide only when encysted, and then the divisions
are in close succession, forming a brood of four, rarely more. This is well
seen in the common _Colpoda cucullus_. In the majority, however, ENCYSTMENT
is resorted to only as a means of protection against drought, etc., or for
quiet rest after a full meal (_Lacrymaria_).
Maupas[164] has made a very full study of the LIFE-CYCLES of the Ciliata.
He cultivated them under the usual conditions for microscopic study, _i.e._
on a slide under a thin glass cover supported by bristles to avoid
pressure, preserved in a special moist chamber; and examined them at
regular intervals.
{148}[Illustration: FIG. 52.—_Paramecium caudatum_, stages in conjugation.
_gul_, Gullet; _mg.nu_, meganucleus; _Mg.nu_, reconstructed meganucleus;
_mi.nu_, micronucleus; _Mi.nu_, reconstructed micronucleus; _o_, mouth.
(From Parker and Haswell, after Hertwig.)]
The animals collect at that zone where the conditions of aeration are most
suitable, usually just within the edge of the cover, and when well supplied
with food are rather sluggish, not swimming far, so that they are easily
studied and counted. When well supplied with appropriate food they undergo
binary fission at frequent intervals, dividing as often as five times in
the twenty-four hours at a temperature of 65-69° F. (_Glaucoma
scintillans)_, so that in this period a single individual has resolved
itself into a posterity of 32; but such a rapid increase is exceptional. At
a minimum and a maximum temperature multiplication is arrested, the optimum
lying midway. If the food-supply is cut off, encystment occurs in those
species capable of the process; but when there is a mixture of members of
different broods of the same species, subject to the limitations that we
shall learn, conjugation ensues. Under the conditions of Maupas'
investigations he found a limit to the possibilities of continuous
fissions, even when interrupted by occasional encystment. The individuals
of a series ultimately dwindle in size, their ciliary apparatus is reduced,
and their nuclear apparatus degenerates. Thus the ultimate members of a
fission-cycle show a progressive decay, notably in the nuclear apparatus,
which Maupas has aptly compared to "senility" or "old age" in the Metazoan.
If by the _timely_ mixture of broods conjugation be induced, these senile
degenerations do not occur.[165] In _Stylonychia {149}mytilus_ the produce
of a being after conjugation died of senility after 336 fissions; in
_Leucophrys_ after 660.
Save in the Peritrichaceae (p. 151) CONJUGATION takes place between similar
mates, either of the general character and size of the species, or reduced
by fissions, in rapid succession, induced by the same conditions as those
of mating. The two mates approach, lying parallel and with their oral faces
or their sides (_Stentor_) together, and partially fuse thereby; though no
passage of cytoplasm is seen it is probable that there is some interchange
or mixture.[166]
[Illustration: FIG. 53.—Diagram of conjugation in _Colpidium colpoda_.
Horizontal line means degeneration; parallel vertical lines, separation of
gametes; broken lines (above), boundary between pairing animals; (below),
first fission; single vertical line, continuity or enlargement. _M_,
Meganucleus; _µ_, micronucleus; _Z_, zygote-nucleus.]
{150}[Illustration: FIG. 54.—Four individuals of _Coleps hirtus_
(Gymnostomaceae) swarming about and ingesting a Vorticella (?) (From
Verworn.)]
The meganucleus lengthens, becomes irregularly constricted, and breaks up
into fragments, which are ultimately extruded or partially digested. The
micronucleus enlarges (Fig. 52, A) and undergoes three successive
divisions, or, strictly speaking, two fissions producing four nuclei, of
which one only undergoes the third. The other three nuclei of the second
fission degenerate like the meganucleus.[167] Of the two micronuclei of
this last division one remains where it is as a "stationary" pairing
nucleus, while its sister passes as a "migratory" pairing-nucleus into the
other mate, and fuses with its stationary pairing-nucleus. Thus in either
mate is formed a "zygote-nucleus," or "fusion-nucleus." All these processes
are simultaneous in the two mates; and the migratory nuclei cross one
another on the bridge of junction of the two mates (Fig. 52, C). Each mate
now has its original cytoplasm (subject to the qualification above),
{151}but its old nuclear apparatus is replaced by the fusion-nucleus. This
new nucleus undergoes repeated fissions; its offspring enlarge unequally,
the larger being differentiated as mega-, the smaller as micro-nuclei. The
mates now separate (Fig. 52, F, G), and by the first (or subsequent)
fission of each, the new mega- and micro-nuclei are distributed to the
offspring. _Colpidium colpoda_ offers the simplest case, on which we have
founded our diagram showing the nuclear relations. During conjugation the
oral apparatus often atrophies, and is regenerated; and in some cases the
pellicle and ciliary apparatus are also "made over."
[Illustration: FIG. 55.—_Paramecium caudatum_ (Aspirotrichaceae). A, The
living animal from the ventral aspect; B, the same in optical section, the
arrows show the course taken by food-particles. _buc.gr_, Buccal groove;
_cort_, cortex; _cu_, cuticle; _c.vac_, contractile vacuole; _f.vac_, food
vacuole; _gul_, gullet; _med_, medulla; _mth_, mouth; _nu_, meganucleus;
_pa.nu_, micronucleus; _trch_, trichocysts discharged. (From Parker's
_Biology_.)]
In the Peritrichaceae the mates are unequal; the larger is the normal cell,
and is fixed; the smaller, mobile, is derived from an ordinary individual
by brood-divisions, which only occur under the conditions that induce
conjugation (Fig. 60). Here, though the two pairs of nuclei are formed, it
is only the migratory {152}nuclei that unite, the stationary ones aborting
in both mates. During the final processes of conjugation the smaller mate
is absorbed into the body of the larger, and so plays the part of male
there. But this process, though one of true binary sex, is clearly derived
from the peculiar type of equal reciprocal conjugation of the other
Infusoria.
The Ciliata are almost all free-swimming animals with the exception of most
of the Peritrichaceae, and of the genera we now cite. _Folliculina_ forms a
sessile tube open at either end; and _Schizotricha socialis_ inhabits the
open mouths of a branching gelatinous tubular stem, obviously secreted by
the hinder end of the animal, and forking at each fission to receive the
produce. A similar habit to the latter characterises _Maryna socialis_; all
three species are marine, and were described by Gruber.[168] _Stentor_
habitually attaches itself by processes recalling pseudopodia, and often
forms a gelatinous sheath.
The majority of the Oligotrichaceous Tintinnidae inhabit free chitinous
tests often beautifully fenestrated, as in _Dictyocystis_.
Many genera are parasitic in the alimentary canal of various Metazoa, but
none appear to be seriously harmful except _Ichthyophtheirius_, which
causes an epidemic in fresh-water fish. Quite a peculiar fauna inhabit the
paunch of Ruminants. _Nyctotherus_ and _Balantidium_ are occasionally found
in the alimentary canal of Man.[169]
The Gymnostomaceae are predaceous, feeding for the most part on smaller
Ciliates. We have described the peculiar character of the mouth and pharynx
in this group, and the mail-like pellicle of _Coleps_ (Fig. 54).
_Loxophyllum_ is remarkable for the absence of cilia from one of the sides
of its flattened body, and the tufts of trichocysts studding its dorsal
edge at regular intervals. _Actinobolus_ has numerous tentacles, exsertile
and retractile, each bearing a terminal tuft of trichocysts, which serve to
paralyse such active prey as _Halteria_. _Ileonema_ has one tentacle
overhanging the mouth; and _Mesodinium_ has four short sucker-like
projections around it.[170] It has only two girdles {153}of cilia, which
are stout and resemble fine-pointed cirrhi. In _Dysteria_ the cilia are
exclusively ventral, and the naked dorsal surface has its pellicle
condensed into a bivalve shell; a posterior motile process ("foot") and a
complex pharyngeal armature add to the exceptional characters of the genus.
The Aspirotrichaceae are well known to every student of "Elementary
Biology" by the "type" _Paramecium_ (Fig. 55), so common in infusions,
especially when containing a little animal matter. _P. bursaria_ often
contains in its endosarc the green symbiotic Flagellate _Zoochlorella_.
_Colpoda cucullus_, very frequent in vegetable infusions, usually only
divides during encystment, and forms a brood of four. _Pleuronema
chrysalis_ (Fig. 57) is remarkable for its habit of lying for long periods
on its side and for its immense undulating membrane, forming a lip on the
left of its mouth; _Glaucoma_ has two, right and left.
[Illustration: FIG. 56.—_Trachelius ovum._ A, general view; B, section
through sucker; C, section through contractile vacuole and its pore of
discharge. _al_, Alveolar layer of ectoplasm; _cil_, cilia; _c.v_,
contractile vacuole; _m_, mouth; _N_, meganucleus; _s_, sucker, from which
pass inwards retractile myonemes. (After Clara Hamburger.)]
{154}[Illustration: FIG. 57.—_Pleuronema chrysalis_ (Aspirotrichaceae). A,
Unstimulated, lying quiet; B, stimulated, in the act of springing by the
stroke of its cilia. (From Verworn.)]
The Heterotrichaceae present very remarkable forms. _Spirostomum_ is nearly
cylindrical, and, a very giant, may attain a length of 4 mm. (1/6").
_Stentor_ can attach itself by its hinder end, which is then finely tapered
and prolonged into a few pseudopodia; its body is trumpet-shaped, with a
spiral peristome forming a coil round its wide end, and leading on the left
side into the mouth. Many species when attached secrete a gelatinous sheath
or tube. _S. polymorphus_ is often coloured green by _Zoochlorella_ (p.
125); _S. coeruleus_[171] and _S. igneus_ owe their names to the brilliant
pigment, blue or scarlet, deposited in granules in lines between the
conspicuous longitudinal myonemes. From their large size and elongated
meganucleus accompanied by numerous micronuclei, these two genera have
frequently been utilised for experiments on regeneration. In _Metopus
sigmoides_ the peristomial area forms a dome above its wreath of
membranellae; and in _M. pyriformis_ this is so great as to form the larger
part of the cell, which is top-shaped, tapering behind to a point.
_Caenomorpha_ (Fig. 58) has the same general form, with a peg-like tail,
and possesses a girdle of cirrhi.[172] The converse occurs in
{155}_Bursaria_; the cell is a half ellipse, something like a common twin
tobacco-pouch when closed: a deep depression thus occupies the whole
ventral surface, and opens by a wide slit extending along the anterior end.
The peristomial area occupies the dorsal side of the pocket so formed, and
the mouth is in the hinder left-hand corner. _Blepharisma_ sp. is parasitic
in the Heliozoon _Raphidiophrys viridis_ (Fig. 20, 1, p. 74).
[Illustration: FIG. 58.—_Caenomorpha uniserialis._ _crh_, Zone of cirrhi;
_c.t_, cilia of tail; _c.v_, contractile vacuole; _c.w_, ciliary wreath;
_g_, granular aggregate; _m_, zone of membranellae; _N_, meganucleus; _n_,
micronucleus; _oe_, pharynx; _t_, tail-spine; _t_^1, accessory spine;
_u.m_, undulating membrane; _v_, vacuole; _z_, precaudal process. (After
Levander.)]
Among Oligotrichaceae, _Halteria_, common among the débris at the bottom of
pools in woods containing dead leaves, is remarkable for an equatorial
girdle of very long fine setae, and for its rapid erratic darting
movements, alternating with a graceful bird-like hover. The Tintinnidae are
mostly marine, pelagic, with the general look of a stalkless _Vorticella_;
some have a latticed chitinous shell.[173]
{156}[Illustration: FIG. 59.—_Stentor polymorphus._ I, Young individual
attached, extended; II, adult in fission, contracted; _cv_ in I, afferent
canal of contractile vacuole; in II, contractile vacuole; N, moniliform
meganucleus (micronuclei omitted); _o_, mouth; the fine lines are the
myoneme fibrils. (From Verworn.)]
Among Peritrichaceae, _Vorticella_ (Fig. 60) and its allies have long been
known as Bell-animalcules to every student of pond-life. The body has
indeed the form of an inverted bell, closed at its mouth by the
"peristome," or oral disc; this is a short, inverted truncate cone set
obliquely so that its wide base hardly projects at one side, but is tilted
high on the other; the edge of the bell is turned out into a rim or
"collar," separated from the disc by a deep gutter. The collar, habitually
everted, or even turned down, contracts over the retracted disc when the
animal is retracted (E^2), which is brought about by any sort of shock, or
when it swims freely backwards. For the latter purpose a posterior ring of
cilia (or rather membranellae) is developed round the hinder end of the
bell (A, _cr_, E^3). The cilia of the adoral wreath are very strong, united
at the base into a continuous membrane, and indeed themselves partake of
the composite nature of membranellae. The wreath forms more than one turn
of a right-handed spiral, the innermost turn ending abruptly on the disc,
the outer leading down into the mouth at the point where the disc is most
tilted and the groove deepest.[174] The pharynx (_p_) is long, and contains
an undulating membrane (_u.m_) on its inner side projecting out through the
mouth, and numerous cilia; it leads deep into the body (_p_). The first
part is distinguished as the "vestibule" (_v_), as into it opens the anus,
and the contractile vacuole (_c.v._), the latter sometimes opening by a
reservoir (_r_). The body contains in the ectoplasm {157}myonema-fibrils
which, by their contraction, withdraw the disc, and at the same time
circular fibrils close the peristome over it. In the type-genus the
pellicle is continued into a long, slender elastic stalk (_s_), of which
the longitudinal myoneme fibrils of the ectoplasm converge to the stalk,
and are prolonged into it as a spirally winding fibre, sometimes
transversely striated.[175] The effect of the contraction of this is to
pull the stalk into a helicoid spiral (like a coil-spring), with the line
of insertion of the muscle along the inner side of the coils, which is, of
course, the shortest path from one end to the other (Fig. 60, B).
[Illustration: FIG. 60.—_Vorticella._ A, expanded; B, stalk in contraction;
_c_, eversible collar below peristome; _cr_, line of posterior ciliary
ring; _c.v_, contractile vacuole; _m_, muscle of stalk; _N_, meganucleus;
_n_, micronucleus; _p_, pharynx; _r_, reservoir of contractile vacuole;
_s_, tubular stalk; _u.m_, undulating membrane in vestibule; _v_, hinder
end of vestibule. E^1, E^2, two stages in binary fission; E^3, free zooid,
with posterior wreath; F^1, F^2 division into mega- and micro-zooids (_m_);
G^1, G^2, conjugation; _m_, microzooid. (Modified from Bütschli, from
Parker and Haswell.)]
The members of the Vorticellidae are very commonly attached to weeds or to
various aquatic Metazoa, each species being more or less restricted in its
haunts. _Vorticella_, the type, is singly {158}attached to a contractile
stalk; fission takes place in the vertical plane, and one of the two so
formed retains the original stalk, while the other swims off (Fig. 60,
E^1-E^3), often to settle close by, so that the individuals are found in
large social aggregates, side by side, fringing water-weeds with a halo
visible to the naked eye, which disappears on agitation by the sudden
contraction of all the stalks. _Carchesium_ and _Zoothamnium_ differ from
_Vorticella_ in the fact that the one daughter-cell remains attached by a
stalk coming off a little below the body of the other, so as to give rise
to large branching colonies.
In _Carchesium_ (Fig. 51) the muscular threads of each cell are separate,
while in _Zoothamnium_ they are continuous throughout the colony.
_Epistylis_ has a solid, rigid stalk, and may give rise to branching
colonies, which often infest the body of the Water-Fleas (Copepoda) of the
genus _Cyclops_. _Opercularia_ is characterised by the depth of the gutter,
the height of the collar, and the tapering downward of the elongated disc.
_Vaginicola_, _Pyxicola_, _Cothurnia_, _Scyphidia_, all inhabit tubes, some
of extreme elegance. _Ophrydium_ is a colonial form, found in ponds and
ditches, resembling _Opercularia_, but inhabiting tubes of jelly[176] that
coalesce by their outer walls into a large floating sphere; it usually
contains the green symbiotic Flagellate _Zoochlorella_. _Trichodina_ is
free, short, and cylindrical, with both wreaths permanently exposed, and is
provided with a circlet of hooks within the aboral wreath. It is often
parasitic, or perhaps rather epizoic, on the surface of _Hydra_ (see p.
254), gliding over its body[177] with a graceful waltzing movement; it
occurs also in the bladder and genito-urinary passages of Newts, and even
in their body-cavity and kidneys.
II. SUCTORIA = TENTACULIFERA
_Infusoria with cilia only in the young state,[178] without mouth or anus,
but absorbing food (usually living Ciliates) by one or more tentacles,
perforated at the apex; mostly attached, frequently epizoic, rarely
parasitic in the interior of other Protozoa._
{159}_Acineta_, Ehrb. (Fig. 61, 2); _Amoebophrya_, Koppen; _Choanophrya_,
Hartog (Fig. 62); _Dendrocometes_, St. (Fig. 61, 4); _Dendrosoma_, Ehrb.
(Fig. 61, 9); _Endosphaera_, Engelm.; _Ephelota_, Str. Wright (Fig. 61,
5, 8); _Hypocoma_, Gruber; _Ophryodendron_, Cl. and L. (Fig. 61, 7);
_Podophrya_, Ehrb. (Fig. 61, 1); _Rhyncheta_, Zenker (Fig. 61, 3);
_Sphaerophrya_, Cl. and L. (Fig. 61, 6), _Suctorella_, Frenzel;
_Tokophrya_, Bütschli.
This group, despite a superficial resemblance to the Heliozoa, show a close
affinity to the Ciliata; the nuclear apparatus is usually double though a
micronucleus is not always seen; the young are always ciliated, and the
mode of conjugation is identical in all cases hitherto studied. Most of the
genera are attached by a chitinous stalk (Fig. 61), continued in _Acineta_
into a cup or "theca" surrounding the cell. The pellicle is firm, often
minutely shagreened or "milled" in optical section by fine radial
processes, whether superficial rods or the expression of the meeting edges
of radial alveoli is as yet uncertain. The pellicle closely invests the
ectosarc, is continued down into a tubular sheath, from the base of which
the tentacle rises, and upwards to invest the tentacle, and is even
prolonged into its cavity in _Choanophrya_, the only genus where the
tentacles are large enough for satisfactory demonstration. These organs may
be one or more, and vary greatly in character. They may be (1) pointed for
prehension, puncture, and suction (_Ephelota_, Fig. 61, 5); (2) nearly
cylindrical, with a slightly "flared" truncate apex (_Podophrya_, Fig. 61,
1_a_); (3) filiform with a terminal knob; (4) "capitate" (_Acineta_, Fig.
61, 2); (5) bluntly truncate and capable of opening into a wide funnel for
the suction of food[179] (_Choanophrya_, Fig. 62; _Rhyncheta_, Fig. 61, 3).
Their movements, too, are varied, including retraction and protrusion, and
a degree of flexion which reaches a maximum in _Rhyncheta_ (Fig. 61, 3),
whose tentacle is as freely motile as an elephant's trunk might be supposed
to be were it as slender in proportion to its length. They are continued
into the body, and in _Choanophrya_ may extend right across it. In
_Podophrya_ trold the pellicle rises into a conical tube about the base of
the tentacle, which is retracted through it completely with the prey in
deglutition. In _Dendrocometes_, _Dendrosoma_, and _Ophryodendron_ (Fig.
61, 4, 9, 7), the tentacles arise from outgrowths of the cell-body.
{160}[Illustration: FIG. 61.—Various forms of Suctoria. 1, _a_ and _b_, two
species of _Podophrya_; _c_, a tentacle much enlarged; 2, _a_, _Acineta
jolyi_; 2, _b_, _A. tuberosa_, with four ciliated buds; in 6 the animal has
captured several small Ciliata; 8, _a_, a specimen multiplying by budding;
8, _b_, a free ciliated bud; 9, _a_, the entire colony; 9, _b_, a portion
of the stem; 9, _c_, a liberated bud. _a_, Organism captured as food;
_b.c_, brood-cavity; _bd_, bud; _c.vac_, contractile vacuole; _l_, test;
_mg.nu_, meganucleus; _mi.nu_, micronucleus; _nu_, nucleus; _t_, tentacle.
(From Parker and Haswell, after Bütschli and Saville Kent.)]
{161}The mechanism of suction is doubtful; but from the way particles from
a little distance flow into the open funnels of _Choanophrya_, it may be
the result of an increase of osmotic pressure. The external pellicle of the
tentacles is marked by a spiral constriction,[180] which may be prolonged
over the part included in the sheath. The endosarc is rich in oil-drops,
often coloured, and in proteid granules which sometimes absorb stains so
readily as to have been named "tinctin bodies." It usually contains at
least one contractile vacuole.
In _Dendrocometes_ (and perhaps others) the whole cell may become ciliated,
detach itself and swim off; this it does when its host (_Gammarus_) moults
its cuticle.
In fission or budding we have to distinguish many modes. (1) In the
simplest, after the nuclear apparatus has divided, the cell divides
transversely; the distal half acquires cilia and swims off to attach itself
elsewhere, while the proximal remains attached. The tentacles have
previously disappeared and have to be formed afresh in both. (2) More
commonly fission passes into budding on the distal face; a sort of groove
deepens around a central prominence which becomes the ciliated larva (Fig.
62, _em_); the tentacles of the "parent" are retained. This process passes
into (3) "internal budding," where a minute pit leads into a bottle-shaped
cavity.[181] (4) Again, the budding may be multiple, the meganucleus
protruding a branch for each bud, while the micronucleus, by successive
divisions, affords the supply requisite. _Sphaerophrya_ (Fig. 61, 6) and
_Endosphaera_ multiply freely by fission within their Ciliate hosts, and
were indeed described by Stein as stages in their life-cycle. Conjugation
of the same type as in most Ciliates has been fully worked out in
_Dendrocometes_ alone, by Hickson,[182] who has found the meganuclei
(though destined to disorganisation) conjugate for a short time by the
bridge of communication before the reciprocal conjugation of the
micronuclei.
We have referred to the endoparasitism of two genera. _Amoebophrya_ lives
in several Acanthometrids, and in the aberrant Radiolarian _Sticholonche_
(see p. 86). The attached species are {162}some of them indifferent to
their base; others are only found on Algae, or again only epizoic on
special Metazoan hosts, or even on special parts of these. Thus _Rhyncheta_
is only found on the couplers of the thoracic limbs of _Cyclops_, and
_Choanophrya_ on the ventral surface of its head and the adjoining
appendages.
[Illustration: FIG. 62.—_Choanophrya infundibulifera._ A, adult; B-D,
tentacles in action in various stages; E, tentacle at rest; F, young, just
settled down, _a_, _a_, _a_, Tentacles in various stages of activity; _c_,
central cavity; _c.v_, contractile vacuole; _em_, ciliated embryo showing
contractile vacuole and nucleus; _f_, spiral ridge; _m_, muscular wall of
funnel; _n_, nucleus; _tr_, opening of funnel. (A-D, F, modified after
Zenker; E, original.)]
We owe our knowledge of this group to the classical works of Ehrenberg,
Claparède and Lachmann, Stein, R. Hertwig, and Bütschli. Plate has shed
much light on _Dendrocometes_, and Hickson has studied its conjugation.
Ischikawa[183] has utilised modern histological methods for the cytological
study of _Ephelota bütschliana_. René Sand has written a useful, but
unequal, and not always trustworthy monograph of the Order,[184] containing
an elaborate bibliography.
PORIFERA (SPONGES)
BY
IGERNA B. J. SOLLAS, B.SC. (LOND.)
Lecturer on Zoology at Newnham College, Cambridge.
{165}CHAPTER VII
PORIFERA (SPONGES)[185]
INTRODUCTION—HISTORY—DESCRIPTION OF _HALICHONDRIA PANICEA_ AS AN EXAMPLE OF
BRITISH MARINE SPONGES AND OF _EPHYDATIA FLUVIATILIS_ FROM FRESH
WATER—DEFINITION—POSITION IN THE ANIMAL KINGDOM.
Sponges occupy, perhaps, a more isolated position than any other animal
phylum. They are not only the lowest group of multicellular animals, but
they are destitute of multicellular relatives. They are all aquatic
and—with the exclusion of a few genera found in fresh water—marine,
inhabiting all depths from between tide marks to the great abysses of the
ocean. They depend for their existence on a current of water which is
caused to circulate through their bodies by the activity of certain
flagellated cells. This current contains their food, it is their means of
respiration, and it carries away effete matters. Consequently sponges
cannot endure deprivation of oxygenated water except for short periods, and
only the hardiest inhabit regions where the supply is intermittent, as
between tide marks. This also renders useless attempts to keep specimens in
tanks, unless the water is frequently renewed.
The outward appearance of sponges has an exceptionally wide range, so that
it is difficult to give a novice any very definite picture of what he is to
expect when searching for these animals. This diversity is in part due to
the absence of organs of sufficient size to determine the shape of the
whole or limit its variation, {166}that is to say, the separate organs are
of an order of size inferior to that of the entire body. The animals are
fixed or lie loose on the sea bottom; there are in no case organs of
locomotion, and again no sense-organs, no segregated organs of sex, and as
a rule no distinction into axis and lateral members. It is by these
negative characters that the collector may easily recognise a sponge.
HISTORY.—Sponges are, then, in many of their characters unique; and they
present a variety of problems for solution, both of special and general
interest, they are widely distributed in time and space, and they include a
host of forms. It therefore causes no little surprise to learn that they
have suffered from a long neglect, even their animal nature having been but
recently established. Though known to naturalists from the time of
Aristotle, sponges have been left for modern workers as a heritage of
virgin soil: it has yielded to them a rich harvest, and is as yet far from
exhausted.
The familiar bath sponge was naturally the earliest known member of the
phylum. It is dignified by mention in the _Iliad_ and in the _Odyssey_, and
Homer, in his choice of the adjective "full of holes," πολύτρητος, shows at
least as much observation as many a naturalist of the sixteenth and
seventeenth centuries. Aristotle based his ideas of sponges entirely upon
the characters of the bath sponge and its near allies, for these were the
only kinds he knew. With his usual perspicuity he reached the conclusion
that sponges are animals, though showing points of likeness to plants.
The accounts of sponges after Aristotle present little of scientific
interest until the last century. Doubtless this is in part due to the
absence of organs which would admit of dissection, and the consequent
necessity of finer methods of study. Like other attached forms, sponges
were plant or animal as it pleased the imagination of the writer, and
sometimes they were "plant animals" or Zoophyta: those who thought them
animal were frequently divided among themselves as to whether they were
"polypous" or "apolypous." An opinion which it is somewhat difficult to
classify was that of Dr. Nehemiah Grew,[186] who says: "No _Sponge_ hath
any Lignous Fibers, but is wholly composed of those which make the Pith and
all the pithy parts {167}of a Plant, ... So that a _Sponge_, instead of
being a _Zoophyton_, is but the one-half of a Plant."
Sponges figure in herbals beside seaweeds and mushrooms, and Gerarde
says:[187] "There is found growing upon rockes near unto the sea a certaine
matter wrought together of the foame or froth of the sea which we call
Spunges ... whereof to speak at any length would little benefit the reader
... seeing the use thereof is so well known." About the middle of the
eighteenth century, authors, especially Peyssonnel, suggested that sponges
were but the houses of worms, which built them much as a bee or wasp builds
nests and cells. This was confuted by Ellis in 1765,[188] when he pointed
out that the sponge could not be a dead structure, as it gave proof of life
by "sucking and throwing out water." To Ellis, then, is due the credit of
first describing, though imperfectly, a current set up by sponges. He
mentions that Count Marsigli[189] had already made somewhat similar
observations.
It was not till 1825 that attention was again turned to the current, when
Robert Grant approached the group in a truly scientific manner, and was
ably supported by Lieberkühn. It would be impossible to do justice to Grant
in the brief summary to which we must limit ourselves. The most important
of his contributions was the discovery that water enters the sponge by
small apertures scattered over the surface, and leaves it at certain larger
holes, always pursuing a fixed course. He made a few rough experiments to
estimate the approximate strength of the current, and, though he failed to
detect its cause, he supposed that it was probably due to ciliary action.
Grant's suggestion was afterwards substantiated by Dujardin (1838), Carter
(1847), Dobie (1852), and Lieberkühn (1857). These five succeeded in
establishing the claims of sponges to a place in the animal kingdom, claims
which were still further confirmed when James-Clark[190] detected the
presence of the protoplasmic collar of the flagellated cells (see pp. 171,
176). Data were now wanted on which to base an opinion as to the position
of sponges within the animal kingdom. In 1878 Schulze[191] furnished
valuable embryological facts, in a description agreeing with an earlier one
of Metschnikoff's, of the amphiblastula larva (p. 226) and its
metamorphosis. {168}Then Bütschli[192] (1884) and Sollas[193] on combined
morphological and embryological evidence (1884) concluded that sponges were
remote from all the Metazoa, showing bonds only with Choanoflagellate
Protozoa (p. 121). This the exact embryological work of Maas, Minchin, and
Delage has done much to prove, but it has to be admitted that unanimity on
the exact position of the phylum has not yet been attained, some
authorities, such as Haeckel, Schulze, and Maas still wishing to include
sponges in the Metazoa.
In this short history we have been obliged to refer only to work helping
directly to solve the problem of the nature of a sponge, hence many names
are absent which we should have wished to mention.
HALICHONDRIA PANICEA.
One of the commonest of British sponges, which may be picked up on almost
any of our beaches, and which has also a cosmopolitan distribution, is
known by the clumsy popular name of the "crumb of bread sponge," alluding
to its consistency; or by the above technical name, with which even more
serious fault may be found.[194]
In its outward form _H. panicea_ affords an excellent case of a peculiarity
common among sponges. Its appearance varies according to the position in
which it has lived. In fact, Bowerbank remarks that it has no specific
form. It may grow in sheets of varying thickness closely attached to a
rock, when it is "encrusting," or it is frequently massive and lying free
on the sea bottom; again, it may be fistular, consisting of a single long
tube, or it may be ridge-like, apparently in this case consisting of a row
of long tubes fused laterally. In this last form it used to be called the
"cockscomb sponge," having been taken for a distinct species.
Bidder has proposed to call the different forms of the same species
"metamps" of the species. Figures of the metamps of _H. panicea_ will be
found in Bowerbank's useful Monograph.[195]
{169}The colour of the species is as inconstant as its form, ranging from
green to light brown and orange. MacMunn concludes from spectroscopic work
that _H. panicea_, contains at least three pigments, a chlorophyll, a
lipochrome, and a histohaematin.[196] Lipochromes vary from red to yellow,
chlorophyll is always associated with one or more of them. Histohaematin is
a respiratory pigment. Proof has not yet been adduced that the chlorophyll
is proper to the sponge and is not contained in symbiotic algae.
In spite of all this inconstancy _H. panicea_, is one of the most easily
determined species. It is only necessary to dry a small fragment, including
the upper surface; a beautiful honeycomb-like structure is then visible on
this surface, and among British sponges this is a property peculiar to the
species (Bowerbank). Whatever the form of the sponge, one or more large
rounded apertures are always present on the exterior; these are the
"oscula." In the encrusting metamp the oscula are flush with the general
surface, while in the other cases they are raised on conical projections;
fistular specimens carry the osculum at the distal end, and the cockscomb
has a row of them along its upper edge. Much more numerous than the oscula
are smaller apertures scattered over the general surface of the sponge, and
known as "ostia."
[Illustration: FIG. 63.—Portion of the surface of _H. panicea_, from dried
specimen. A, natural size; B, magnified. The large shaded patches are
ostia.]
If the sponge be placed in a shallow glass dish of sea water the function
of the orifices can be made out with the naked eye, especially if a little
powdered chalk or carmine be added to the water. If the specimen has been
gathered after the retreating tide has left it exposed for some time, this
addition is unnecessary, for as soon as it is plunged into water its
current bursts vigorously forth, and is rendered visible by the particles
of detritus that have accumulated in the interior during the period of
{170}exposure and consequent suspended activity. The oscula then serve for
the exit of currents of water carrying particles of solid matter, while the
entrance of water is effected through the ostia.
Sections show that the ostia lead into spaces below the thin superficial
layer or "dermal membrane"; these are continued down into the deeper parts
of the sponge as the "incurrent canals," irregular winding passages of
lumen continually diminishing as they descend. They all sooner or later
open by numerous small pores—"prosopyles"—into certain subspherical sacs
termed flagellated chambers. Each chamber discharges by one wide
aperture—"apopyle"—into an "excurrent canal." This latter is only
distinguishable from an incurrent canal by the difference in its mode of
communication with the chambers.
[Illustration: FIG. 64.—_H. panicea_: the arrows indicate the direction of
the current, which is made visible by coloured particles. (After Grant.)]
The excurrent canals convey to the osculum the water which has passed
through the ostia and chambers. All the peripheral parts of the sponge from
which chambers are absent are termed the "ectosome," while the
chamber-bearing regions are the "choanosome."
The peculiar crumb-of-bread consistency is due to the nature of the
skeleton, which is formed of irregular bundles and strands of minute
needles or spicules composed of silica hydrate, a substance familiar to us
in another form as opal: they are clear and transparent like glass. They
are scattered through the tissues in great abundance.
The classes of cellular elements in the sponge are as follows: Flattened
cells termed "pinacocytes" cover all the free surfaces, that is to say, the
external surface and the walls of the {171}excurrent and incurrent canals.
The flagellated chambers are lined by "choanocytes" (cf. Fig. 70, p. 176);
these are cells provided at their inner end with a flagellum and a collar
surrounding it. They resemble individuals of the Protozoan sub-class
Choanoflagellata, and the likeness is the more remarkable because no other
organisms are known to possess such cells. Taken together the choanocytes
constitute the "gastral layer," and they are the active elements in
producing the current. The tissue surrounding the chambers thus lying
between the excurrent and incurrent canals consists of a gelatinous matrix
colonised by cells drawn from two distinct sources. In the first place, it
contains cells which have a common origin with the pinacocytes, and which
together with them make up the "dermal layer"; these are the "collencytes"
and "scleroblasts"; secondly, it contains "archaeocytes," cells of
independent origin.
Collencytes are cells with clear protoplasm and thread-like pseudopodial
processes; they are distinguished as stellate or bipolar, according as
these processes are many or only two. Scleroblasts or spicule cells are at
first rounded, but become elongated with the growth of the spicule they
secrete, and when fully grown are consequently fusiform.
[Illustration: FIG. 65.—Diagrammatic section of a siliceous Sponge. _a.p_,
Apopyle; _d.o_, dermal ostia; _ex.c_, excurrent, or exhalant canal; _in.c_,
incurrent canal; _o_, osculum. (Modified from Wilson.)]
Each spicule consists of an organic filamentar axis or axial fibre around
which sheaths of silica hydrate are deposited successively by the
scleroblast. Over the greater length of the spicule the sheaths are
cylindrical, but at each end they taper to a point. The axial canal in
which the axial fibre lies is open at both ends, and the fibre is
continuous at these two points with an organic sheath, which invests the
entire spicule. From this structure we may conclude that the spicule grows
at both ends—_i.e._ it grows in two opposite directions along one line—it
has two rays lying in one axis, and is classed among uniaxial diactinal
spicules. Being {172}pointed at both ends it receives the special name
_oxea_. The lamination of the spicule is rendered much more distinct by
heating or treatment with caustic potash.[197]
[Illustration: FIG. 66.—Cut end of a length of a siliceous spicule from
_Hyalonema sieboldii_, with the lamellar structure revealed by solution. ×
104. (After Sollas.)]
The archaeocytes are rounded amoeboid cells early set apart in the larva;
they are practically undifferentiated blastomeres. Some of them become
reproductive elements, and thus afford a good instance of "continuity of
germ plasm," others probably perform excretory functions.[198]
[Illustration: FIG. 67.—Free-swimming larva of _Gellius varius_, in optical
section. _a_, Outer epithelium; _pi_, pigment; _x_, hinder pole. (After
Maas.)]
The reproductive elements are ova and spermatozoa, and are to be found in
all stages in the dermal jelly. Dendy states that the eggs are fertilised
in the inhalant canals, to which position they migrate by amoeboid
movements, and there become suspended by a peduncle.
The larva has unfortunately not been described, but as the course of
development among the near relatives of _H. panicea_ is known to be fairly
constant, it will be convenient to give a description of a "Halichondrine
type" of larva based on Maas' account of the development of _Gellius
varius_.[199] The free-swimming larvae escape by the osculum; they are
minute oval bodies moving rapidly by means of a covering of cilia. The
greater part of the body is a dazzling white, while the hinder pole is of a
brown violet colour. This coloured patch is non-ciliate, the general
covering of cilia ending at its edge in a ring of cilia twice the length of
the others. Forward {173}movement takes place in a screw line; when this
ceases the larva rests on its hinder pole, and the cilia cause it to turn
round on its axis.
Sections show that the larva is built up of two layers:—
1. "The inner mass," consisting of various kinds of cells in a gelatinous
matrix.
2. A high flagellated epithelium, which entirely covers the larva with the
exception of the hinder pole.
[Illustration: FIG. 68.—Longitudinal section through the hinder pole of the
larva of _G. varius_. _a_, Flagellated cells; _ma_^1, undifferentiated
cell; _ma_^2, differentiated cell; _pi_, pigment; _x_, surface of hinder
pole. (After Maas.)]
The cells in the inner mass are classified into (1) undifferentiated cells,
recognised by their nucleus, which possesses a nucleolus; these are the
archaeocytes; (2) differentiated cells, of which the nucleus contains a
chromatin net; these give rise to pinacocytes, collencytes, and
scleroblasts. Some of them form a flat epithelium, which covers the hinder
pole. Some of the scleroblasts already contain spicules. Fixation occurs
very early. The front pole is used for attachment, the pigmented pole
becoming the distal end (Fig. 69). The larva flattens out, the margin of
the attached end is produced into radiating pseudopodial processes. The
flagellated cells retreat to the interior, leaving the inner mass exposed,
and some of its cells thereupon form a flat outer epithelium. This is the
most important process of the metamorphosis; it is followed by a pause in
the outward changes, coinciding in time with rearrangements of the internal
cells to give rise to the canal system; that is to say, lacunae arise in
the inner mass, pinacocytes pass to the surface of the lacunae, and form
their lining; the flagellated cells, which have lain in confusion, become
grouped in small clusters. These become flagellated chambers,
communications are established between the various portions of the canal
system, and its external apertures arise. There is at first only one
osculum. The larvae may be obtained by keeping the parent sponge in a dish
of sea water, shielded from too bright a light, and surrounded by a second
dish of water to keep the temperature constant. They will undergo
{174}metamorphosis in sea water which is constantly changed, and will live
for some days.
We have said that the young sponge has only one osculum. This is the only
organ which is present in unit number, and it is natural to ask whether
perhaps the osculum may not be taken as a mark of the individual; whether
the fistular specimens, for example, of _H. panicea_ may not be solitary
individuals, and the cockscomb and other forms colonies in which the
individuals are merged to different degrees. Into the metaphysics of such a
view we cannot enter here. We must be content to refer to the views of
Huxley and of Spencer on Individuality.
But it is advisable to avoid speaking of a multi-osculate sponge as a
colony of many individuals, even in the sense in which it is usual to speak
of a colony of polyps as formed of individuals. The repetition of oscula is
probably to be regarded as an example of the phenomenon of repetition of
parts, the almost universal occurrence of which has been emphasised by
Bateson.[200] Delage[201] has shown that when two sponge larvae fixed side
by side fuse together, the resulting product has but one osculum. This,
though seeming to bear out our point of view, loses weight in this
connexion, when it is recalled that two Echinoderm larvae fused together
give rise in a later stage to but one individual.
[Illustration: FIG. 69.—Larva of _Gellius varius_ shortly after fixation.
The pigmented pole, originally posterior, is turned towards the reader.
_R_, Marginal membrane with pseudopodia; _x_, hinder pole. (After Maas.)]
EPHYDATIA FLUVIATILIS.
In the fresh water of our rivers, ponds, and lakes, sponges are represented
very commonly by _Ephydatia (Spongilla) fluviatilis_, a cosmopolitan
species. The search for specimens is most likely {175}to be successful if
perpendicular timbers such as lock-gates are examined, or the underside of
floating logs or barges, or overhanging branches of trees which dip beneath
the surface of the water.
The sponge is sessile and massive, seldom forming branches, and is often to
be found in great luxuriance of growth, masses of many pounds weight having
been taken off barges in the Thames. The colour ranges from flesh-tint to
green, according to the exposure to light. This fact is dealt with in a
most interesting paper by Professor Lankester,[202] who has shown not only
that the green colour is due to the presence of chlorophyll, but that the
colouring matter is contained in corpuscles similar to the chlorophyll
corpuscles of green plants, and, further, that the flesh-coloured specimens
contain colourless corpuscles, which, though differing in shape from those
which contain the green pigment, are in all probability converted into
these latter under the influence of sufficient light. The corpuscles, both
green and colourless, are contained in amoeboid cells of the dermal
layer;[203] and in the same cells but not in the corpuscles are to be found
amyloid substances.
The anatomy of _Ephydatia fluviatilis_ is very similar to that of
_Halichondria panicea_, differing only in one or two points of importance.
The ectosome is an aspiculous membrane of dermal tissue covering the whole
exterior of the sponge and forming the roof of a continuous subdermal
space. This dermal membrane is perforated by innumerable ostia, and is
supported above the subdermal cavity by means of skeletal strands, which
traverse the subdermal cavity and raise the dermal membrane into tent-like
elevations, termed conuli. The inhalant canals which arise from the floor
of the subdermal cavity are as irregular as in _H. panicea_, and
interdigitate with equally irregular exhalant canals; these latter
communicate with the oscular tubes. Between the two sets of canals are the
thin folds of the choanosome with its small subspherical chambers provided
with widely open apopyles (Fig. 70). The soft parts are supported on a
siliceous skeleton of oxeas, which may have a quite smooth surface or may
{176}be covered in various degrees with minute conical spines (Fig. 72,
_a_, _b_). These spicules are connected by means of a substance termed
_spongin_ deposited around their overlapping ends, so as to form an
irregular network of strands, of which some may be distinguished as main
strands or fibres, others as connecting fibres. In the main fibres several
spicules lie side by side, while in the connecting fibres fewer or
frequently single spicules form the thickness of the fibre. The fibres are
continuous at the base with a plate or skin of spongin, which is secreted
over the lower surface of the sponge and intervenes between it and the
substratum. Of the chemical composition of spongin we shall speak later
(see p. 237). It is a substance which reaches a great importance in some of
the higher sponges, and forms the entire skeleton of certain kinds of bath
sponge. Lying loose in the soft parts and hence termed flesh spicules, or
microscleres, are minute spicules of peculiar form. These are the
amphidiscs, consisting of a shaft with a many-rayed disc at each end (Fig.
72).
[Illustration: FIG. 70.—_Ephydatia fluviatilis._ Section of flagellated
chamber, showing the choanocytes passing through the apopyle. (After
Vosmaer and Pekelharing.)]
In addition to its habitat the fresh-water sponge is worthy of attention on
account of its methods of reproduction, which have arisen in adaptation to
the habitat. A similar adaptation is widespread among fresh-water members
of most aquatic invertebrates.[204]
{177}_Ephydatia fluviatilis_ normally produces not only free-swimming
larvae of sexual origin, but also internal gemmules arising asexually.
These bodies appear in autumn, distributed throughout the sponge, often
more densely in the deeper layers, and they come into activity only after
the death of the parent, an event which happens in this climate at the
approach of winter.
[Illustration: FIG. 71.—Portion of the skeletal framework of _E.
fluviatilis_. _a_, Main fibres; _b_, connecting fibres. (After Weltner.)]
[Illustration: FIG. 72.—Spicules of _E. fluviatilis_. _a._ _b._ _c._ Oxeas,
spined and smooth; _d._ _e_, amphidiscs, side and end views. (After
Potts.)]
Weltner[205] has shown that on the death and disintegration of the mother
sponge some of the gemmules remain attached to the old skeleton, some sink
and some float. Those which remain attached are well known to reclothe the
dead fibres with living tissue. They inherit, as it were, the advantages of
position, which contributed to the survival of the parent, as one of the
selected fittest. The gemmules which sink are doubtless rolled short
distances along the bottom, while those which float have the opportunity of
widely distributing the species with the risk of being washed out to sea.
But even these floating gemmules are exposed to far less dangers than the
delicate free-swimming larvae, for their soft parts are protected from
shocks by a thick coat armed with amphidiscs.
The gemmules are likewise remarkable for their powers of {178}resistance to
climatic conditions, powers which must contribute in no small way to the
survival of a species exposed to the variable temperatures of fresh water.
Thus, if the floating gemmules or the parent skeleton with its attached and
dormant offspring should chance to be included in the surface layer of ice
during the winter, so far from suffering any evil consequences they appear
to benefit by these conditions. Both Potts and Weltner have confirmed the
truth of this statement by experiments. Weltner succeeded in rearing young
from gemmules which had suffered a total exposure of 17 days to a
temperature "under 0° C."
Of important bearing on the question of the utility of the gemmules are
certain instances in which _E. fluviatilis_ has been recorded as existing
in a perennial condition.[206] The perennial individuals may or may not
bear gemmules, which makes it evident that, with the acquisition of the
power to survive the winter cold, the prime necessity of forming these
bodies vanishes.
The perennial specimens are described as exhibiting a diminished vegetative
activity in winter, the flagellated chambers may be absent (Lieberkühn), or
present in unusually small numbers (Weltner), the entire canal system may
be absent (Metschnikoff), or, on the other hand, it may be complete except
for the osculum.
[Illustration: FIG. 73.—Gemmule of _E. fluviatilis_. _b_, Amphidisc. (After
Potts.)]
In tropical countries gemmulation occurs as a defence against the ravages
caused by the dry season when the waters recede down their banks, exposing
all or most of their sponge inhabitants to the direct rays of the sun. The
sponges are at once killed, but the contained gemmules being thoroughly
dried, become efficient distributing agents of the species; they are light
enough to be carried on the wind. It is probable that those individual
sponges which escape desiccation survive the dry season without forming
gemmules.
It has been shown experimentally that gemmules are not injured by
drying—Zykoff found that gemmules kept dry for a period of two years had
not lost the power of germination.
{179}The mature gemmules consist of a more or less spherical mass of cells,
which we shall refer to as yolk cells, and of a complex coat. The latter is
provided with a pore or pore tube (Fig. 74) which is closed in winter by an
organic membrane.
There are three layers in the coat: an inner chitinous layer surrounded by
an air-chamber layer, which is finely vesicular, showing a structure
recalling plant tissue, and containing amphidiscs arranged along radii
passing through the centre of the gemmule. One of the discs of each
amphidisc lies in the inner chitinous coat, while the other lies in a
similar membrane which envelopes the air-chamber layer and is termed the
outer chitinous coat.
Marshall has suggested that one function of the amphidiscs is to weight the
gemmules and thus protect them against the force of the river current; and
no doubt the sinking or floating of individual gemmules depends on the
relative degree of development of the air-chambers and of the amphidiscs.
A study of the development of _Ephydatia_ gemmules vividly illustrates
various characters of the inner processes of sponges. Specially noteworthy
are the migrations of cells and the slight extent to which division of
labour is carried: one and the same cell will be found to perform various
functions.
[Illustration: FIG. 74.—Part of a longitudinal section of a gemmule of
_Ephydatia_ sp. passing through the pore (_a_). (After Potts.)]
The beginning of a gemmule is first recognisable[207] as a small cluster of
amoeboid archaeocytes in the dermal membrane. These move into the deeper
parts of the sponge to form larger groups. They are the essential part of
the gemmule, the yolk cells, which, when germination takes place, give rise
to a new sponge. They are followed by two distinct troops of actively
moving cells. Those forming the first troop arrange themselves round the
yolk cells and ultimately assume a columnar form so that they make an
epithelioid layer. They then secrete the inner chitinous coat. The cells of
the second troop are entrusted with the nutrition of the gemmule.
Consequently they pass in among the yolk cells, distribute their food
supplies, and make their escape {180}by returning into the tissues of the
mother sponge, before the columnar cells have completed the chitinous coat.
Yet another migration now occurs, the cells—"scleroblasts"—which have been
occupied in secreting amphidiscs at various stations in the sponge, carry
the fully formed spicules to the gemmules and place them radially round the
yolk cells between the radially lying cells of the columnar layer. The
scleroblasts themselves remain with the amphidiscs, and becoming modified,
contribute to the formation of the air-chamber layer. The columnar cells
now creep out between the amphidiscs till their inner ends rest on the
outer ends of these spicules. They then secrete the outer chitinous coat
and return to the mother sponge.
Carter gives directions[208] for obtaining young sponges from the gemmules.
The latter should be removed from the parent, cleaned by rolling in a
handkerchief, and then placed in water in a watch-glass, protected with a
glass cover and exposed to sunlight. In a few days the contents of the
gemmule issue from the foramen and can be seen as a white speck. A few
hours later the young sponge is already active and may be watched producing
aqueous currents. At this age the sponge is an excellent object for
studying in the living condition: being both small and transparent it
affords us an opportunity of watching the movements of particles of carmine
as they are carried by the current through the chambers.
Potts[209] describes how he has followed the transportal of spicules by
dermal cells, the end of each spicule multiplying the motion, swaying like
an oscillating rod.
In _E. fluviatilis_ reproduction also occurs during the warmer months in
this climate by means of sexual larvae. These are interesting for certain
aberrant features in their metamorphosis.[210] While some of the
flagellated chambers are formed in the normal way from the flagellated
cells of the larva, others arise each by division of a single archaeocyte.
This, it is suggested, is correlated with the acquisition of the method of
reproduction by gemmules, the peculiarities (_i.e._ development of organs
from archaeocytes) of which are appearing in the larvae.
DEFINITION.—We may now define sponges as multicellular, {181}two-layered
animals; with pores perforating the body-walls and admitting a current of
water, which is set up by the collared cells of the "gastral" layer.
POSITION IN THE ANIMAL KINGDOM.—Sponges are the only multicellular animals
which possess choanocytes, and their mode of feeding is unique. Since they
are two-layered it has been sought to associate them with the Metazoan
phylum Coelenterata, but they are destitute of nematocysts or any other
form of stinging cell, and their generative cells arise from a class of
embryonic cells set apart from the first, while the generative cells of
Coelenterata are derived from the ectoderm, or in other cases from the
endoderm. These weighty differences between sponges and that group of
Metazoa to which they would, if of Metazoan nature at all, be most likely
to show resemblance, suggest that we should seek a separate origin for
sponges and Metazoa. We naturally turn to the Choanoflagellate Infusorian
stock (see p. 121) as the source of Porifera, leaving the Ciliate stock as
the progenitors of Metazoa.
That both Porifera and Metazoa are reproduced by ova and spermatozoa is no
objection to this view, seeing that the occurrence of similar reproductive
cells has been demonstrated in certain Protozoa (see pp. 100, 128).
Let us now see which view is borne out by facts of embryology. Suppose, for
the moment, we regard sponges as Metazoa, then if the sponge larva be
compared with the Metazoan larva we must assign the large granular cells to
the endoderm; the flagellated cells to the ectoderm; and we are led to the
anomalous statement that the digestive cells in the adult are ectodermal,
the covering, outer cells endodermal; or conversely, if we start our
comparisons with the adults, then it follows that the larval ectoderm has
the characters of an endoderm, and the larval endoderm those of an
ectoderm.
Thus both embryology and morphology lead us to the same point, they both
show that in the absence of any fundamental agreement between Porifera and
Metazoa it is necessary to regard the two stocks as independent from the
very first, and hence the name PARAZOA (Sollas) has been given to the group
which contains the Porifera as its only known phylum.
Interesting in connexion with the phylogeny of Parazoa is the
Choanoflagellate genus _Proterospongia_ (Fig. 75), described by
{182}Saville Kent, and since rediscovered both in England and abroad.[211]
This is a colony of unicellular individuals embedded in a common jelly. The
individuals at the surface are choanoflagellate, while in the interior the
cells are rounded or amoeboid, and some of them undergo multiple fission to
form reproductive cells. This is just such a creature as we might imagine
that ancestral stage to have been of which the free-swimming sponge larva
is a reminiscence: for we have seen that the flagellated cells of the larva
are potential choanocytes.
[Illustration: FIG. 75.—_Proterospongia haeckeli._ _a_, Amoeboid cell; _b_,
a cell dividing; _c_, cell with small collar; _z_, jelly. × 800. (After S.
Kent.)]
{183}CHAPTER VIII
PORIFERA (_CONTINUED_): FORMS OF SPICULES—CALCAREA—HOMOCOELA—HETEROCOELA
—HEXACTINELLIDA—DEMOSPONGIAE—TETRACTINELLIDA—MONAXONIDA—CERATOSA—KEY TO
BRITISH GENERA OF SPONGES
Sponges fall naturally into two branches differing in the size of their
choanocytes: in the MEGAMASTICTORA these cells are relatively large,
varying from 5µ to 9µ in diameter; in MICROMASTICTORA they are about 3µ in
diameter.[212] For further subdivision of the group the SPICULES are such
important weapons in the hands of the systematist that it is convenient to
name them according to a common scheme. This has been arrived at by
considering first the number of axes along which the main branches of the
spicules are distributed, and secondly whether growth has occurred in each
of these axes in one or both directions from a point of origin.[213]
I. _Monaxons._—Spicules of rod-like form, in which growth is directed from
a single origin in one or both directions along a single axis. The axis of
any spicule is not necessarily straight, it may be curved or undulating.
The ray or rays are known as actines.
Biradiate monaxon spicules are termed "rhabdi" (Fig. 76, _a_). A rhabdus
pointed at both ends is an "oxea," rounded at both ends a "strongyle,"
knobbed at both ends a "tylote." By branching a rhabdus may become a
"triaene" (Fig. 110, _k_, _l_).
Uniradiate monaxon spicules are termed "styli."
II. _Tetraxons._—Spicules in which growth proceeds from an {184}origin in
one direction only, along four axes arranged as normals to the faces of a
regular tetrahedron. Forms produced by growth from an origin in one
direction along three axes lying in one plane are classed with tetraxons.
III. _Triaxons._—Spicules in which growth is directed from an origin in
both directions along three rectangular axes. One or more actines or one or
two axes may be suppressed.
IV. _Polyaxons._—Spicules in which radiate growth from a centre proceeds in
several directions.
V. _Spheres._—Spicules in which growth is concentric about the origin.
A distinction more fundamental than that of form is afforded by the
chemical composition: all sponges having spicules composed of calcium
carbonate belong to a single class, CALCAREA, which stands alone in the
branch Megamastictora.
[Illustration: FIG. 76.—Types of megascleres. _a_, Rhabdus (monaxon
diactine); _b_, stylus (monaxon monactine); _c_, triod (tetraxon
triactine); _d_, calthrop (tetraxon tetractine); _e_, triaxon hexactine;
_f_, euaster.]
BRANCH I. MEGAMASTICTORA
CLASS CALCAREA
Calcarea are marine shallow-water forms attached for the most part directly
by the basal part of the body or occasionally by the intervention of a
stalk formed of dermal tissue. They are almost all white or pale grey brown
in colour. Their spicules are either monaxon or tetraxon or both. The
tetraxons are either quadriradiate and then called "calthrops," or
triradiate when the fourth actine is absent. The triradiates always lie
more or less tangentially in the body-wall; similarly three rays of a
calthrop are tangentially placed, the fourth lying across the thickness of
the wall. It is convenient to include the triradiate and the three
tangentially placed rays of a calthrop under the common {185}term
"triradiate system" (Minchin). The three rays of one of these systems may
all be equal in length and meet at equal angles: in this case the system is
"regular." Or one ray or one angle may differ in size from the other rays
or angles respectively, which are equal: in either of these two cases the
system is bilaterally symmetrical and is termed "sagittal." A special name
"alate" is given to those systems which are sagittal in consequence of the
inequality in the angles. Thus all equiangular systems whether sagittal or
not are opposed to those which are alate. This is the natural
classification.[214]
SUB-CLASS I. HOMOCOELA.
The Homocoela or Ascons possess the simplest known type of canal system,
and by this they are defined. The body is a sac, branched in the adult, but
simple in the young; its continuous cavity is everywhere lined with
choanocytes, its wall is traversed by inhalant pores, and its cavity opens
to the exterior at the distal end by an osculum. The simple sac-like young
is the well-known Olynthus of Haeckel—the starting-point from which all
sponges seem to have set out. Two processes are involved in the passage
from the young to the adult, namely, multiplication of oscula and branching
of the original Olynthus tube or sac. If the formation of a new osculum is
accompanied by fission of the sac, and the branching of the latter is
slight, there arises an adult formed of a number of erect, well separated
main tubes, each with one osculum and lateral branches. Such is the case in
the LEUCOSOLENIIDAE. In the CLATHRINIDAE, on the other hand, branching of
the Olynthus is complicated, giving rise to what is termed reticulate body
form, that is, a sponge body consisting of a network of tubules with
several oscula, but with no external indication of the limits between the
portions drained by each osculum. These outward characters form a safe
basis for classification, because they are correlated with other
fundamental differences in structure and development.[215]
As in _Halichondria_, and in fact all sponges, the body-wall is formed of
two layers; the gastral layer, as we have said, forming a continuous lining
to the Ascon tube and its branches. The {186}dermal layer includes a
complete outer covering of pinacocytes, which is reflected over the oscular
rim to meet the gastral layer at the distal end of the tube; a deeper
gelatinous stratum in which lie scleroblasts and their secreted
products—calcareous spicules; and finally porocytes.[216] These last are
cells which traverse the whole thickness of the thin body-wall, and are
perforated by a duct or pore. The porocytes are contractile, and so the
pores may be opened or closed; they are a type of cell which is known only
in Calcarea. It will be noticed that the fusiform or stellate "connective
tissue cells" are absent. The layer of pinacocytes as a whole is highly
contractile, and is capable of diminishing the size of the sponge to such
an extent as quite to obliterate temporarily the gastral cavity.[217]
The choanocytes show certain constant differences in structure in the
families Clathrinidae and Leucosoleniidae respectively. In the former, the
nucleus of the choanocyte is basal; in the latter, it is apical, and the
flagellum can be traced down to it (Fig. 77).
[Illustration: FIG. 77.—The two types of Asconid collar cells. A, of
_Clathrina_, nucleus basal; B, of _Leucosolenia_, nucleus not basal,
flagellum arising from the nuclear membrane. (A, after Minchin; B, after
Bidder.)]
The tetraxon spicules have "equiangular" triradiate systems in the
Clathrinidae, while in Leucosoleniidae they are "alate." Finally, the larva
of Clathrinidae is a "parenchymula" (see p. 226), that of Leucosoleniidae
an "amphiblastula."
The fact that it is possible to classify the Calcarea Homocoela largely by
means of histological characters is in accordance with the importance of
the individual cell as opposed to the cell-layers generally throughout the
Porifera, and is interesting in serving to emphasise the low grade of
organisation of the Phylum. The organs of sponges are often unicellular
(pores), or the products of the activity of a single cell (many skeletal
elements); and even in the gastral layer, which approaches nearly to an
epithelium, comparable with the epithelia of Metazoa, the component cells
{187}still seem to assert their independence, the flagella not lashing in
concert,[218] but each in its own time and direction.
SUB-CLASS II. HETEROCOELA.
[Illustration: FIG. 78.—Transverse section of the body-wall of _Sycon
carteri_, showing articulate tubar skeleton, gastric ostia (_a.p_), tufts
of oxeas at the distal ends of the chambers (_fl.ch_), and pores (_p_).
(After Dendy.)]
[Illustration: FIG. 79.—_Sycon coronatum._ At a a portion of the wall is
removed, exposing the paragaster and the gastric ostia of the chambers
opening into it.]
The Heterocoela present a series of forms of successive grades of
complexity, all derivable from the Ascons, from which they differ in having
a discontinuous gastral layer. The simplest Heterocoela are included in the
family SYCETTIDAE, of which the British representative is _Sycon_ (Fig.
79). In _Sycon_ numerous tubular flagellated chambers are arranged radially
round a central cavity, the "paragaster," into which they open (Figs. 78,
79). The chambers, which are here often called radial tubes, are close set,
leaving more or less quadrangular tubular spaces, the {188}inhalant canals,
between them; and where the walls of adjacent chambers come in contact,
fusion may take place. Pores guarded by porocytes put the inhalant canals
into communication with the flagellated chambers. The paragaster is lined
by pinacocytes; choanocytes are confined to the flagellated chambers.
The skeleton is partly defensive, partly supporting; one set of spicules
strengthens the walls of the radial tubes and forms collectively the "tubar
skeleton." It is characteristic of Sycettidae that the tubar skeleton is of
the type known as "articulate"—_i.e._ it is formed of a number of
successive rings of spicules, instead of consisting of a single ring of
large spicules which run the whole length of the tube.
[Illustration: FIG. 80.—_Sycon setosum._ Young Sponge. × 200. _d_, Dermal
cell; _g_, gastral cell; _o_, osculum; _p_, pore cell; _sp__{1}, monaxon;
_sp__{3}, triradiate spicule. (After Maas.)]
The walls of the paragaster are known as the "gastral cortex"; they contain
quadriradiate spicules, of which the triradiate systems lie tangentially in
the gastral cortex, while the apical ray projects into the paragaster, and
is no doubt defensive. The distal ends of the chambers bristle with tufts
of oxeate spicules, and the separate chambers are distinguishable in
surface view. It is interesting to notice that in some species of _Sycon_,
the gaps between the distal ends of the chambers are covered over by a
delicate perforated membrane, thus leading on, as we shall see presently,
to the next stage of advance.[219] The larva of _Sycon_ is an amphiblastula
(see p. 227). Fig. 80 is a drawing of the young sponge soon after fixation;
it would pass equally well for an ideally simple Ascon or, neglecting the
arrangement of the spicules, for an isolated radial tube of _Sycon_. Figs.
81, 82 show the same sponge, somewhat older. From them it is seen that the
_Sycon_ type is produced from the young individual, in what {189}may be
called its Ascon stage, by a process of outgrowth of tubes from its walls,
followed by restriction of choanocytes to the flagellated chambers. Minute
observation has shown[220] that this latter event is brought about by
immigration of pinacocytes from the exterior. These cells creep through the
jelly of the dermal layer and line the paragaster as fast as its original
covering of choanocytes retreats into the newly formed chambers.
[Illustration: FIG. 81.—_S. setosum._ Young Sponge, with one whorl of
radial tubes. _o_, Osculum; _p_, pore; _sp__{1}, monaxon; _sp__{4},
quadriradiate spicule. (After Maas.)]
With a canal system precisely similar to that of _Sycon_, _Ute_ (Fig. 83)
shows an advance in structure in the thickening of the dermal layers over
the distal ends of the chambers. The dermal thickenings above neighbouring
chambers extend laterally and {190}meet; and there results a sheet of
dermal tissue perforated by dermal ostia, which open into the inhalant
canals, and strengthened by stout spicules running longitudinally. This
layer is termed a cortex; it covers the whole sponge, compacting the radial
tubes so that they form, together with the cortex, a secondary wall to the
sponge, which is once more a simple sac, but with a complex wall. The
cortex may be enormously developed, so as to form more than half the
thickness of the wall (Fig. 84). The chambers taken together are spoken of
as the chamber layer.
[Illustration: FIG. 82.—_Sycon raphanus._ A, Longitudinal section of young
decalcified Sponge at a stage somewhat later than that shown in Fig. 81. B,
Transverse section of the same through a whorl of tubes. _d_, Dermal
membrane; _g_, gastral membrane; _H_, paragaster; _sp__{4}, tetraradiate
spicule; _T_, radial tube. (After Maas.)]
[Illustration: FIG. 83.—Transverse section of the body-wall of _Ute_,
passing longitudinally through two chambers. _a.p_, Apopyle; _d.o_, dermal
ostium; _fl.ch_, flagellated chamber or radial tube; _i.c_, inhalant canal;
_p_, prosopyle. (After Dendy.)]
We have already alluded to the resemblance between a young Ascon person and
a radial tube of _Sycon_—a comparison which calls to mind the somewhat
strange view of certain earlier authors, that the flagellated chambers are
really the sponge individuals. If now we suppose each Ascon-like radial
tube of _Sycon_ to undergo that same process of growth by which the
{191}_Sycon_ itself was derived from the Ascon, we shall then have a sponge
with a canal system of the type seen in _Leucandra_ among British forms,
but more diagrammatically shown in the foreign genus _Leucilla_ (Fig. 85).
The foregoing remarks do not pretend to give an account of the transition
from _Sycon_ to _Leucilla_ as it occurred in phylogeny. For some indication
of this we must await embryological research.
In _Leucandra_ the fundamental structure is obscured by the irregularity of
its canal system. It shows a further and most important difference from
_Leucilla_ in the smaller size and rounded form of its chambers. This
change of form marks an advance in efficiency; for now the flagella
converge to a centre, so that they all act on the same drop of water, while
in the tubular chamber their action is more widely distributed and
proportionately less intense (see p. 236).
[Illustration: FIG. 84.—Transverse section through the body-wall of
_Grantiopsis_. _d.o_, Dermal ostium; _fl.ch_, flagellated chamber; _i.c_,
long incurrent canal traversing the thick cortex to reach the chamber
layer; _p_, apopyle. (After Dendy.)]
[Illustration: FIG. 85.—Transverse section through the body-wall of
_Leucilla_. _d.o_, Dermal ostium; _ex.c_, exhalant canal; _fl.ch_, chamber;
_i.c_, inhalant canal. (After Dendy.)]
Above are described three main types of canal system—that of Homocoela, of
_Sycon_, and of _Leucandra_ and _Leucilla_. These are conveniently termed
the first, second, and third types respectively, and may be briefly
described as related to one another somewhat in the same way as a scape,
umbel, and compound umbel among {192}inflorescences. These types formed the
basis of Haeckel's famous classification.[221] It has, however, been
concluded[222] that the skeleton is a safer guide in taxonomy, at any rate
for the smaller subdivisions; and in modern classifications genera with
canal systems of the third type will be found distributed among various
families; while in the Grantiidae, _Ute_ and _Leucandra_ stand side by
side. This treatment implies a belief that the third type of canal system
has been independently and repeatedly evolved within the Calcarea—an
example of a phenomenon, homoplasy, strikingly displayed throughout the
group. It is, remarkably enough, the case that all the canal systems found
in the remainder of the Porifera are more or less modified forms of one or
other of the second two types of canal system above described.
The families Grantiidae, Heteropidae, and Amphoriscidae, all possessing a
dermal cortex, are distinguished as follows:—The GRANTIIDAE by the absence
of subdermal sagittal triradiate spicules and of conspicuous subgastral
quadriradiates; the HETEROPIDAE by the presence of sagittal triradiates;
the AMPHORISCIDAE by the presence of conspicuous subgastral quadriradiates.
Two families of Calcarea, possibly allied, remain for special mention—the
Pharetronidae, a family rich in genera, and containing almost all the
fossil forms of the group, and the Astroscleridae.
The PHARETRONIDAE are with one, or perhaps two exceptions, fossil forms,
having in common the arrangement of the spicules of their main skeletal
framework in fibres. The family is divided into two sub-families:—
I. DIALYTINAE.—The spicules are not fused to one another; the exact mode of
their union into fibres is unknown, but an organic cement may be present.
_Lelapia australis_, a recent species, should probably be placed here as
the sole living representative. Dendy has shown[223] that this remarkable
species has a skeleton of the same fibrous character as is found in typical
Dialytinae, and that the triradiate spicules in the fibres undergo a
modification into the "tuning-fork" type (Fig. 86, C), to enable them to be
compacted into smooth fibres. {193}"Tuning-forks," though not exclusively
confined to Pharetronids, are yet very characteristic of them.
[Illustration: FIG. 86.—Portions of the skeleton of _Petrostroma schulzei._
A, Framework with ensheathing pellicle; B, quadriradiate spicules with
laterally fused rays; C, a "tuning-fork." (After Doederlein.)]
II. LITHONINAE.—The main skeletal framework is formed of spicules fused
together, and is covered by a cortex containing free spicules.
[Illustration: FIG. 87.—A spicule from the skeleton framework of
_Plectroninia_, showing the terminally expanded rays. (After Hinde.)]
The sub-family contains only one living genus and a few recently described
fossil forms. _Petrostroma schulzei_[224] lives in shallow water near
Japan; _Plectroninia halli_[225] and _Bactronella_ were found in Eocene
beds of Victoria; _Porosphaera_[226] long known from the Chalk of England
and of the Continent, has recently been shown by Hinde[226] to be nearly
allied to _Plectroninia_; finally, _Plectinia_[227] is a genus erected by
Počta for a sponge from Cenomanian beds of Bohemia. Doederlein, in 1896,
expressed his opinion that fossil representatives of Lithoninae would most
surely be discovered. The fused spicules are equiangular quadriradiates;
they are united in _Petrostroma_ by lateral fusion of the rays, in
_Plectroninia_ (Fig. 87) and _Porosphaera_ by {194}fusion of apposed
terminal flat expansions of the rays, and in some, possibly all, genera a
continuous deposit of calcium carbonate ensheaths the spicular reticulum.
Thus they recall the formation of the skeleton on the one hand of the
Lithistida and on the other of the Dictyonine Hexactinellida (see pp. 202,
211). "Tuning-forks" may occur in the dermal membrane.
[Illustration: FIG. 88.—_Astrosclera willeyana_, Lister. A, the Sponge, ×
about 3. _p_, The ostia on its distal surface. B, a portion of the skeleton
showing four polyhedra with radiating crystalline fibres. C, an ostium; the
surrounding tissue contains young stages of polyhedra. (After Lister.)]
The ASTROSCLERIDAE, as known at present, contain a single genus and
species, apparently the most isolated in the phylum. _Astrosclera
willeyana_[228] was brought back from the Loyalty Islands, and from
Funafuti of the Ellice group. Its skeleton is both chemically and
structurally aberrant. In other Calcarea the calcium carbonate of the
skeleton is present as calcite, in _Astrosclera_ as aragonite, and the
elements are solid polyhedra, {195}united by their surfaces to the total
exclusion of soft parts (Fig. 88). Each element consists of crystalline
fibres radially disposed around a few central granules, and terminating
peripherally in contact with the fibres of adjacent elements. Young
polyhedra are to be found free in the soft parts at the surface. The
chambers are exceptionally minute, especially for a calcareous sponge,
comparing with those of other sponges as follows:—
_Astrosclera_ chambers, 10µ × 8µ to 18µ × 11µ.
Smallest chambers in Silicea, 15µ × 18µ to 24µ × 31µ.
Smallest chambers in Calcarea, 60µ × 40µ.
In its outward form _Astrosclera_ resembles certain Pharetronids. The
minute dimensions of the ciliated chambers relegate _Astrosclera_ to the
Micromastictora, and the fortunate fact that the calcium carbonate of its
skeleton possesses the mineral characters not of calcite, but of aragonite,
renders it less difficult to conceive that its relations may be rather with
the non-calcareous than the calcareous sponges.
BRANCH II. MICROMASTICTORA
All sponges which do not possess calcareous skeletons are characterised by
choanocytes, which, when compared with those of Calcarea, are conspicuous
for their smaller size. The great majority (Silicispongiae) of the
non-calcareous sponges either secrete siliceous skeletons or are connected
with siliceous sponges by a nicely graded series of forms. The small
remainder are entirely askeletal. All these non-calcareous sponges are
included, under the title Micromastictora, in a natural group, opposed to
the Megamastictora as of equal value.
The subdivision of the Micromastictora is a matter of some difficulty. The
Hexactinellida alone are a well circumscribed group. After their separation
there remains, besides the askeletal genera, an assemblage of forms, the
Demospongiae, which fall into two main tribes. These betray their
relationship by series of intermediate types, but a clue is wanting which
shall determine decisively the direction in which the series are to be
read. The askeletal genera are the _crux_ of the systematist. It is perhaps
safest, while recognising that many of them bear a likeness of {196}one
kind or another to various Micromastictora, to retain them together in a
temporary class, the Myxospongiae.
CLASS I. MYXOSPONGIAE
The class Myxospongiae is a purely artificial one, containing widely
divergent forms, which possess a common negative character, namely, the
absence of a skeleton. As a result of this absence they are all encrusting
in habit.
One genus, _Hexadella_, has been regarded by its discoverer Topsent[229] as
an Hexactinellid. The same authority places _Oscarella_ with the
Tetractinellida; it is more difficult to suggest the direction in which we
are to seek the relations of the remaining type, _Halisarca_.
_Hexadella_, from the coast of France, is a remarkable little rose-coloured
or bright yellow sponge, with large sac-like flagellated chambers and a
very lacunar ectosome.
_Oscarella_ is a brightly coloured sponge, with a characteristic velvety
surface; it is a British genus, but by no means confined to our shores. Its
canal system has been described by some authors as diplodal, by others as
eurypylous. Topsent[230] has shown, and we can confirm his statement, that
though the chambers have usually the narrow afferent and efferent ductules
of a diplodal system, yet since each one may communicate with two or three
canals, the canal system cannot be described as diplodal. The hypophare
attains a great development, and in it the generative products mature. The
pinacocytes, like those of Plakinidae, and perhaps of _Aplysilla_, are
flagellated.
_Halisarca_, also British, is easily distinguished from _Oscarella_ by the
presence of a mucus-like secretion which oozes from it, and by the absence
of the bright coloration characteristic of _Oscarella_. It naturally
suggests itself that the coloration in the one case and the secretion in
the other are protective, and in this respect perform one of the functions
of the skeleton of other sponges. The chambers are long, tubular, and
branched. There is no hypophare.
{197}CLASS II. HEXACTINELLIDA[231]
_Silicispongiae, defined by their spicules, of which the rays lie along
three rectangular axes. The canal system is simple, with thimble-shaped
chambers. The body-wall is divided into endosome, ectosome, and
choanosome._
Some authors would elevate the Hexactinellida to the position of a third
main sub-group of Porifera, thus separating them from other siliceous
sponges. In considering this view it is important to realise at the outset
that they are deep-water forms. They bear evident traces of the influence
of their habitat, and like others of the colonists of the deep sea, are
impressed with marked archaic features. Yet they are still bound to other
Micromastictora, first by the small size of their choanocytes, and secondly
by the presence of siliceous spicules. This second character is really a
double link, for it involves not merely the presence of silica in the
skeleton, but also the presence in each spicule of a well-marked axial
filament. Now this axial filament is a structure which is gaining in
importance, for purposes of classification, in proportion as its absence in
Calcarea is becoming more probable. The Hexactinellida are the only
sponges, other than the bath sponge, which are at all generally known. They
have won recognition by their beauty, as the bath sponge by its utility,
and, like it, one of their number—the Venus's Flower-Basket—forms an
important article of commerce, the chief fishery being in the Philippine
Islands. This wonderful beauty belongs to the skeleton, and is greatly
concealed when the soft parts are present.
We have said that the Hexactinellids are deep-sea forms; they are either
directly fixed to the bottom or more often moored in the ooze by long tufts
of rooting spicules. In the "glass-rope sponge," the rooting tuft of long
spicules, looking like a bundle of spun glass, is valued by the Japanese,
who export it to us. In _Monorhaphis_ the rooting tuft is replaced by a
single giant spicule,[232] three metres in length, and described as "of the
thickness of a little finger"! Probably it is as a result of their fixed
life in the calm waters of the deep sea[233] that {198}Hexactinellids
contrast with most other sponges by their symmetry. It should not, however,
be forgotten that many of the Calcarea which inhabit shallow water exhibit
almost as perfect a symmetry.
[Illustration: FIG. 89.—Longitudinal section of a young specimen of
_Lanuginella pupa_ O.S., with commencing formation of the oscular area. ×
35. _d.m_, Dermal membrane; _g.m_, gastral membrane; _pg_, paragaster;
_sd.tr_, subdermal trabeculae; _Sg.tr_, subgastral trabeculae. (After F. E.
Schulze.)]
The structure of the body-wall in Hexactinellida is so constant as to make
it possible to give a general description applicable to all members of the
group. It is of considerable thickness, but a large part is occupied by
empty spaces, for the actual tissue is present in minimum quantity. In the
wall the chamber-layer is suspended by trabeculae of soft tissue, between a
dermal membrane on the outside and a similar gastral membrane on the inner
side (Fig. 89). Thus the water entering the chambers through their numerous
pores has first passed through the ostia in the dermal membrane and
traversed the subdermal trabecular space; on leaving the chambers it flows
through the subgastral trabecular space and the ostia in the gastral
membrane, to enter the paragaster and leave the body at the osculum. The
trabeculae and the dermal and gastral membranes together constitute the
dermal layer. This conclusion is based on comparison with adults of the
other groups, for in the absence of embryological knowledge no direct
evidence is available. According to {199}the Japanese investigator, Isao
Ijima,[234] the dermal and gastral membranes are but expansions of the
trabeculae, and the trabeculae themselves are entirely cellular, containing
none of the gelatinous basis met with in the dermal layer of all other
sponges. There is no surface layer of pinacocytes, the cells forming the
trabeculae being all of one type, namely, irregularly branching cells,
connected with one another by their branches to form a syncytium. In the
trabeculae are found scleroblasts and archaeocytes.
The chambers have a characteristic shape: they are variously described as
"thimble-shaped," "tubular," or "Syconate," and they open by wide mouths
into the subgastral trabecular space. Their walls have been named the
_membrana reticularis_ from the fact that, when preserved with only
ordinary precautions, they are seen as a regular network of protoplasmic
strands, with square meshes and nuclei at the nodes. This appearance
recently found an explanation when Schulze, for the first time, succeeded
in preserving the collared cells of Hexactinellids.[235] Schulze was then
able to show that the choanocytes are not in contact with one another at
their bases, where the nuclei are situated, but communicate with one
another by stout protoplasmic strands. The form of the choanocyte can be
seen in Fig. 91.
[Illustration: FIG. 90.—Portion of the body-wall of _Walteria_ sp., showing
the thimble-shaped flagellated chambers, above which is seen the dermal
membrane. (After F. E. Schulze.)]
To Schulze's description of the chamber, Ijima has added the important
contributions that every mesh in the reticulum functions as a chamber pore
or prosopyle; and that porocytes, such as are found in Calcarea, are
wanting. This structure of the chamber-walls, the absence of gelatinous
basis in the dermal layer, and the slight degree of histological
differentiation in {200}the same layer, added to the more obvious character
of thimble-shaped chambers, are the chief archaic features of Hexactinellid
morphology.
[Illustration: FIG. 91.—Portion of a section of the membrana reticularis or
chamber-wall of _Schaudinnia arctica_, × 1500. (After F. E. Schulze.)]
The skeleton which supports the soft parts is, like them, simple and
constant in its main features. It is secreted by scleroblasts, which lie in
the trabeculae, and is made up of only one kind of spicule and its
modifications. This is the hexactine, a spicule which possesses six rays
disposed along three rectangular axes. Each ray contains an axial thread,
which meets its fellow at the centre of the spicule, where they together
form the axial cross. Modifications of the hexactine arise either by
reduction or branching, by spinulation or expansion of one or more of the
rays. The forms of spicule arising by reduction are termed pentactines,
tetractines, and so on, according to the number of the remaining rays.
Those rays which are suppressed leave the proximal portion of their axial
thread as a remnant marking their former position (Fig. 94). Octactine
spicules seem to form an exception to the above statements, but Schulze has
shown that they too are but modifications of the hexactine arising by (1)
branching of the rays of a hexactine, followed by (2) recombination of the
secondary rays (Fig. 92).
[Illustration: FIG. 92.—A, discohexaster, in which the four cladi _a_,
_a'_, _b_, _b'_, _c_ of each ray start directly from a central nodule. B,
disco-octaster, resulting from the redistribution of the twenty-four cladi
of A into eight groups of three. (After Schulze, from Delage.)]
The various spicules are named, irrespective of their form, according to
their position and corresponding function. The {201}arrangement of the
spicules is best realised by means of a diagram (Fig. 93).
[Illustration: FIG. 93.—Scheme to show the arrangement of spicules in the
Hexactinellid skeleton. _Canalaria_, microscleres in the walls of the
excurrent canals; _Dermalia Autoderm[alia]_, microscleres in the dermal
membrane; _D. Hypoderm[alia]_, more deeply situated dermalia;
_Dictyonalia_, parenchymalia which become fused to form the skeletal
framework of Dictyonina; _Gastralia Autogastr[alia]_, microscleres in the
gastral membrane; _Gastralia Hypogastr[alia]_, more deeply situated
gastralia; _Parenchymalia Principalia_, main supporting spicules between
the chambers; _P. Comitalia_, slender diactine or triactine spicules
accompanying the last; _P. Intermedia_, microscleres between the P.
principalia; _Prostalia_, projecting spicules; _P. basalia_, rooting
spicules, from the base; _P. marginalia_, defensive spicules, round the
oscular rim; _P. pleuralia_, defensive spicules, from the sides. (From
Delage and Hérouard, after F. E. Schulze.)]