CHAPTER V.--ONTOGENY OR INDIVIDUAL DEVELOPMENT._Report on the Radiolaria Collected by H.M.S. Challenger During the Years 1873-1876, First Part: Porulosa (Spumellaria and Acantharia) Report on the Scientific Results of the Voyage of H.M.S. Challenger During the Years 1873-76, Vol. XVIII

(§§ 141-152.)

141. _Individual Developmental Stages._--The germinal history of the

Radiolaria presents great obstacles to direct observation, and hence is

very incompletely known. The fragmentary observations, however (having been

made on Radiolaria of very various groups and supplemented by comparative

anatomical considerations), allow us to draw a general picture of the

essential developmental processes in this great class. It may probably be

assumed that in all Radiolaria, after maturation, the central capsule

discharges the function of a sporangium, and its contents are broken up

into numerous flagellate swarm-spores (zoospores). After these flagellate

swarm-spores (resembling _Astasia_) have emerged from the ruptured central

capsule, they probably pass over into a _Heliozoan_-stage (_Actinophrys_)

and then after the formation of a jelly-veil into the condition of

_Sphærastrum_. Afterwards, when a membrane is formed between the outer

jelly-veil and the inner nucleated cell-body, an _Actissa_-stage arises,

which exhibits in its simplest form the differentiation of the spherical

unicellular body into the central capsule and calymma. _Actissa_ thus

represents both ontogenetically and phylogenetically the primitive

condition of the Radiolarian organism, and may thus be regarded as the

point of departure of all other forms.

142. _The Astasia-Stage._--The formation of flagellate zoospores in the

mature central capsule is probably to be regarded as the common form of

individual development in all Radiolaria; since the whole contents are

utilised in the formation of these swarm-spores, and since the

extracapsulum takes no share in the process and perishes after they are

evacuated, the _central capsule_ may be regarded as a _sporangium_ (see

note A, below). The zoospores of the Radiolaria generally arise in the

following way:--the nucleus of the unicellular organism, sometimes early,

sometimes late (and in several different ways, §§ 63-70) breaks up into

numerous small nuclei, and each of these surrounds itself with a small

portion of the endoplasm. Very often, perhaps generally, this endoplasm

contains one or several fat-granules and sometimes also a small oblong

crystal; from the protoplasm {xciv}of the small roundish or ovoid cells

protrudes one or more vibratile flagella. The fully developed spores, which

commence their vibrations even within the central capsule, emerge when it

ruptures, and swim about freely in the surrounding water by means of the

flagellum. At this stage of its existence the young Radiolarian represents

essentially the simplest form of the Flagellata, such as _Astasia_ or

_Euglena_; the unicellular body is for the most part ovoid or

subcylindrical, sometimes fusiform or reniform, usually from 0.004 to 0.008

mm. in diameter (Pl. 1, fig. 1_c_; Pl. 129, fig. 11). In the anterior part

of the flagellate cell, immediately behind the base of the flagellum, lies

a homogeneous, spherical nucleus, whilst in the posterior part are usually

several small fat-granules and often also a small oblong crystal (hence the

name "crystal-spore," "Krystall-Schwärmer"). The number of vibrating

flagella, which are extremely long and fine, seems to be variable, usually

one, sometimes two, occasionally perhaps three, or even four or more (see

note B).

A. The formation of the motile spores in the central capsule was first

observed by J. Müller in _Acanthometra_ (1856, L. N. 10, p. 502), then by

A. Schneider in _Thalassicolla_ (1858, L. N. 13, p. 41), and finally by

myself in _Sphærozoum_ (1859, L. N. 16, p. 141). These older observations

were, however, incomplete, for the origin of the motile corpuscles from

the contents of the central capsule was not observed. The first complete

and detailed observations upon the formation of spores in the Radiolaria

were published in 1871 by Cienkowski (L. N. 22, p. 372, Taf. xxix.); they

relate to two different Polycyttaria, _Collosphæra_ and _Collozoum_.

These investigations were supplemented by R. Hertwig on _Collozoum_ and

_Thalassicolla_ (1876, L. N. 26, pp. 28, 43, &c.); on _Collozoum_ he made

the important discovery that the Polycyttaria form two kinds of spores,

one with and the other without crystals, and that the latter are divided

into macrospores and microspores (compare the chapter on "Reproduction,"

§§ 212-216). Quite recently Karl Brandt has confirmed these observations,

and has extended them to all the genera of Polycyttaria (1881, L. N. 38,

p. 393, and 1885, _loc. cit._).

B. The number of flagella, projecting from each spore, is very difficult

to determine, owing to their extraordinary length and slenderness. It

appeared to me that in the majority of those Radiolaria whose spores I

investigated only a single flagellum could be demonstrated with

certainty, although sometimes two, springing from a common base, seemed

to be present. Compare the chapter on "Reproduction," (§ 215) and the

recent work of Karl Brandt on Sphærozoea (1885, L. N. 52, pp. 145-174).

143. _The Actinophrys-Stage._--The fate of the flagellate zoospores which

emerge from the mature central capsule of the Radiolaria has not hitherto

been decided by actual observation; all attempts to rear the swarming

zoospores have been in vain, for they have soon died. From what we know,

however, of the comparative morphology of the Protista, the hypothesis is

fully justified, that between the _Astasia_-stage of the flagellate

swarm-spores, and the well-known _Actissa_-stage of the simplest

Radiolaria, there lies an intermediate developmental stage, which may be

regarded as being essentially the simplest Heliozoan form, _Actinophrys_ or

_Heterophrys_. The swarm-spore is very probably converted directly in to a

simple floating _Heliozoon_ by its elongated or ovoid body {xcv}becoming

spherical and by fine pseudopodia protruding all round instead of a single

flagellum; the nucleus at the same time assuming a central position.

144. _The Sphærastrum-Stage._--The _Actinophrys_-stage of the young

Radiolaria, which proceeds immediately from the flagellate zoospore, is

probably connected with the _Actissa_-stage by an intermediate form, which

may be regarded as a simple skeletonless _Heliozoon_ with a jelly-veil; a

well-known example of such a form is _Sphærastrum_ (in the solitary, not

the social condition) and _Heterophrys_. This important intermediate form

has arisen from the simple _Actinophrys_-stage by the excretion of an

external structureless jelly-veil, such as is formed in many other Protista

(_e.g._, in the encystation of many Infusoria). The young Radiolarian in

this second _Heliozoon_-stage becomes a simple cell with pseudopodia

radiating on all sides; its body consists of three concentric spheres, the

central nucleus, the protoplasmic body proper, and the surrounding calymma

or jelly-veil. When a firm membrane is developed between the last two

spheres this _Sphærastrum_-stage passes over into the _Actissa_.

The gap in our empirical knowledge which still exists between the

flagellate stage (§ 142) and the simplest Radiolarian stage (_Actissa_, §

145), can be filled hypothetically only by the assumption of several

_Heliozoon_-stages following one upon another. It is possible also that

the capsule-membrane is not formed between the endoplasm and exoplasm (as

here supposed), but that the membrane was formed first outside the cell

and the extracapsulum subsequently secreted around it.

145. _The Actissa-Stage._--The first SPUMELLARIAN genus, _Actissa_, is not

only the simplest form actually observed among the Radiolaria, and the true

prototype of the whole class, but also the simplest form under which the

Radiolarian organisation can be conceived. It is therefore extremely

probably that _Actissa_ not only forms the common stem-form of the whole

class in a phylogenetic sense, but is also its common ontogenetic or

germinal form. Probably in all Radiolaria the _Sphærastrum_-stage develops

immediately into the typical _Actissa_-stage, by the formation of a firm

membrane between the protoplasmic body of the spherical Heliozoan cell and

its jelly-veil. Thus arises the characteristic central capsule, which is

wanting in the nearly related Heliozoa. It is further probable that all

Radiolaria in their early stage will so far conform to the state of things

in _Actissa_ as to have the capsule-membrane of the spherical skeletonless

cell perforated everywhere by fine pores. This structure is retained in all

SPUMELLARIA, whilst in the other three legions those structural relations

of the capsule which are characteristic of each develop from the

_Actissa_-stage.

146. _The Ontogeny of the Spumellaria._--In the simplest case the

individual development in the SPUMELLARIA ceases with the _Actissa_-stage.

In all other genera of this legion diverging forms proceed from this, of

which the different growth of the three dimensive {xcvi}axes on the one

hand (§§ 44, 45), and the differentiation of the various parts of the

unicellular organism with the formation of the skeleton on the other, are

of pre-eminent significance. Even in the varying growth of the central

capsule in the different dimensions of space in the skeletonless

#Colloidea#, four different modes may be distinguished, which further, in

the corresponding development of the skeleton, furnish the basis for the

origin of the four orders of #Sphærellaria#. The most primitive and

simplest form of growth, equal extension in all directions, is found in the

spherical central capsule and the concentric spherical skeletons

(_Procyttarium_, #Sphæroidea#). When the growth of the central capsule

proceeds more rapidly in the direction of the vertical main axis than in

any other direction, the ellipsoidal or cylindrical central capsule

(_Actiprunum_) arises, and the vertically elongated skeleton of the

#Prunoidea#, which is derived from it. When, on the contrary, the growth of

the central capsule and lattice-shell is less in the direction of the

vertical main axis than in any other direction, the lenticular or discoid

central capsule (_Actidiscus_) arises, and the corresponding lenticular

shell of the #Discoidea#. Finally, even quite early in many SPUMELLARIA,

the growth of the central capsule and of the corresponding lattice-shell in

the three dimensive axes is different, and hence arise the lentelliptical

forms whose geometrical type is the triaxial ellipsoid or the rhombic

octahedron (_Actilarcus_, #Larcoidea#). Thus the origin of the four orders

of #Sphærellaria# is simply explained by a varying growth in the different

dimensive axes. The _primary_ (innermost) lattice-shell is in this legion

always _simultaneously_ developed (suddenly excreted at the moment of

lorication from the sarcodictyum). The _secondary_ lattice-shells, on the

other hand, which surround the former concentrically, and are united with

it by radial bars, arise _successively_ from within outwards.

147. _The Ontogeny of the Acantharia._--The individual development of the

ACANTHARIA in the simplest case (_Actinelius_) stops at a point which

differs from the _Actissa_-stage only in the change of radial axial threads

into acanthin spines. In the small group #Actinelida#, their number remains

variable and usually indeterminate (Adelacantha), whilst in the great

majority of the legion (#Acanthonida# and #Acanthophracta#) the number is

constantly twenty, and those spines are regularly arranged according to the

Müllerian law in five parallel circles, each containing four crossed spines

(Icosacantha). The simplest form among these latter is _Acanthometron_,

which may be regarded both ontogenetically and phylogenetically as the

common starting-point of all the Icosacantha. Within this extensive group

variations in the length of the dimensive axes appear, similar to those

observed in the SPUMELLARIA. In the Astrolonchida and #Sphærophracta# the

central capsule remains spherical, extending equally in all directions; and

correspondingly the lattice-shell, which is excreted on the surface of the

spherical calymma, remains spherical. In the Belonaspida (just as in the

#Prunoidea#) {xcvii}this form passes over into an ellipsoid by prolongation

of one axis; on the contrary, in the Hexalaspida (as in the #Discoidea#)

the discoidal or lenticular form arises by shortening of an axis. Finally,

in the Diploconida, and in some Hexalaspida in which the growth is

different in all three dimensive axes (as in the #Larcoidea#), both the

central capsule and the shell assume the lentelliptical form. The

lattice-shell of the #Acanthophracta# is usually successive in its

development, since from each of the twenty radial spines two or four

tangential apophyses proceed, whose branches subsequently unite and combine

to form the lattice-shell. Only in the peculiar Sphærocapsida can the

pavement-like shell arise simultaneously or in a moment of lorication.

148. _The Ontogeny of the Nassellaria._--The individual development of the

NASSELLARIA in the simplest instance remains stationary at the skeletonless

Nasselid stage (_Cystidium_, _Nassella_), which can be immediately derived

from the foregoing _Actissa_-stage by the disappearance of the pores in the

upper (apical) hemisphere of the central capsule, whilst in the lower

(basal) portion they are modified to form a porochora; the podoconus is

developed within the endoplasm upon this latter. Usually the spherical form

of the central capsule passes over into an ovoid or ellipsoidal one, the

vertical axis which passes through the centre of the porochora being

elongated. From the skeletonless Nassellida the other NASSELLARIA may be

derived both ontogenetically and phylogenetically by the excretion of an

extracapsular siliceous skeleton. Unfortunately, the earliest stages in the

formation of this skeleton are unknown, and hence no answer can at present

be given to the important question, in what order the three primary

skeletal elements of the NASSELLARIA (the basal tripod, sagittal ring, and

latticed cephalis) appear (compare §§ 111 and 182). If, for example, in

_Cortina_ and _Tripospyris_ the basal tripod were to appear first in the

ontogeny, and the sagittal ring were developed from this, then the

#Plectoidea# would be rightly considered to be the oldest forms in the

phylogeny of the skeleton-forming NASSELLARIA; and in the contrary case the

#Stephoidea# would be so regarded. The relations of growth in the three

dimensive axes are very variable in the NASSELLARIA; the three most

important factors in this respect (partly separately and partly in

combination) are; (1) the development of the basal tripod to a triradial

stauraxon form (the ground-form being a three-sided pyramid); (2) the

development of the sagittal ring in the median plane of the body (the

vertical axis having the poles different); (3) the development of the

latticed cephalis outside the central capsule (the poles of the vertical

axis being again different). Since the development both of the skeleton and

of the malacoma is characterised in most NASSELLARIA by the stronger growth

of the vertical axis and the differentiation of the two poles, the

allopolar monaxon ground-form acquires a predominant significance in this

legion (§ 32); the starting point of most of the further modifications is

the basal pole of the vertical main axis. Next to this the sagittal axis is

usually the most important determining factor (its dorsal and ventral poles

being {xcviii}usually different), more rarely the frontal axis (with equal

right and left poles). In the zygothalamous #Spyroidea# (as in the

#Stephoidea#) the formation of the shell proceeds from the sagittal ring,

whilst in the polythalamous #Cyrtoidea# the latticed cephalis is always the

starting point, from which a series of joints (thorax, abdomen, and in the

Stichocyrtida, the numerous post-abdominal joints) successively arise

(unipolar growth).

149. _The Ontogeny of the Phæodaria._--The individual development of the

PHÆODARIA in the simplest case stops with the skeletonless condition of the

Phæodinida (_Phæodina_, _Phæocolla_), which can be immediately derived from

the foregoing _Actissa_-stage by the disappearance of the pores in the

greater part of the central capsule, the characteristic astropyle being

developed at the basal pole (§ 60). Since this particular form and

structure of the spheroidal central capsule remains the same in all

PHÆODARIA, whilst the formation of their skeleton follows very different

directions, it follows that further common paths of development are

excluded both ontogenetically and phylogenetically. What will be laid down

in this respect as regards the phylogeny of the different groups of

PHÆODARIA (§§ 194-199) holds true also of their ontogeny. The relations of

growth in the three dimensive axes are hence very different in the

skeletons of the various groups of PHÆODARIA. This difference is best

marked in the #Phæoconchia#, whose bivalved lattice-shells have as their

ground-form the rhomboid pyramid of Ctenophora. In most #Phæogromia# the

monaxon lattice-shell may develop simultaneously by sudden excretion at a

particular moment of lorication; this is also the case with the polyaxon

lattice-shells of the #Phæosphæria#. In their future growth the development

of basal or radial apophyses is of special importance. In the majority of

the PHÆODARIA these apophyses are tubes of silicate filled with jelly

(often provided with an axial siliceous thread); thus their development is

distinguished by complications which are absent in the case of the other

three legions.

150. _Growth._--The growth of the body in the Radiolaria, as in all other

organisms, is the fundamental function of individual development (see note

A). All structural relations which this richest class of the Protista

exhibits may be referred to different forms of growth, either of the

unicellular malacoma or of the skeleton which it produces. In general the

special development of the skeleton is dependent upon that of the central

capsule, and of the sarcodictyum on the surface of the calymma; in the

further growth, however, the conditions are reversed, and the condition of

the skeleton already formed directly determines the further development of

the central capsule and of the calymma with its sarcodictyum. The four

legions of Radiolaria show, speaking generally, certain characteristic

differences in growth, which are due in great part to the different

structure and ground-form of their central capsule. In the two legions of

the Porulosa (SPUMELLARIA and ACANTHARIA), in which the central capsule is

originally spherical and {xcix}the ground-form of the skeleton either

polyaxon or isopolar monaxon, two fundamental and variously combined

directions of growth are recognisable; firstly, the _concentric_ growth

(equal increase of volume in all directions), and secondly, multipolar or

_diametral_ growth (hypertrophy of certain parts in the direction of

definite pairs of radii). A different state of things obtains, however, for

the most part, in the two legions of the Osculosa (NASSELLARIA and

PHÆODARIA), in which the central capsule possesses a vertical main axis

with different poles, and the structure of the skeleton is determined by

this allopolar monaxon ground-form. The two fundamental directions of

growth here combined in the most various ways are, firstly, _unipolar_

growth (starting from the basal pole of the vertical main axis), and

secondly, radial or _pyramidal_ growth (characterised by the different

development of separate parts in the direction of definite radii). Whilst

the growth of the _malacoma_ is dependent on intussusception (as in most

organic structures capable of imbibing), the growth of the _skeleton_ in

all Radiolaria takes place by apposition (see note B).

A. The earliest investigations into the modes of growth in the Radiolaria

are due to J. Müller (L. N. 12, pp. 21-33). More detailed communications

I gave myself in my Monograph (L. N. 16, pp. 150-159). The relations

there sketched have now, in consequence of the examination of the

Challenger collection, undergone many important additions, and in some

divisions, important modifications; these are for the most part treated

of in the general account of the separate families.

B. The view here maintained, that the skeleton of all Radiolaria grows

only by apposition, appeared formerly to have certain exceptions. I

thought I had shown that in _Coelodendrum_ the thin-walled tubes grew not

only in length but also in thickness, with continuous increase in the

lumen (L. N. 16, pp. 152, 360). Further K. Brandt concluded, from the

varying size of the median bars in the twin-spicules of _Sphærozoum_,

that these siliceous structures grow by intussusception (L. N. 38, p.

401). Both suppositions have been proved erroneous and I have come to the

opinion that in all Radiolaria the skeleton grows by apposition.

151. _Regeneration._--Whilst the general course of individual development

(perhaps without any exception in the Radiolaria), begins with the

formation of zoospores in the central capsule, there yet occurs in some

groups a different form of ontogeny, introduced by simple division of the

unicellular organism, and coming under the term "regeneration" in its wider

sense. This spontaneous division occurs quite commonly in the Polycyttaria

(or social SPUMELLARIA), and produces their colonies (compare the chapter

on Reproduction, § 213). On the contrary, it has not been observed in the

solitary SPUMELLARIA, nor in the ACANTHARIA and NASSELLARIA; possibly,

however, the peculiar ACANTHARIAN family, Litholophida, has arisen by the

division of Acanthonida (compare p. 734). Among the PHÆODARIA division is

commonly observed in the order #Phæocystina# (which have an incomplete

Beloid skeleton or none), and also in the #Phæoconchia#. In all these cases

the increase by division is nothing else than an ordinary case of

cell-division, in which bisection of the nucleus precedes that of the

central capsule. The regeneration by {c}which each of the two

daughter-cells develops to a complete mother-cell depends upon simple

growth. Another form of regeneration, different from this, has been

observed in _Thalassicolla_. If the central capsule be extracted

artificially from the large concentric calymma, the enucleated central

capsule produces a new extracapsulum, with sarcomatrix, pseudopodia, and

calymma. This experiment may be repeated several times with the same

result. (Compare A. Schneider, 1867, L. N. 20.)

152. _The Formation of Colonies._--The individual development of colonies

takes place in all three families of the Polycyttaria (Collozoida,

Sphærozoida, Collosphærida) in the same simple way, by the repeated

division of a single monozootic SPUMELLARIAN. Since these divisions only

affect the central capsule and not the extracapsulum, the sister-cells,

which arise by repeated division of the mother, remain enclosed in a common

rapidly growing calymma. Probably in all Polycyttaria the commencement of

the formation of colonies immediately follows the _Actissa_-stage of the

monozootic mother-cell (or takes place in the _Thalassicolla_-stage, which

arises from the former by the development of alveoles in the calymma). The

simple central nucleus separates (by direct nuclear division) into two

halves, and the central capsule follows this process of bisection, becoming

constricted in the middle between the two daughter nuclei (Pl. 3, fig. 12).

In the further growth of the colony the process of division proceeds in the

older, now multinucleate, central capsules, in which an oil-globule has

taken the place of the original nucleus; then the division of the

oil-globules precedes that of the central capsule (Pl. 5, fig. 1). Another

mode of growth of the colonies is the multiplication of the central

capsules by gemmulation, or the formation of the so-called "extracapsular

bodies" (Gemmulæ, § 214). The characteristic skeletal structure of the

different species appears at a later stage. Whether ripe central capsules

can emerge from the social bond of a coenobium, and, having become

isolated, establish the formation of a new colony, is very doubtful. The

various forms which the coenobium assumes in the different species of

Polycyttaria, are due partly to simple growth, partly to the development of

large vacuoles in the calymma.

The _form and size_ of the coenobia appear in many fully developed

Polycyttaria to exhibit specific differences, which require further

investigation; in the young stage, on the contrary, they are simple

spheres or ellipsoids, often cylindrical or sausage-shaped (Pl. 3, figs.

1, 4, 6, 11). In some species the cylindrical gelatinous bodies become

moniliform, and separated by transverse constrictions into many segments,

each of which encloses a large alveole (Pl. 3, fig. 10). The rare

ring-shape (Pl. 4, fig. 1) which I figured in 1862 in the case of

_Collozoum_ (L. N. 16, p. 522, Taf. xxxv. fig. 1), I have recently

observed in different species of Polycyttaria; it is capable of a very

simple mechanical explanation, both ends of a sausage-shaped colony

having been accidentally brought into contact by a wave and having united

by agglutination. Quite recently Brandt has given a very complete account

of the development, form, and growth of Polycyttarian colonies in his

work on the colonial Radiolaria of the Bay of Naples (1885, L. N. 52, pp.

71-85).

{ci}CHAPTER VI.--PHYLOGENY OR GENEALOGICAL DEVELOPMENT.

(§§ 153-200.)

153. _Sources of Phylogenetic Knowledge._--For the purpose of constructing

a hypothetical genealogical tree of the Radiolaria, as of all other

organisms, three sources of information are open to us, viz., palæontology,

comparative ontogeny, and comparative anatomy. In the present case,

however, these three sources are of very different value; the first two are

at present only very inadequately known and have only been partially

investigated, hence they can only be utilised to a very slight extent. The

comparative anatomy of the Radiolaria, on the other hand, is so completely

known, and affords such certain glimpses into the morphological relations

of the related groups, that by its aid we are in a position at all events

to lay down the general features of their phylogeny with some probability,

and to lay the foundation of a natural system.

154. _Natural and Artificial Systems._--Although in the classification of

the Radiolaria, as in the case of all other organisms, the natural system

must be regarded as the goal of systematic classification, our phylogenetic

knowledge of the Radiolaria is too fragmentary and inadequate to admit of

the systematic arrangement here adopted being regarded as a thoroughly

consistent natural system, that is, as representing the true genealogical

tree of the class. Owing, however, to the extraordinary variety of form of

the Radiolaria, and the complicated relationships of the larger and smaller

groups, a synoptical grouping of the different categories and the erection

of a complete, even if to some extent artificial, system, becomes a logical

necessity. Under these circumstances, and regard being had to both these

conditions, the following systematic treatment of the Radiolaria will

appear as a _compromise between the natural and artificial systems_, like

all other zoological and botanical classificatory attempts. On the one

hand, the attempt is made to arrange the larger and smaller groups as

nearly as possible according to their phylogenetic relationships, whilst,

on the other hand, the practice of circumscribing each by a definition as

clear and logical as possible has been carried out. Since these two efforts

naturally often come into contact, the insufficiency of many parts of the

arrangement is obvious, hence its hypothetical and provisional character is

emphatically stated.

155. _Systematic Categories._--The categories or different orders of

divisions have in the Radiolaria, as in all other organisms, no _absolute_

significance, but only a _relative_ value. In itself it is quite

unimportant whether the whole group be regarded, as at first, as a _family_

(Ehrenberg, 1847), or as an _order_ (J. Müller, 1858), or as a _class_

(Haeckel, {cii}1881). These different views are regulated, on the one hand,

by the known extent of the group and by the amount of our acquaintance with

it, and on the other, by comparison with related groups and by reference to

their conventional disposition. When, therefore, the whole class,

Radiolaria, is here divided into two subclasses, four legions, eight

orders, eighty-five families, &c., these artificial categories are drawn up

only in the conviction that by this means the easiest survey and most

thorough insight into the system as a whole may be attained; this latter

will indeed approach as far as possible the ideal of a natural system, but

must on numerous practical grounds always remain more or less artificial.

Since it is to be expected that with the progress of our systematic

knowledge the rank of the various categories will rise, it is possible that

in the future the arrangement of the group may be somewhat as

follows:--_Phylum_, RADIOLARIA; _Four Classes_, SPUMELLARIA, NASSELLARIA,

ACANTHARIA, PHÆODARIA; _Eight Legions_ (Nos. I.-VIII. in the following

Table); _Twenty Orders_ (Nos. 1-20 in the Table), &c.

Four Legions. Eight Sublegions. Twenty Orders. Typical Families.

{ {1. Colloidea, { 1a. Thalassicollida.

{I. COLLODARIA { { 1b. Collozoida.

{ (Spumellaria {

{ palliata) {2. Beloidea, { 2a. Thalassosphærida.

{ { { 2b. Sphærozoida.

I. Legion {

(or Subclass){ { { 3a. Ethmosphærida.

SPUMELLARIA { {3. Sphæroidea, { 3b. Collosphærida.

(PERIPYLEA) { {

{ {4. Prunoidea, { 4a. Ellipsida.

[Porulosa {II. SPHÆRELLARIA { { 4b. Zygartida.

peripylea.] { (Spumellaria {

{ loricata) {5. Discoidea, { 5a. Phacodiscida.

{ { { 5b. Porodiscida.

{ {

{ {6. Larcoidea, { 6a. Larnacida.

{ { { 6b. Pylonida.

{ {7. Actinelida, { 7a. Astrolophida.

{ { { 7b. Litholophida.

{III. ACANTHOMETRA { { 7c. Chiastolida.

{ (Acantharia {

II. Legion { palliata) { { 8a. Astrolonchida.

(or Subclass){ {8. Acanthonida, { 8b. Quadrilonchida.

ACANTHARIA { { { 8c. Amphilonchida.

(ACTIPYLEA) {

{ { { 9a. Sphærocapsida.

[Porulosa { {9. Sphærophracta, { 9b. Dorataspida.

actipylea.] {IV. ACANTHOPHRACTA { { 9c. Phractopeltida.

{ (Acantharia {

{ loricata) { {10a. Belonaspida.

{ {10. Prunophracta, {10b. Hexalaspida.

{ { {10c. Diploconida.

{ {11. Nassoidea, 11. Nassellida.

{ {

{V. PLECTELLARIA {12. Plectoidea, {12a. Plagonida.

{ (Nassellaria { {12b. Plectanida.

{ palliata) {

{ {13. Stephoidea, {13a. Stephanida.

{ { {13b. Tympanida.

{

III. Legion { {14. Spyroidea, {14a. Zygospyrida.

(or Subclass){ { {14b. Androspyrida.

Nassellaria { {

(MONOPYLEA) { { {15a. Cannobotryida.

{VI. CYRTELLARIA {15. Botryodea, {15b. Lithobotryida.

[Osculosa { (Nassellaria { {15c. Pylobotryida.

monopylea.] { loricata) {

{ { {16a. Monocyrtida.

{ {16. Cyrtoidea, {16b. Dicyrtida.

{ { {16c. Tricyrtida.

{ { {16d. Stichocyrtida.

{ { {17a. Phæodinida.

{ {17. Phæocystina, {17b. Cannorrhaphida.

{ { {17c. Aulacanthida.

{VII. PHÆOCYSTINA {

IV. Legion { (Phæodaria { {18a. Orosphærida.

(or Subclass){ palliata) {18. Phæosphæria, {18b. Aulosphærida.

Phæodaria { { {18c. Cannosphærida.

(CANNOPYLEA).{

{ { {19a. Challengerida.

[Osculosa { {19. Phæogromia, {19b. Castanellida.

cannopylea.]{VIII. PHÆOCOSCINA { {19c. Circoporida.

{ (Phæodaria {

{ loricata) { {20a. Concharida.

{ {20. Phæoconchia, {20b. Coelodendrida.

{ { {20c. Coelographida.

156. _Formation of Species._--The totality of similar forms, which we unite

in one species, and which in the earlier dogmatic systems was regarded as a

category of absolute value, possesses only a _relative value_ like all

other systematic categories (§ 155). According to the individual views of

the systematist and the general survey which he has attained of the smaller

and larger systematic groups, the conception of a species adopted in his

practical work will be wider or narrower. In the present systematic

arrangement a medium extent has been adopted. It is shown that in the

Radiolaria, as in all other extensive groups of organisms, the constancy of

the species is very variable in the different groups. Many families of

Radiolaria are very rich in "bad species," _i.e._, very _variable_ forms,

in which the process of the formation of species is seen in progress; such,

for example, are--among the SPUMELLARIA, the Sphærozoida, Stylosphærida,

Phacodiscida and Pylonida; among the ACANTHARIA, the Amphilonchida and

Phractopeltida; among the NASSELLARIA, the #Stephoidea# and #Botryodea#;

and among the PHÆODARIA, the Aulacanthida, Sagosphærida, Castanellida and

Concharida. On the {civ}other hand, in some families numerous "good

species" may be distinguished, since the intermediate connecting forms are

no longer present and the forms have become _relatively constant_. As

instances of such families may be mentioned, among the SPUMELLARIA, the

Astrosphærida, Cyphinida, Porodiscida and Tholonida; among the ACANTHARIA

the Quadrilonchida and Dorataspida; among the NASSELLARIA, the #Spyroidea#

and #Cyrtoidea#; among the PHÆODARIA, the Challengerida, Medusettida,

Circoporida and Coelographida. The more carefully the different groups are

studied, the more numerous the individuals of each species under

comparison, the greater becomes the number of "bad" species among the

Radiolaria, and the smaller the number of good ones. Originally, no doubt,

all "species bonæ" were "malæ." There may be observed in the manifold

skeletal forms of the Radiolaria, on the one hand, the utmost accuracy of

configuration, and on the other, the greatest variability, and hence a

careful comparative study of them leads to a firm conviction of the gradual

"Transformation of Species," and of the truth of the "Theory of Descent."

157. _Palæontological Development._--The palæontology of the Radiolaria

already offers very considerable material for study; but in consequence of

its incompleteness this is of little value for the study of the phylogeny

of the class. By far the larger portion of the fossil Radiolaria belong to

the Tertiary period; only quite recently have numerous well-preserved

fossil Radiolaria been described from the Mesozoic period, and especially

from the Jura. Of Palæozoic Radiolaria (from the coal measures) only slight

traces are known. Moreover, the fossil Radiolaria hitherto known have been

found only in very circumscribed and widely separated localities. The

majority of all the species belong to the small island of Barbados.

Although our palæontological acquaintance with the Radiolaria must

necessarily be incomplete for this reason, it is still more so since at

least thirty out of the eighty-five families (that is more than a third)

could not possibly leave any fossil remains, either because they possess no

skeleton, or because of its chemical composition.

Of the four legions of the Radiolaria, the ACANTHARIA (on account of the

solubility of their astroid acanthin skeletons) have entirely vanished

and have never been found fossil. Of the PHÆODARIA, whose silicate

skeleton is not as a rule capable of fossilisation, only one section

(Dictyochida) of a single family (Cannorrhaphida) has been observed

fossil. Hence the fossil remains of the Radiolaria belong almost

exclusively to the two legions, SPUMELLARIA and NASSELLARIA, which were

formerly united under the term "Polycystina." Among these, however, the

skeletonless Thalassicollida, Collozoida, and Nassellida could leave no

traces. Hence there only remain fifty-five families of which we might

expect to find fossil siliceous skeletons. Even of these, however,

scarcely the half are certainly known in the fossil condition, whilst of

the remainder nothing certain is known; for example, of the large order

#Larcoidea# (among the SPUMELLARIA) and of the #Stephoidea# (among the

NASSELLARIA) with a few isolated exceptions, no fossils are known. The

great majority of fossil Radiolaria belong to the two NASSELLARIAN orders

#Cyrtoidea# and #Spyroidea# (two relatively very highly developed

groups); next to these follow the orders {cv}#Discoidea# and #Sphæroidea#

among the SPUMELLARIA. From these palæontological facts it is obvious

that our present very incomplete acquaintance with the fossil Radiolaria

is quite insufficient to warrant us in drawing any conclusions from it

regarding the phylogenetic development or palæontological succession of

the individual groups.

158. _Origin of the Four Legions._--The agreement of all Radiolaria in

those constant and essential characters of the unicellular body, which

distinguish them from all other Protista (especially the differentiation of

the malacoma into a central capsule and extracapsulum), justifies the

conclusion that all members of this class have been developed from a common

undifferentiated stem-form. Only the simplest form of the SPUMELLARIA, a

skeletonless spherical cell with concentric spherical nucleus and calymma,

can be regarded as such. The simplest form of the Thalassicollida which is

now extant (_Actissa_, _Procyttarium_, p. 12), corresponds so exactly to

the morphological idea of that hypothetical stem-form that it may

unhesitatingly be regarded in a natural system as the common point of

origin of the whole class. On the other hand, _Actissa_ is so closely

related to the simple Heliozoa (_Actinophrys_, _Actinosphærium_,

_Heterophrys_, _Sphærastrum_, &c.) that its origin from this group of

Rhizopoda is exceedingly probable. The three legions ACANTHARIA,

NASSELLARIA, and PHÆODARIA are to be regarded as three main diverging

branches of the genealogical tree, which have been developed in different

directions and are only connected by their simplest stem-forms

(_Actinelius_, _Nassella_, _Phæodina_) with the stem-form of the

SPUMELLARIA, the primordial _Actissa_.

159. _Phylogeny of the Spumellaria._--The legion SPUMELLARIA or PERIPYLEA

is to be regarded as the common stem-group of the Radiolaria, and its

simplest form, _Actissa_, as the primitive genus or radical form of the

whole class; for it possesses in the simplest and most undifferentiated

form all those characters by which the Radiolaria are distinguished from

other Protista; all the other genera of the class may be derived from it by

successive modifications. Considered as a legion the whole group

SPUMELLARIA is undoubtedly monophyletic, for all its members possess those

essential characters by which it is distinctively marked off from the other

three legions, more especially a simple capsule-membrane, which is

everywhere evenly perforated by innumerable small pores; the nucleus lies

originally in the centre of the spherical central capsule. Furthermore, all

SPUMELLARIA lack those positive characters which distinguish the three

remaining legions--the centrogenous acanthin skeleton of the ACANTHARIA,

the basal porochora and the monaxon podoconus of the NASSELLARIA, the

astropyle and phæodium of the PHÆODARIA.

160. _Origin of the Spumellaria._--The genus _Actissa_ (p. 12, Pl. 1, fig.

1) presents the Radiolarian type in its simplest and most primitive form--a

spherical central capsule, which encloses in its middle a spherical

nucleus, and which is surrounded by a spherical calymma. The whole

unicellular body consists, therefore, of three concentric spheres, {cvi}and

possesses neither skeleton nor alveoles, nor other differentiated parts.

The innumerable fine pseudopodia, which issue from the central capsule

through the evenly distributed pores in its membrane, radiate in all

directions through the calymma and pass out over its surface. _Actissa_

can, therefore, be directly derived phylogenetically from the simplest

skeletonless Heliozoa (_Actinophrys_, _Heterophrys_, _Actinosphærium_,

_Sphærastrum_). The only essential difference between the two consists in

the development of the _central capsule_, which in _Actissa_ separates as a

distinct membrane the endoplasm from the exoplasm. This differentiation

which we regard is the most important distinguishing character of the

Radiolaria, has been transmitted by inheritance, along with the formation

of flagellate spores in the central capsule, from _Actissa_, the primitive

parent to all the other Radiolaria.

{cvii}161. _Hypothetical Genealogical Tree of the Spumellaria_:--

LARCOIDEA DISCOIDEA

~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~

Streblonida PHACODISCARIA

| Coccodiscida

| THOLONIDA |

| | |

PRUNOIDEA | | |

~~~~~~~~~~~~~~~~~~ Soreumida| | |

| | | |

Zygartida | | | |

| | | | |

| | | | |~~~~~~~~~~~~~ | Spongodiscida

| | | | | | | | |

| Artiscida| +----+---+ | | | |

| | | | | | | |

| | | | | Staurosphærida | +-----+

| | | | | | | |

Cyphinida | | +---------+ | | |

+-----+ | | | | | |

Druppulida | | | | | +--------+

| | | | | | |

| | | | | | | |

| | | LARNACILLA +-----+-----+ |

+------+-------+ | | |

| | | |

Ellipsida Larcarida Liosphærida Cenodiscida

(CENELLIPSIS) (CENOLARCUS) (CENOSPHÆRA) (CENODISCUS)

| | | |

[Actiprunum?] [Actilarcus?] [Procyttarium] [Actidiscus?]

| | | |

+---------------+------+-------+--------------+

CENOSPHÆRA (Common stem-form of all Sphærellaria?)

| |

| | POLYCYTTARIA

| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

|Collosphærida Collozoida Sphærozoida }

| | | | } BELOIDEA

+-----+ | Thalassosphærida }

| | |

Ethmosphærida | |

| | |

+----------------+----------+

COLLOIDEA

Thalassicollida

ACTISSA

{cviii}162. _Collodaria and Sphærellaria._--Whilst in all SPUMELLARIA the

malacoma agrees in possessing the characteristic features of the legion,

and thus justifies its derivation monophyletically from the common

stem-form _Actissa_, the different forms of skeleton, on the other hand,

cannot all be referred to the same fundamental form. More especially the

_spherical lattice-shell_, from which all the numerous skeletal forms of

the #Sphærellaria# may be derived, cannot have arisen from the incomplete

Beloid skeleton which characterises the #Beloidea# among the #Collodaria#.

It is probable rather that the formation of the skeleton has taken place

independently in those two groups of SPUMELLARIA. From the skeletonless

#Colloidea#, as the common stem-group of the SPUMELLARIA, two different

main groups have diverged, on the one hand the #Beloidea#, whose skeleton

consists of separate spicules scattered in the extracapsulum, and on the

other hand, the #Sphærellaria#, which have formed a simple lattice-sphere

around the central capsule; from this the manifold forms of the remaining

SPUMELLARIA may be derived.

163. _Descent of the Sphærellaria._--The extensive order #Sphærellaria#,

which includes all SPUMELLARIA with a complete lattice-shell, develops an

extraordinary variety of skeletal structures; these may, nevertheless, all

be derived without violence from a common stem-form, or simple spherical

lattice-shell, _Cenosphæra_. The main stem of the order, the extensive

suborder #Sphæroidea# (Pls. 5-30), is derived immediately from _Cenosphæra_

(p. 61, Pl. 12); three diverging branches of it being represented by the

other three suborders, the #Prunoidea# (Pls. 16, 17, 39, 40) being

developed by elongation, and the #Discoidea# (Pls. 31-48) by shortening of

the vertical main axis, whilst the #Larcoidea# (Pls. 9, 10, 49, 50) have

originated by the modification of the spherical lattice-shell into a

lentelliptical or triaxial ellipsoidal one. Although the monophyletic

derivation of all #Sphærellaria# from _Cenosphæra_ is exceedingly probable,

the possibility of a polyphyletic origin for the group is by no means

excluded. For even in the skeletonless primitive genus of all the

SPUMELLARIA, _Actissa_ (as well as in the social _Collozoum_), there are

found, in addition to the usual spherical types, other species (or

subgenera, p. 12) whose central capsule is not spherical but a modification

of the sphere; in _Actiprunum_ ellipsoidal; in _Actidiscus_ lenticular; in

_Actilarcus_ lentelliptical; if such modified forms of _Actissa_ were to

develop their lattice-shells independently, then their form would

correspond to that of the central capsule; and such simple ellipsoidal,

discoidal, and lentelliptical lattice-shells might have been the primitive

forms of the #Prunoidea#, #Discoidea# and #Larcoidea#.

164. _Genealogical Tree of the Sphæroidea._--_Cenosphæra_, the simplest

form of the spherical lattice-shell, may be unhesitatingly regarded as the

common stem-form of all the #Sphæroidea# (pp. 50-284, Pls. 5-30).

_Cenosphæra_ (p. 61, Pl. 12) arose directly from _Actissa_ simply by the

silicification of the spherical exoplasmatic network of the sarcodictyum

around the central capsule, on the surface of the concentric calymma. From

this simple siliceous extracapsular lattice-sphere all other forms of

#Sphæroidea# have arisen, in the main by the manifold combination of two

simple processes, first by the formation of radial spines on the surface of

the lattice-sphere, and second, the addition of concentric spherical

lattice-shells. Both processes may be utilised as the foundation for a

systematic treatment of the #Sphæroidea# (compare pp. 52-58).

If in the #Sphæroidea# the characteristic number and disposition of the

_radial spines_ be regarded as the most important heritable peculiarity

of the different families, then we have the following natural

arrangement:--(1) Liosphærida, without radial spines; (2) Cubosphærida,

with six radial spines (opposite in pairs in three axes perpendicular to

each other); (3) Staurosphærida, with four radial spines (in two axes

crossed at right angles); (4) Stylosphærida, with two opposite radial

spines (in the vertical main axis); and (5) Astrosphærida, with numerous

regularly or irregularly distributed radial spines (eight to twenty or

more). If, on the contrary, more stress be laid upon the number of the

concentric lattice-shells, then we have the following artificial

grouping:--(1) Monosphærida, with one simple lattice-sphere; (2)

Dyosphærida, with two concentric lattice-spheres; (3) Triosphærida, with

three; (4) Tetrasphærida, with four; (5) Polysphærida, with numerous

(five to twenty or more) concentric lattice-shells; (6) Spongosphærida,

with a spongy spherical shell. In general the former arrangement appears

more natural than the latter, since the number of primary radial spines,

which grow out from the primary lattice-sphere, determines their

ground-form from the outset, whatever may be the number of secondarily

added shells. Strictly speaking, according to the view adopted, these

Liosphærida which have several shells, on the outer surface of which

there are no radial spines, ought to be classified according to the

number and arrangement of their internal radial connecting beams and

distributed among the other families. The practical application of this

correct principle meets, however, with great difficulties. Also in many

cases the phylogenetic relations of the different #Sphæroidea# are more

complicated than would appear from both these classificatory principles.

In general their phylogeny will quite correspond with their ontogeny,

since from the innermost first formed {cix}lattice-shell (primary

medullary shell) a number of radial spines arises, and upon these the

secondary shells are formed from within outwards.

165. _Genealogical Tree of the Prunoidea._--The suborder #Prunoidea# is

very closely related to the #Sphæroidea#, and is distinguished from it by

the elongation of one axis; from the simple lattice-sphere (_Cenosphæra_)

is developed a latticed ellipsoid (_Cenellipsis_, Pl. 39, fig. 1). The

development of this vertical isopolar main axis is foreshadowed even among

the #Sphæroidea#, in that family in which two opposite radial spines grow

out of the primary lattice-sphere at the two poles of the vertical main

axis (Stylosphærida, Pls. 13, 14). These latter pass over without any sharp

boundary into those forms of #Prunoidea# whose ellipsoidal lattice-shell

bears two opposite main-spines (Stylatractida, Pls. 15, 16). Other very

intimate relationships between the #Sphæroidea# and #Prunoidea# are

indicated in certain of the latter by the fact that of the two concentric

lattice-shells the inner (medullary) shell is spherical, the outer

(cortical) shell ellipsoidal (Pl. 39, figs. 3, 7, 8, 14, 19); often three

concentric lattice-shells are present, of which the two inner are spherical

intracapsular medullary shells, whilst the outer is an extracapsular

cortical shell, ellipsoidal or cylindrical in form (Pl. 39, figs. 4, 12,

17, 18). Owing to the manifold nature of these phylogenetical relations and

the variety of their combinations, the derivation of the individual

#Prunoidea# from the #Sphæroidea# is rendered very difficult; in addition

to which it is possible that the simplest #Prunoidea# (_Cenellipsis_,

_Ellipsidium_) have been directly developed from the skeletonless

_Actiprunum_ (a form of _Actissa_ with ellipsoidal central capsule, p. 14)

by the excretion of a simple ellipsoidal lattice-shell on the surface of

their calymma.

The phylogeny of the #Prunoidea# is especially complicated by the

formation of peculiar transverse constrictions, perpendicular to the

longitudinal axis. They are wanting only in the Monoprunida (Ellipsida,

Druppulida, and Spongurida); the Dyoprunida (Artiscida and Cyphinida, Pl.

39, figs. 9-19) possess only one such constriction (in the equatorial

plane); the Polyprunida, on the other hand, have three, five, or more

parallel constrictions (Panartida and Zygartida, Pl. 40). The chambers,

which are separated off by these constrictions, may be regarded as polar

sections of incomplete cortical shells.

166. _Genealogical Tree of the Discoidea._--The suborder #Discoidea# is

closely related to the #Sphæroidea#, but separated from it by shortening of

one axis; from a simple lattice-sphere (_Cenosphæra_) a latticed lens or

flattened spheroid is developed, whose circular equatorial plane is larger

than any other section (_Cenodiscus_, Pl. 48, fig. 1). The formation of

this horizontal equatorial plane is perhaps indicated in that family of

#Sphæroidea# in which four crossed radial spines, lying in one plane, are

developed (Staurosphærida, Pls. 15, 31, 42). The morphological and

phylogenetical relations of the #Discoidea# to the #Sphæroidea# are

precisely the converse of those of the #Prunoidea#; in the latter the

vertical axis appears longer, in the former shorter than any {cx}other axis

of the body. The #Discoidea# are probably polyphyletic, having originated

from several different groups of #Sphæroidea#; at least two essentially

different main groups may be distinguished among them; of these the one is

characterised by the formation of a large extracapsular lenticular cortical

shell (Phacodiscaria), whilst in the other this typical "Phacoid shell" or

lattice-lens is wanting (Cyclodiscaria, compare pp. 403-409).

The Phacodiscida (Pls. 31-35) perhaps constitute the primitive group of

the Phacodiscaria, their lenticular or Phacoid cortical shell being

connected by radial bars with one or two concentric spherical medullary

shells; they may have originated directly from the Dyosphærida or

Triosphærida by flattening of the spheroidal cortical shell. From the

Phacodiscida the Cenodiscida (if indeed they be not the primitive

stem-form) have been developed by retrogression and loss of those

medullary shells. The Coccodiscida (Pls. 36-38), on the other hand, have

been developed from the Phacodiscida by the addition of concentric rings

of chambers, which may be regarded as incomplete cortical shells, only

the equatorial portion of which is developed. Perhaps the Porodiscida,

the primitive group of the Cyclodiscaria, have arisen in a similar way;

they lack, however, the typical Phacoid shell, the concentric rings of

chambers being directly applied to a small spherical medullary shell in

the equatorial plane (Pls. 41-46). If those rings from the commencement

be interrupted by three interradial gaps (gates) the family Pylodiscida

arises (Pl. 38, figs. 6-20). If, on the contrary, the concentric radially

divided chambers of the Porodiscida become quite irregular and spongy,

they pass over into the Spongodiscida (Pls. 46, 47). It is not, however,

impossible that part of the #Discoidea# (especially the Cenodiscida) have

originated directly from skeletonless #Collodaria# with a lenticular

central capsule, such as are found in a subgenus of _Actissa_

(_Actidiscus_, p. 15).

167. _Genealogical Tree of the Larcoidea._--The suborder #Larcoidea#

presents in the structure, composition, and development of its variously

formed lattice-shells much more complicated relations than the other

#Sphærellaria#; it is essentially distinguished from them by the

characteristic ground-form of its lattice-shells, which is a "lentellipsis"

or a triaxial ellipsoid (also the ground-form of the rhombic

crystallographic system, the rhombic octahedron). Hence all parts of the

body are regularly disposed with respect to three different dimensive axes;

all three axes, perpendicular one to another, are isopolar but of different

lengths; the longest is the vertical main axis, the mean the horizontal

frontal axis, the shortest the horizontal sagittal axis. In the great

majority of the #Larcoidea# the lentelliptical ground-form is indicated in

the central capsule, even when it is not at once obvious in the skeleton.

Since such lentelliptical central capsules are developed even in _Actissa_

(_Actilarcus_, p. 16), it is possible that the simplest #Larcoidea# may

have arisen directly from these by deposition of a simple lentelliptical

lattice-shell in the sarcodictyum, on the surface of the calymma

(_Cenolarcus_, Pl. 50, fig. 7). It is more probable, however, that these

simplest forms (_Cenolarcus_, _Larcarium_) have been developed from the

simplest #Sphæroidea# (_Cenosphæra_), by the spherical body growing

unequally in the three dimensions of space. It appears especially likely

{cxi}from a study of the concentrically disposed lattice-shells of some

#Larcoidea# (_Coccolarcus_, _Larcidium_, Pl. 50, fig. 8), in which the

inner medullary shell is spherical, the outer cortical shell more or less

elliptical. In the great majority of #Larcoidea# the latter arises in quite

a peculiar manner, three broad lattice-zones, which are developed in three

planes at right angles to each other, growing out from a small spherical or

lentelliptical medullary shell, _Trizonium_, _Larnacilla_ (compare pp. 600,

615, 628, &c.).

The trizonal _Larnacilla_-shell commences by the formation of a

transverse girdle, by the union of two lateral latticed processes, which

spring right and left in the equatorial plane from the poles of the

frontal axis of a lentelliptical medullary shell (_Monozonium_, p. 633,

Pl. 9, fig. 1). This is followed by a second lateral girdle, which lies

in the frontal plane and proceeds from its lateral poles (_Dizonium_, p.

634, Pl. 9, figs. 2, 3). Finally the sagittal girdle is formed, lying in

the sagittal plane and arising from the lateral girdle on the two poles

of the main axis (_Trizonium_, p. 637, Pl. 9, fig. 4). Whilst the gaps

between the three zones of this trizonal shell remain open in the

Pylonida, in _Larnacilla_, the important primitive form of the Larnacida,

they are closed by lattice-work (Pl. 50, figs. 3-8). From this trizonal

_Larnacilla_-shell the great majority of Larcoid shells may be derived.

Such a system of zones may be repeated (Diplozonaria) or even developed a

third time (Triplozonaria, p. 632). In most #Larcoidea# the zones are

secondarily connected by lattice-work. In the Tholonida (Pl. 10) each of

the two opposite latticed wings of a zone becomes a closed dome. In the

Zonarida (Pl. 50, figs. 9-12) these domes are partially or wholly

bisected by constrictions or latticed septa which are developed in the

three dimensive planes. The Lithelida (Pl. 49, figs. 1-7) are

characterised by the fact that one of each pair of opposite latticed

processes (or half zones) grows more strongly than the other, and that

the larger completely embraces the smaller so as to form a complicated

spiral. Whilst in this case the spiral lies in a plane, in the

Streblonida (Pl. 49, figs. 8, 9) it becomes turbinoid like a gastropod

shell and forms an ascending spiral. Finally, two small families of

#Larcoidea# are characterised by quite irregular growth (a very rare

occurrence among the Radiolaria); these are the simple-chambered

Phorticida (Pl. 49, figs. 10, 11) and the many chambered Soreumida (Pl.

49, figs. 12, 13). The phylogenetic relationship of these families of

#Larcoidea# is probably very complicated and demands closer investigation

(compare pp. 599-604).

168. _Descent of the Polycyttaria._--The polyzootic or colonial Radiolaria,

which we unite in the group Polycyttaria (sometimes known as "Sphærozoea"),

belong without doubt to the legion SPUMELLARIA, for they possess all the

peculiarities by which these PERIPYLEA are distinguished from the other

legions of the Radiolaria. Only the morphological position of the

Polycyttaria in that legion, and their phylogenetic relation to the

monozootic or solitary SPUMELLARIA, can be variously interpreted. The three

families which we distinguish among the Polycyttaria are so closely related

to three different families of the Monocyttaria, that they may be directly

derived from them by the formation of colonies. According to this

_triphyletic hypothesis_ the social skeletonless Collozoida (Pl. 3) would

be descended from the solitary Thalassicollida (Pl. 1), the polyzootic

Sphærozoida with a Beloid skeleton (Pl. 4) from the monozootic

{cxii}Thalassosphærida (Pl. 2), and the colonial Collosphærida with a

Sphæroid skeleton (Pls. 5-8) from the solitary Ethmosphærida (Pl. 12, &c.).

Many species of monozootic and polyzootic forms in all three groups are so

alike that they can only be distinguished by the fact that the one series

are colonial, the others solitary. On the other hand, there are some

reasons which would justify a monophyletic hypothesis for the Polycyttaria,

_e.g._, the precocious nuclear division; in this case it would be most

natural to hold that the Sphærozoida and Collosphærida have arisen as two

diverging branches from the Collozoida, whilst the latter are nothing else

than colonial Thalassicollida.

169. _Phylogeny of the Acantharia._--The legion ACANTHARIA or ACTIPYLEA is

distinguished by its peculiar acanthin skeleton, which develops

centrogenously, as well as by the disposition in groups of the pores in its

central capsule, and its excentric usually precocious nucleus; it is thus

so different from all other Radiolaria as undoubtedly to furnish,

phylogenetically considered, an independent stem (§ 7). This stem is only

connected at the root by _Actinelius_ with the primitive form of the

SPUMELLARIA, _Actissa_. The stem is monophyletic, since all the forms

belonging to it may be derived without violence from _Actinelius_ as a

common primitive form.

170. _Origin of the Acantharia._--The genus _Actinelius_ (p. 730, Pl. 129,

fig. 1), which may naturally be regarded as the common primitive form of

all ACANTHARIA, possesses a spherical central capsule, which in consequence

of the early division of the nucleus (§ 63), encloses numerous small

nuclei; from its centre arise many simple radial spines of equal size,

which penetrate the central capsule. A large number of radial pseudopodia

issue between the spines from the sarcomatrix which surrounds the capsule.

_Actinelius_ may have been directly derived from _Actissa_, the common

stem-form of all Radiolaria, by the division of the pseudopodia into two

groups, myxopodia, which remained soft, and axopodia, which became firm (§

95A). As the latter became changed into strong acanthin rods, and touched

each other in the centre, they forced the nucleus from its originally

central position and brought about its early division. _Actinelius_ is also

of all Radiolaria the form which, next to _Actissa_, most nearly approaches

the Heliozoa. If the stiff axial threads of _Actinosphærium_ be conceived

of as partially converted into acanthin spines, and its nucleated medullary

substance as separated from the alveolar cortical layer by a membrane

(central capsule), then _Actinelius_ would be produced.

{cxiii}171. _Hypothetical Genealogical Tree of the Acantharia_:--

Diploconida

Phractopeltida Hexalaspida Cenocapsida

| | |

| Phatnaspida | Lychnaspida |

| | | | |

| | | | Porocapsida

| | Coleaspida | |

| | | | |

| Ceriaspida +-----+-----+ | |

| | | | |

+--+--+ | | |

| Belonaspida | |

| | | |

Phractaspida | Stauraspida |

| | | |

| | | Astrocapsida

| | | Sphærocapsida

+-------+----------+ ------+------ |

| | |

Diporaspida Tessaraspida |

(Dorataspida dipora) (Dorataspida tetrapora) |

| | | | |

| +--------+--------+ | |

| | | |

| [Dorataspida] | |

| | |

| Quadrilonchida | |

| | | |

Phractacanthida | Stauracanthida |

| | | |

| Amphilonchida | +--+--+

| | | |

| | | Acanthonia

| | | |

+---------+--------+--+------------------+

Astrolonchida

Litholophida | Chiastolida

| | |

| Zygacanthida |

| Acanthonida Actinastrum

| Acanthometron |

Astrolophida | | | Acanthochiasmida

| | | | |

| | | | Acanthometron

| | | | |

+----------+-----------------+-------------------+---------+

Actinelida

Actinelius

Actissa

{cxiv}172. _Adelacantha and Icosacantha._--The numerous forms of

ACANTHARIA, here disposed in twelve families and sixty-five genera, may be

divided phylogenetically into two main groups of very different

extent--_Adelacantha_ and _Icosacantha_. The more primitive group,

_Adelacantha_, have an indefinite and variable number of radial spines,

which are always quite simple in form and usually irregularly distributed;

this main division includes only the one order #Actinelida#, with six

genera, among which is _Actinelius_, the common stem-form of all the

ACANTHARIA. The more recent group, Icosacantha, includes all the other

ACANTHARIA (fifty-nine genera), and is very markedly distinguished from the

Adelacantha by the fact that the radial spines are always twenty in number,

and arranged according to Müller's law (compare pp. 717-725, and § 110).

Since this regular disposition (in five alternating zones each of four

spines) has been retained by inheritance in the whole of the Icosacantha,

it is probable that this large group has been developed monophyletically

from a twig of the Adelacantha; _Actinastrum_ (p. 732) and _Chiastolus_ (p.

738) still present connecting links between the former and the latter,

between _Actinelius_ and _Acanthometron_.

173. _Acanthonida and Acanthophracta._--The extensive main division

Icosacantha (§ 110), which embraces all ACANTHARIA with twenty radial

spines, disposed according to Müller's law, may be subdivided into two

large groups or orders:--the #Acanthonida# (p. 740, Pls. 130-132) and the

#Acanthophracta# (p. 791, Pls. 133-140). The latter possess a complete

extracapsular lattice-shell, which the former have not. The more recent

#Acanthophracta# may be derived phylogenetically from the more primitive

#Acanthonida# simply by the development of this lattice-shell, with which

process are usually (perhaps always) connected certain alterations in the

malacoma, _e.g._, degeneration of the myophriscs (§ 96). The most primitive

form of all Icosacantha is the genus _Acanthometron_ (p. 324), in which all

the twenty acanthin spines are of the simplest constitution and of equal

dimensions.

174. _Differentiation of the Acanthonida._--The order #Acanthonida#, which

embraces all Icosacantha which have no complete lattice-shell, divides

early into three main branches, the three families Astrolonchida,

Quadrilonchida, and Amphilonchida (p. 727, Pls. 130-132). The first of

these constitutes the common stem-group from which the other two as well as

the whole group #Acanthophracta# have been developed; the common stem-form

of all is _Acanthometron_ (§ 173). All the Astrolonchida (p. 740, Pl. 130)

have twenty radial spines of equal size and similar form. On the other

hand, in the Quadrilonchida (p. 766, Pl. 131) the four equatorial spines

differ from the others in size and sometimes also in form. In the

Amphilonchida (p. 781, Pl. 132) two opposite equatorial spines (lying in

the hydrotomical axis) are much larger than the other eighteen and of a

different shape. Of the three families of the #Acanthonida# the most

important is the primitive group Astrolonchida, for from this the various

stem-forms of the #Acanthophracta# arise. They are subdivided according to

the formation of the spines into three subfamilies: the Zygacanthida, with

simple spines without apophyses (or transverse processes); the

Phractacanthida, with two opposite apophyses on each radial {cxv}spine, and

the Stauracanthida, with four crossed apophyses on each radial spine. The

three genera of the Zygacanthida represent the stem-forms of the three

families, since the radial spines in _Acanthometron_ (the most primitive

form of #Acanthonida#) are cylindrical, in _Zygacantha_ two-edged, and in

_Acanthonia_ four-edged (p. 741).

175. _Capsophracta and Cladophracta._--The extensive order

#Acanthophracta#, which embraces all ACANTHARIA with a complete

lattice-shell, is polyphyletic, its main subdivisions have been developed

independently from different branches of the #Acanthonida#. The whole order

may be divided directly into two main groups, the #Capsophracta# and

#Cladophracta# (p. 793), which differ in the structure and the origin of

their lattice-shell. The group (or suborder) #Capsophracta# includes only

the single family Sphærocapsida (p. 795, Pl. 133, figs. 7-11; Pl. 135,

figs. 6-10); the lattice-shell arises independently of the twenty radial

spines, being made up like a pavement of innumerable small acanthin plates,

united by a kind of cement; each plate being perforated by a fine pore. In

addition twenty larger main pores (or groups of four pores each) are

present, corresponding to the twenty radial spines; these are always equal,

quadrangular prismatic, without transverse processes as in _Acanthonia_. In

the #Cladophracta#, which include the five remaining families of the

#Acanthophracta#, the structure and origin of the lattice-shell are quite

different; the lattice-shell is here made up of the branches of the

transverse processes, which radiate tangentially from the twenty radial

spines and are only united secondarily.

176. _Ascent of the Dorataspida._--The group #Cladophracta#, or those

ACANTHARIA whose lattice-shell arises by the union of transverse

processes of the twenty radial spines, includes five different

families, whose stem-group is the family Dorataspida, with a simple

spherical lattice-shell. This family itself is, however, diphyletic in

origin, being composed of two essentially and originally different

subfamilies--Diporaspida and Tessaraspida (p. 803). The Diporaspida (p.

808, Pls. 137, 138) have been developed from the Phractacanthida, and as

each radial spine of the latter bears two opposite apophyses, so the

lattice-shell of the former has forty primary aspinal pores (two on the

base of each spine). On the other hand, the Tessaraspida (p. 830, Pls. 135,

136) have been developed from the Stauracanthida, and as each radial spine

of the latter bears four crossed apophyses, so the lattice-shell of the

former has eighty primary aspinal pores (four at the base of each spine).

177. _Descent of the Diporaspida._--Whilst the Tessaraspida (§ 176) have

given rise to no new groups which could take rank as independent families,

no less than four separate families of ACANTHARIA have arisen from the

Diporaspida. The Phractopeltida (Pl. 133, figs. 1-6) are distinguished from

all other ACANTHARIA by the possession of two concentric spherical

lattice-shells, and have probably been developed from the {cxvi}Diporaspida

in the same way as the Dyosphærida from the Monosphærida among the

#Sphæroidea#; in that case the smaller inner lattice-sphere (medullary

shell) would be the primary, and the larger outer sphere (cortical shell)

the secondary; this latter shows forty primary aspinal pores like those of

the Diporaspida. The possibility is not excluded, however, that the small

inner lattice-sphere of the Phractopeltida is a secondary product. The

three remaining families, which must be regarded as descendants of the

Diporaspida, form together a single phylogenetic series, and are separated

from the primitive group mainly by the fact that the original spherical

form of the lattice-shell has been modified into one distinguished by an

elongated equatorial axis (the hydrotomical axis); hence the #Prunophracta#

(pp. 794-859). The ellipsoidal Belonaspida have arisen directly by

hypertrophy of the two opposite equatorial spines of this hydrotomical axis

(p. 859, Pl. 136, figs. 6-9; Pl. 139, figs. 8, 9; perhaps they have also

arisen directly from the Amphilonchida). In the lentelliptical Hexalaspida

(Pl. 139) all six spines which lie in the hydrotomical meridian plane (two

equatorial and four polar) are very strongly developed, the remaining

fourteen being rudimentary. Finally, in the Diploconida the two conical

sheaths of the two opposite hydrotomical equatorial spines are so

predominant that they take the chief part in the formation of the

hour-glass-shaped shell.

178. _Phylogeny of the Nassellaria._--The legion NASSELLARIA or MONOPYLEA

is so clearly characterised by the peculiar porochora, which closes the

osculum at the oral pole of the monaxon central capsule, and by the

podoconus connected with it, that there can be no doubt that

phylogenetically it represents an independent stem (§ 8). This stem is only

connected at its base by means of _Cystidium_ and _Nassella_ with _Actissa_

and _Thalassicolla_, the stem-forms of the SPUMELLARIA. This stem is

monophyletic, inasmuch as all its members may be derived without violence

from the skeletonless Nassellida (_Nassella_, _Cystidium_, p. 896, Pl. 91,

fig. 1).

179. _Origin of the Nassellaria._--The Nassellida (p. 896), which may

naturally be considered as the common stem-group of the NASSELLARIA, are

most nearly related among other Radiolaria to the Thalassicollida, and in

both these skeletonless families the simplest forms, _Cystidium_ and

_Actissa_ correspond; on the other hand, those which have arisen from them

by the formation of alveoles in the calymma (_Nassella_ and

_Thalassicolla_) also correspond. The origin of the simplest Nassellida

from these primitive Thalassicollida may be explained by supposing that the

numerous (formerly evenly distributed) pores of the capsule membrane became

obliterated in the upper (apical) half of the central capsule, whilst in

the lower (basal) half they became correspondingly more strongly developed;

hence the porochora was formed at the oral pole of the vertical main axis,

and a differentiation of the endoplasm proceeding from this gave rise to

the characteristic podoconus. Both these organs still at present exhibit

very various degrees of progressive development.

{cxvii}180. _Hypothetical Genealogical Tree of the Nassellaria._

CYRTOIDEA.

~~~~~~~~~~~~~~~~~~~~~~~~~

BOTRYODEA _Triradiata_

~~~~~~~~~~~~~

Pylobotryida Podocampida

| |

| _Eradiata_ | _Multiradiata_ SPYROIDEA

| | | ~~~~~~~~~~~~~~~~~

| Lithocampida | Phormocampida Androspyrida

| | | | |

Lithobotryida | Podocyrtida | |

| | | | |

| Theocyrtida | Phormocyrtida |

Tholospyrida

| | | | | |

Cannobotryida | Tripocyrtida | Phormospyrida |

| | | | | |

| Sethocyrtida | Anthocyrtida | |

| | | | | |

| | | | | |

| Cyrtocalpida | Phænocalpida +---+----+

| | | | |

STEPHOIDEA | +---------+------------+ Zygospyrida

~~~~~~~~~~~~ | | (Spyroidea

triradiata)

Tympanida | Tripocalpida |

| | (Cyrtoidea triradiata monocyrtida) |

| | | |

Coronida | +------------------+-----------------------------+

| | |

| Semantida |

| | |

+--+---+ Cyrtellaria

| |

| Cortiniscus |

| | |

Stephanida | |

| | |

+-----+------+ |

| |

Cortina *--------- Cortinida (PLECTELLARIA)

(Cortina)

Plectaniscus ---- Plagoniscus ---- }

| }

Tetraplecta ---- Tetraplagia ---- } PLECTOIDEA

| } Plectanida

Plectophora ---- Plagiacantha ---- } |

| } |

Triplecta ---- Triplagia ---- } Plagonida

| }

Nassoidea

(Nassellida)

Nassella

(Cystidium)

Actissa

{cxviii}181. _Plectellaria and Cyrtellaria._--The extensive legion

NASSELLARIA far surpasses the other three legions in the endless variety of

its skeletal structures, and owing to the complicated relationships of its

numerous families presents no lack of difficult phylogenetic problems. All

NASSELLARIA may be divided first into two main groups or sublegions,

#Plectellaria# and #Cyrtellaria#; the latter having a complete

lattice-shell, the former not. Probably the #Cyrtellaria# have been

polyphyletically developed from several different groups of #Plectellaria#.

These groups are, however, connected in such manifold ways that a

monophyletic origin of all the NASSELLARIAN skeletons from one original

element is possible. Such a primitive element may have been furnished by

any one of three different skeletal parts, the sagittal ring, the basal

tripod, and the latticed cephalis (compare pp. 891-895, Bütschli, L. N. 40,

41).

182. _Phylogenetic Skeletal Elements of the Nassellaria._--The multiform

skeleton of the NASSELLARIA may be referred in different ways to one of the

three above-mentioned structural elements. Each of these (p. 891) may by

itself form the skeleton; the sagittal ring in the simplest #Stephoidea#

(_Archicircus_, _Lithocircus_), the basal tripod in the simplest

#Plectoidea# (_Triplagia_, _Plagiacantha_), the latticed cephalis in the

simplest #Cyrtoidea# (_Cyrtocalpis_, _Archicapsa_). In the great majority

of the NASSELLARIA, however, two of these elements, or even all three, are

found combined. In most #Cyrtellaria#, more especially, both the sagittal

ring and the basal tripod may be recognised in the lattice-shell, though

often only in slight rudiments or scarcely perceptible traces. In the

#Plectellaria# also (which possess no latticed cephalis) there are

individual genera with complete development both of the sagittal ring and

basal tripod; this important combination is especially well represented in

the Cortinida (_Cortina_, _Cortiniscus_, _Stephanium_, _Stephaniscus_,

_Tripocoronis_, &c.). The greatest difficulty as regards the phylogeny of

the NASSELLARIA lies in the fact that the most various combinations of the

three elements are presented by closely related or very similar forms. If,

in spite of this, a monophyletic hypothesis as to the origin of the

NASSELLARIA seems essential all sides of the three possible hypotheses must

receive full consideration and critical comparison (§§ 183-191).

183. _Ascent of the Nassellaria from the Plectoidea._--The monophyletic

hypothesis (No. 2, p. 893) which regards the basal tripod as the common

origin of the skeleton of all NASSELLARIA, starts from the simplest forms

of the #Plectoidea# (_Triplagia_, _Plagoniscus_, _Triplecta_,

_Plectaniscus_, &c., Pl. 91). All #Plectoidea# may be immediately derived

as diverging twigs of these, as well as all triradial and multiradial forms

of #Cyrtoidea# and #Spyroidea#; for in all these cases the distinctive

triradial (or the derived multiradial) form of skeleton appears directly

derivable from the simple basal tripod of the former. The same is perhaps

also true of many #Botryodea#. {cxix}Furthermore, certain important forms

of #Stephoidea# (_Cortina_, _Cortiniscus_, _Stephanium_, _Stephaniscus_,

&c.), which have a characteristic combination of the sagittal ring and

basal tripod, may be immediately derived from such forms of #Plectoidea# as

_Plagoniscus cortinaris_, _Plagiocarpa procortina_, _Plectaniscus

cortiniscus_, &c. On the contrary, those #Stephoidea# and #Cyrtoidea# in

which the basal tripod is wanting can only be derived from the #Plectoidea#

by the assumption that this structure has disappeared in consequence of

phylogenetic degeneration. The monophyletic derivation of the NASSELLARIA

from the #Plectoidea# has more internal probability than that from the

#Stephoidea#, since it is easier to suppose that the Cortinida (_Cortina_,

_Stephanium_, &c.) have been derived from the #Plectoidea# (_Plagoniscus_,

_Plagiocarpa_) than the converse. This view is the basis of the

hypothetical tree shown in § 180.

184. _Ascent of the Nassellaria from the Stephoidea._--The monophyletic

hypothesis (No. 1, p. 893) which regards the primary sagittal ring as the

common starting point of the skeleton in all NASSELLARIA, starts from the

simplest forms of #Stephoidea# (_Archicircus_, _Lithocircus_, &c., Pl. 81).

All #Stephoidea# and #Spyroidea# may be immediately derived from these, as

also the majority of the #Cyrtoidea# and probably of the #Botryodea#. Those

numerous forms of the last two groups, however, which possess no trace of a

sagittal ring, can only be derived from the former by the supposition that

the latter has completely disappeared in in consequence of gradual

phylogenetic degeneration. The same holds true also of the #Plectoidea#,

although certain forms (_e.g._, _Plagiocarpa procortina_, Pl. 91, fig. 5;

_Plectaniscus cortiniscus_, Pl. 91, fig. 9) appear to indicate the

commencing formation of the sagittal ring by the concrescence of two

branches, which approach each other from the upper part of the apical rod

and the ventral part of the basal rod. In any case, it is a fact of great

phylogenetic significance, that the primary sagittal ring in the cephalis

of the #Cyrtoidea# shows all conceivable stages of degeneration (compare

Bütschli, L. N. 40, 41, as well as the general account of and critical

comparison of the NASSELLARIA, pp. 889-895, &c.).

185. _Ascent of the Nassellaria from the Cyrtoidea._--The monophyletic

hypothesis (No. 3, p. 894) which regards the latticed cephalis as the

common point of origin of all the skeletons of the NASSELLARIA, starts from

the simplest forms of the #Cyrtoidea#, that is, from the Cyrtocalpida or

eradial Monocyrtida (_Archicorida_, _Archicapsida_, Pls. 51, 52, 98). All

#Cyrtoidea# and #Botryodea# may be regarded as divergent forms of these

monothalamous #Cyrtoidea#; the polythalamous simply by the addition of

fresh joints at the basal pole, the triradiate and multiradiate by the

development of three or more apophyses. The origin of the sagittal ring

(which presents every stage of development and degeneration in the

#Cyrtoidea#) may be regarded as a mechanical thickening of the latticed

plate in the sagittal circumference of the cephalis. By stronger

{cxx}development of this ring and coincident sagittal constriction of the

cephalis the order #Spyroidea# may be derived from the #Cyrtoidea#. On the

other hand, the #Plectellaria#, which possess no cephalis, and indeed no

complete lattice-shell whatever, may be derived from the Monocyrtida by the

assumption of a degeneration of this structure; the sagittal ring having

been preserved in the #Stephoidea#, and the tripod of the Tripocalpida in

the #Plectoidea#. Although such a monophyletic derivation of the

NASSELLARIA from the Cyrtocalpida is possible, and though here, too, the

Cortinida play an important part as connecting links, this hypothesis has

less internal probability than that of the derivation from the #Stephoidea#

(§ 184) or #Plectoidea# (§ 183).

186. _Genealogical Tree of the Plectoidea._--The order #Plectoidea#

includes those NASSELLARIA whose rudimentary skeleton does not contain the

characteristic sagittal ring of the #Stephoidea#, but consists of several

(at least three) radial spines, which proceed from a point in the centre of

the porochora. The branches of these radial spines remain free in the

Plagonida, whilst in the Plectanida they unite with each other to form a

loose meshwork (not, however, a complete lattice-shell). The number and

arrangement of the radial spines, which serve for generic distinctions, are

the same in both families, so that each genus of the Plectanida has arisen

from a corresponding genus of the Plagonida. The simplest Plagonida, which

possess a basal tripod (_Triplagia_ or _Plagiacantha_ with three rays,

_Tetraplagia_ with four rays) are probably to be regarded as forming the

common origin of the whole order. These agree with certain three- and

four-rayed skeletal pieces of the #Beloidea# (Thalassosphærida and

Sphærozoida); and also the four and six-rayed twinned pieces of the latter

(spicula bigemina and trigemina) repeat in the same fashion the skeleton of

the former (_Plagonidium_, _Plagonium_). This similarity, however, is a

mere analogy and possesses no phylogenetic significance. On the other hand,

certain Plagonida (_Plagoniscus_, _Plagiocarpa_), and the corresponding

genera of Plectanida (_Plectaniscus_, _Periplecta_) seem to have important

phylogenetic relations to certain #Stephoidea# (_Cortina_, _Cortiniscus_,

&c.); the sagittal ring of the latter having perhaps arisen by the vertical

apical spine of the former having been connected with their horizontal

basal rod by two ventral apophyses growing out opposite to each other

(compare pp. 902, 914, _Plagiocarpa procortina_, Pl. 91, fig. 5). In this

case the Plectanida would belong to the simplest stem-forms of the

NASSELLARIA.

187. _Genealogical Tree of the Stephoidea._--The order #Stephoidea#

includes all those NASSELLARIA whose skeleton does not form a complete

lattice-shell, but consists of one or more rings, and often of a loose

meshwork which arises by the union of branches of the rings. A _vertical

sagittal ring_ is constantly present, embracing the central capsule in the

median sagittal plane, and forming at its basal pole various processes, the

starting point for other skeletal forms. The most important of these is the

tripodal _Cortina_ {cxxi}(p. 950, § 182). The Stephanida are the most

archaic family among the #Stephoidea# (p. 937, Pl. 81), perhaps indeed

among all the NASSELLARIA (§ 184); in them the sagittal ring and its

processes alone constitute the skeleton; secondary rings and meshes are

wanting. Two diverging families, the Semantida and Coronida, have been

developed from the Stephanida, and from one of them the family Tympanida

has arisen.

The Semantida (p. 953, Pl. 92) develop a horizontal basal ring at the

oral side of the vertical sagittal ring; the basal meshes or lattice

gates, which remain between the former and the latter, are the important

cortinar pores (one pair jugular, one pair cardinal, p. 954); they

usually appear inherited in the cortinar septum of the #Cyrtellaria#. In

the Coronida (p. 967, Pls. 82, 94) a second vertical ring (the frontal

ring) appears in addition to the sagittal ring; it lies in the frontal

plane at right angles to the latter. Finally the Tympanida (p. 987, Pls.

93, 94) have probably arisen from the Semantida by the formation of a

second horizontal ring (mitral ring) parallel to the basal and attached

to the upper portion of the sagittal ring.

188. _Genealogical Tree of the Spyroidea._--The extensive order #Spyroidea#

is of especial interest in connection with the phylogeny of the

NASSELLARIA, since all its members show two well-developed skeletal

elements in combination, the sagittal ring of the #Stephoidea# and the

latticed cephalis of the #Cyrtoidea#; the majority possess also the basal

tripod of the #Plectoidea# (or a radial skeleton derived from it). Hence

there is a possibility of deriving the stem-forms of the #Spyroidea# from

each of these three groups. The four families of this order exhibit similar

relationships to those of the four families of #Cyrtoidea#; the common

stem-group is the family Zygospyrida; from this the Tholospyrida have

arisen by the development of a galea on the apical pole, the Phormospyrida

by the addition of a thorax on the basal pole. The Androspyrida may be

derived either from the Tholospyrida by the formation of a basal thorax, or

from the Phormospyrida by the development of an apical galea. Some groups,

however, such as the peculiar Nephrospyrida (Pl. 90) have probably been

developed directly from the #Stephoidea#.

189. _Genealogical Tree of the Botryodea._--The peculiar order #Botryodea#

(p. 1103), which is both difficult to investigate and insufficiently known,

presents great phylogenetic difficulties both as to its ascent and descent.

Probably the different genera of this order have been polyphyletically

developed from different groups of #Cyrtoidea# (perhaps also to some extent

of #Spyroidea#) by the formation of lobes in the cephalis. The three

families of #Botryodea# are related to each other in the same way as are

the three first families of the #Cyrtoidea#. From the single-jointed

Cannobotryida (corresponding to the Monocyrtida), the two-jointed

Lithobotryida (corresponding to the Dicyrtida), may be derived by the

development of a basal thorax, and from the latter the three-jointed

Pylobotryida (like the Tricyrtida) by the addition of an abdomen. In the

last two families the forms with an open basal mouth {cxxii}(Botryopylida

and Botryocyrtida) are to be regarded as primitive: the Botryocellida and

Botryocampida have arisen by the closure of this mouth with a basal

lattice-plate.

190. _Genealogical Tree of the Cyrtoidea._--The multiform and extensive

group #Cyrtoidea# presents the greatest difficulties to be found in the

phylogeny of the NASSELLARIA, because their morphological relations are

most complicated, and because similar forms very often appear to be of

quite different origin. The great majority of the #Cyrtoidea# show more or

less clearly a combination of the three structural elements: sagittal ring,

basal tripod, and latticed cephalis (p. 891). There are also, however,

numerous #Cyrtoidea#, whose skeleton no longer shows any trace of the

sagittal ring. Many of these show as the basis of the skeleton a strong

basal tripod with an apical spine, around which the cephalis has obviously

been secondarily developed, _e.g._, the remarkable Euscenida (p. 1146, Pls.

53, 97) and the interesting Callimitrida (p. 1217, Pls. 63, 64). These may

have been derived immediately from the #Plectoidea# without any relation to

the #Stephoidea#. There are also numerous true Monocyrtida, whose shell

consists of a simple latticed cephalis without a trace of the sagittal ring

or basal tripod (Cyrtocalpida, Pl. 51, figs. 9-13; Pl. 98, fig. 13); these

may have been developed directly from the skeletonless Nassellida by the

formation of a simple ovoid _Gromia_-like shell, and may have no relation

either to the #Stephoidea# or #Plectoidea#. On these grounds, as well as

from the complicated relationships of the many smaller groups of

#Cyrtoidea#, it is probable that the whole order has been developed

polyphyletically from different divisions of the #Plectellaria#.

191. _Systematic Arrangement of the Cyrtoidea._--Although for the reasons

just given no systematic arrangement of the #Cyrtoidea# can at present, or

for a long time in the future, be regarded as other than artificial, yet

some general principles of classification for this extensive group can be

laid down, which may serve as starting points for some future natural

disposition. This is especially true of the relations which in an

artificial system (p. 1129) were primarily utilised for the distinction of

twelve families and twenty-four subfamilies; the number of segments in the

shell, the number of radial apophyses (and parameres), and the constitution

of the basal aperture of the shell.

As regards the _number of segments_, separated by transverse

constrictions, of which the shell is composed, it is dependent upon the

secondary addition of new joints at the basal pole of the main axis.

Hence all many-jointed #Cyrtoidea# are to be derived from single-jointed

ones, and the four sections thus distinguished (Monocyrtida, Dicyrtida,

Tricyrtida, Stichocyrtida) form a phylogenetic series. Very often,

however, the primary cephalis disappears owing to retrograde

metamorphosis; and in such cases the single joint of the apparent

Monocyrtida is formed of the thorax (_e.g._, {cxxiii}Pls. 52, 54, &c.).

As regards the _number of radial apophyses_, three sections of

#Cyrtoidea# may be distinguished; the Pilocyrtida with three, the

Astrocyrtida with numerous apophyses, and the Corocyrtida with none (p.

1129). The last two may in general be regarded as two divergent branches

from the first, for the eradiate Corocyrtida have probably been formed

from the triradial Pilocyrtida by entire loss of the radial apophyses,

whilst on the other hand the multiradiate Astrocyrtida have arisen from

them by additions to the primary apophyses (interpolation of interradial

between the perradial ones). As regards the _constitution of the

shell-aperture_, the #Cyrtoidea# may be divided into Cyrtaperta and

Cyrtoclausa (p. 1129); in general the Cyrtoclausa (with latticed

shell-aperture) have arisen from the Cyrtaperta (with simple open mouth);

in many Monocyrtida the converse may be supposed, the simple basal mouth

having been formed by degeneration of a basal lattice.

192. _Phylogeny of the Phæodaria._--The legion PHÆODARIA or CANNOPYLEA is

so clearly marked off from other Radiolaria by the double membrane of the

central capsule and the astropyle at its oral pole, as well as by the

extracapsular phæodium, that it must be regarded phylogenetically as an

independent stem (§ 9). This stem is only connected at its root by

_Phæodina_ with the stem-form of the SPUMELLARIA, _Actissa_. The stem

itself is monophyletic, inasmuch it its members may be derived without

violence from the skeletonless Phæodinida (_Phæodina_, _Phæocolla_). On the

other hand, the formation of the skeleton of the PHÆODARIA is undoubtedly

polyphyletic, different Phæodinida having independently commenced the

formation of a skeleton and having carried it out in very different ways.

193. _Origin of the Phæodaria._--The Phæodinida (p. 1544, Pl. 101), which

may naturally be regarded as the common stem-group of the PHÆODARIA, have

their nearest relations among other Radiolaria in the Thalassicollida (p.

10); and since this family is to be regarded as the primitive group of all

Radiolaria, they may be directly derived from them phylogenetically. The

essential modifications by which the primitive Phæodinida have arisen from

the more archaic Thalassicollida are of three kinds; (1) the doubling of

the membrane of the central capsule; (2) the reduction of the numerous fine

pores in the membrane and the formation of an osculum, and of an astropyle

closing it, at the oral pole of the main axis; (3) the production of an

extracapsular phæodium. This last may, perhaps, be regarded as a

unilateral hypertrophy of the voluminous pigment masses which are deposited

in the sarcomatrix of certain Thalassicollida. Of the two genera of

Phæodinida hitherto known, probably _Phæodina_ (Pl. 101, fig. 2) approaches

the original stem of the PHÆODARIA more nearly than _Phæocolla_ (Pl. 101,

fig. 1), for the latter exhibits only the large main opening of the central

capsule (astropyle), whilst the former possesses also a pair of accessory

openings (parapylæ). The hypothetical stem-form (_Phæometra_) presumably

had a larger number of small parapylæ (like many Circoporida and

Tuscarorida), and the astropyle was probably but little differentiated from

them.

{cxxiv}194. _Hypothetical Genealogical tree of the Phæodaria:_--

PHÆOCONCHIA

~~~~~~~~~~~~~~~~~~~~

PHÆOSPHÆRIA Coeloplegmida PHÆOGROMIA

~~~~~~~~~~~~~~~~~~~ | ~~~~~~~~~~~~~~~~~~

Aularida | Tuscarorida

| | |

|Aulonida | Coelodrymida |

| | | | |

+---+ Coelotholida | |

| Coelographida | | Haeckelinida

| | | | |

| Conchopsida | Coelodorida | |

Aulosphærida | | Coelodendrida |Circogonida|

| | +----+---+ | | |

| Sagmarida +-----------+ | +--+--+

| | | | |

Cannosphærida | |Castanellida | Circoporida

| | | | | |

| |Oroscenida Concharida| +------+-+

| | | | | |

| | | | | | |

| | | | | | | |

+--------+-----+ | | | | |

| | | | | |

+--------------+-+ | | | |

| |Euphysettida| |

| |Medusettida | |

| | | | |

| | |Lithogromida |

| | |Challengerida|

Phæodinida | | | |

| | | | |

PHÆOCYSTINA | +-----+-+----+ |

~~~~~~~~~~~~~~~~~~~~ | | |

Aulacanthida | | |

|Cannobelida Catinulida | +------+----+

| | | | |

| | | | | |

| +-----+-------+ | |

| | | Phæodinida

| Cannorrhaphida | Phæodinida |

| | | | |

+----------+------------+----+------+----------+

Phæodina

(Phæometra)

Actissa

{cxxv}195. _Phæocystina and Phæocoscina._--Whilst the malacoma of all

PHÆODARIA possesses the characteristics of the legion, and hence justifies

the assumption of a monophyletic origin, the skeleton, on the other hand,

shows in the different groups such manifold and fundamental variations that

a polyphyletic origin of the latter is indubitable. Different Phæodinida

have commenced the formation of the skeleton independently, and it has

progressed in different directions. In the #Phæocystina# it remained

incomplete and led to the formation of various Beloid skeletons, whilst the

#Phæocoscina# developed complete lattice-shells. Both of these divisions

too are to be regarded as polyphyletic, since the skeletal forms of the

different groups cannot be derived without violence from a common primitive

form.

196. _Phæocystina with a Beloid Skeleton._--The order #Phæocystina#

includes all PHÆODARIA which have no complete lattice-shell; it contains,

firstly, the skeletonless Phæodinida (the common stem-group of the legion),

and secondly, the Phæacanthida, or PHÆODARIA with a Beloid skeleton (§

115). The latter are divisible into several very different groups (at least

two or three) which are probably different in origin. The Aulacanthida

(Pls. 102-105) form radial tubes which perforate the calymma, their

proximal end resting upon the surface of the central capsule, whilst the

distal extremity projects freely outwards. The skeleton of the

Cannorrhaphida, on the other hand, is composed of many separate portions

which are never radially arranged but are either placed tangentially to the

surface of the calymma or scattered irregularly in its gelatinous mass.

Furthermore, in the three subfamilies of which this family is composed, the

individual skeletal portions are so different that they have probably

arisen independently of each other; in the Cannobelida they form

cylindrical tangential tubes (Pl. 101, figs. 3-5), in the Catinulida flat

basin or cap-like structures (Pl. 117, fig. 8), in the Dictyochida hollow

rings, from which small pyramids are developed by unilateral formation of

lattice-work (Pl. 101, figs. 9-14; Pl. 114, figs. 7-12).

197. _Phæosphæria with a Sphæroid Skeleton._--The order #Phæosphæria#

includes those PHÆODARIA which possess a spherical (sometimes slightly

modified) lattice-shell without the characteristic aperture of the

#Phæogromia#. They have probably arisen independently of these, though they

may have been derived from the Castanellida by loss of the shell-aperture,

which was present originally. The four families which we have distinguished

among the #Phæosphæria#, are so different in the structure of their

lattice-shell that their phylogenetic connection is doubtful. In the

Orosphærida (Pls. 106, 107) and the Sagosphærida (Pl. 108) the whole

lattice-shell consists of a single piece and is unjointed (without astral

septa); in the former it is very firm and massive, with thick laminated

trabeculæ and polygonal meshes; in the latter it is very delicate and

brittle, with filiform trabeculæ and large {cxxvi}triangular meshes. On

the other hand, the voluminous shell of the Aulosphærida (Pls. 109-111),

and of the Cannosphærida (Pl. 112), is characterised by a very peculiar

system of joints; it is composed of numerous separate cylindrical tubes,

which are placed tangentially and united at the nodes by stellate

partitions or astral septa. The Cannosphærida possess further a simple

central Cyrtoid shell, connected with the outer jointed shell by hollow

radial trabeculæ. Since many Aulosphærida possess rudiments of such

centripetal trabeculæ it is possible that these latter have been derived

from the former by the loss of the central Cyrtoid shell; the formation of

this monaxon shell perhaps indicates descent from the #Phæogromia#

(Castanellida).

198. _Phæogromia with a Cyrtoid Skeleton._--That order of the PHÆODARIA

which we designate #Phæogromia#, contains many very different forms, all

agreeing in the possession of a Cyrtoid skeleton, or a monaxon

lattice-shell, which has a large aperture at one pole of its vertical main

axis (§ 123). This Cyrtoid skeleton is sometimes ovoid or conical,

sometimes lentiform or helmet-shaped, sometimes polyhedral or almost

spherical. Although the principle of its structure is simple and often like

that of the Monocyrtida among the NASSELLARIA, yet the structure of the

wall and of the apophyses is so different in the various groups of the

#Phæogromia#, that the order is probably polyphyletic, and its Cyrtoid

shells have arisen independently of each other. Only in the Castanellida

(Pl. 113) does the shell-wall usually consist of simple lattice-work; in

the Challengerida, on the other hand (Pl. 99), it has an extremely fine

Diatom-like structure; in the Medusettida (Pls. 118-128) a peculiar

alveolar structure, and in the Circoporida (Pls. 114-117) and Tuscarorida

(Pl. 100) it possesses a characteristic porcellanous constitution (with

tangential spicules in a porous cement-mass); in the latter of these groups

the surface is smooth, in the former peculiarly tabulate; the two families

have also different stem-forms.

199. _Phæoconchia with a Conchoid Shell._--The order #Phæoconchia# (Pls.

121-128) is separated not only from all other PHÆODARIA, but also from all

other Radiolaria, by the possession of a bivalved shell resembling that of

a Lamellibranch; the two valves of this Conchoid skeleton are to be

interpreted as dorsal and ventral (§ 128). Probably these bivalved shells

are independent products, but possibly they may have been formed by the

bisection of a simple spherical lattice-shell; in the former case the

#Phæoconchia# would be directly descended from the Phæodinida, in the

latter from the Castanellida. The three families which we have

distinguished among the #Phæoconchia#, probably constitute a connected

stem, the most primitive group of which are the Concharida (Pls. 123-125).

From these the Coelodendrida (Pls. 121, 122) have next arisen by the

formation of a "galea" upon the apex of each valve, and the growth of

hollow tubes from this helmet-like structure. Finally, the Coelographida

{cxxvii}(Pls. 120-128) have been developed from the Coelodendrida by the

formation of a basal nasal tube (rhinocanna) from each galea, and the

formation of a median or paired frenulum, which connects the opening of the

nasal tube with the apex of the galea. In the Coelodendrida, as well as in

the Coelographida, there are two different subfamilies, of which the more

primitive (Coelodorida, Coelotholida) have free branches from the hollow

radial tubes, whilst the more recent (Coelodrymida, Coeloplegmida) form an

outer bivalved shell by anastomosis of the branches of the tubes.

200. _The Fundamental Biogenetic Law._--The causal connection between

ontogeny and phylogeny, which finds its most precise statement in the

fundamental biogenetic law, holds in general for the Radiolaria as for all

other organisms. In order to furnish direct proof of this, however, a

complete empirical knowledge both of individual and of palæontological

development would be necessary. In both these directions, as has been shown

in the foregoing chapters, our knowledge of the Radiolaria is very

incomplete and fragmentary, but still we are able to convince ourselves

indirectly of the validity of the law as applied to Radiolaria by the aid

of comparative anatomy. This is now so fully known to us (§§ 1-140) that we

are able not only to draw a complete and satisfactory picture of their

morphology, but also to arrive at most important conclusions regarding the

ontogeny and phylogeny of the individual groups. As regards the formation

of the multiform skeleton of the Radiolaria, most of the ontogenetic series

of forms, with which we have become acquainted by comparative anatomy, are

of _palingenetic_ nature; that is, they are primarily due to inheritance

and thus of direct phylogenetic significance. On the other hand, among the

ontogenetic phenomena of the Radiolaria, as far as they have yet been

investigated, only very few are _cenogenetic_, that is, brought about by

adaptive modification and without direct significance as regards phylogeny.

{cxxviii}PHYSIOLOGICAL SECTION.

----

Report on the Radiolaria Collected by H.M.S. Challenger During the Years 1873-1876, First Part: Porulosa (Spumellaria and Acantharia) Report on the Scientific Results of the Voyage of H.M.S. Challenger During the Years 1873-76, Vol. XVIII · 第 5 / 27 章