complete equatorial ring round the ovum, the procephalic and caudal
lobes being only separated by a very narrow space, the undeveloped
dorsal region of the embryo. This is shewn in longitudinal section in
fig. 204. In this condition the embryo may be spoken of as having a
_dorsal flexure_. By the time that this stage is reached (fig. 200 C)
the full number of segments and appendages has become established.
There are in all sixteen segments (including the caudal lobe). The
first six of these bear the permanent appendages of the adult; the
next four are provided with provisional appendages; while the last six
are without appendages. The further features of this stage which
deserve notice are (1) the appearance of a shallow depression
(_st_)--the rudiment of the stomodæum--between the hinder part of the
two procephalic lobes; (2) the appearance of raised areas on the inner
side of the six anterior appendage-bearing segments. These are the
rudiments of the ventral ganglia. It deserves to be especially noted
that the segment of the cheliceræ, like the succeeding segments, is
provided with ganglia; and that the ganglia of the cheliceræ are quite
distinct from the supra-oesophageal ganglia derived from the
procephalic lobes. (3) The pointed form of the caudal lobe. In Pholcus
(Claparède, No. 442) the caudal lobe forms a projecting structure
which, like the caudal lobe of the Scorpion, bends forward so as to
face the ventral surface of the part of the body immediately in front.
In most Spiders such a projecting caudal lobe is not found. While the
embryo still retains its dorsal flexure considerable changes are
effected in its general constitution. The appendages (fig. 200 D and
E) become imperfectly jointed, and grow inwards so as to approach each
other in the middle line. Even in the stage before this, the ventral
integument between the rudiments of the ganglia had become very much
thinner, and had in this way divided the ventral plate into two
halves. At the present stage the two halves of the ventral plate are
still further separated, and there is a wide space on the ventral side
only covered by a delicate layer of epiblast. This is shewn in surface
view (fig. 200 D) and in section in fig. 203 C.
The stomodæum (_st_) is much more conspicuous, and is bounded in front
by a prominent upper lip, and by a less marked lip behind. The upper
lip becomes less conspicuous in later stages, and is perhaps to be
compared with the provisional upper lip of Chelifer. Each procephalic
lobe is now marked by a deep semicircular groove.
The next period in the development is characterised by the gradual
change in the flexure of the embryo from a dorsal to a ventral one;
accompanied by the division of the body into an abdomen and
cephalo-thorax, and the gradual assumption of the adult characters.
The change in the flexure of the embryo is caused by the elongation of
the dorsal region, which has hitherto been hardly developed. Such an
elongation increases the space on the dorsal surface between the
procephalic and caudal regions, and therefore necessarily separates
the caudal and procephalic lobes; but, since the ventral plate does
not become shortened in the process, and the embryo cannot straighten
itself in the egg-shell, it necessarily becomes ventrally flexed.
If there were but little food-yolk this flexure would naturally cause
the whole embryo to be bent in so as to have the ventral surface
concave. But instead of this the flexure is at first confined to the
two bands which form the ventral plate. These bands, as shewn in fig.
201 A, acquire a true ventral flexure, but the yolk forms a
projection--a kind of yolk-sack as Barrois (No. 441) calls
it--distending the thin integument between the two ventral bands. This
yolk-sack is shewn in surface view in fig. 201 A and in section in
fig. 206. At a later period, when the yolk has become largely
absorbed, the true nature of the ventral flexure becomes quite
obvious, since the abdomen of the young Spider, while still in the
egg, is found to be bent over so as to press against the ventral
surface of the thorax (fig. 201 B).
[FIG. 201. TWO LATE STAGES IN THE DEVELOPMENT OF AGELENA
LABYRINTHICA.
A. Embryo from the side at the stage when there is a large ventral
protuberance of yolk. The angle between the line of insertion of
the permanent and provisional appendages shews the extent of the
ventral flexure.
B. Embryo nearly ready to be hatched. The abdomen which has not
quite acquired its permanent form is seen to be pressed against
the ventral side of the thorax.
_pr.l._ procephalic lobe; _pd._ pedipalpi; _ch._ cheliceræ; _c.l._
caudal lobe; _pr.p._ provisional appendages.]
The general character of the changes which take place during this
period in the development is shewn in fig. 201 A and B representing
two stages in it. In the first of these stages there is no
constriction between the future thorax and abdomen. The four pairs of
provisional appendages exhibit no signs of atrophy; and the extent of
the ventral flexure is shewn by the angle formed between the line of
their insertion and that of the appendages in front. The yolk has
enormously distended the integument between the two halves of the
ventral plate, as is illustrated by the fact that, at a somewhat
earlier stage than that figured, the limbs cross each other in the
median ventral line, while at this stage they do not nearly meet. The
limbs have acquired their full complement of joints, and the pedipalpi
bear a cutting blade on their basal joint.
The dorsal surface between the prominent caudal lobe and the
procephalic lobes forms more than a semicircle. The terga are fully
established, and the boundaries between them, especially in the
abdomen, are indicated by transverse markings. A large lower lip now
bounds the stomodæum, and the upper lip has somewhat atrophied. In the
later stage (fig. 201 B) the greater part of the yolk has passed into
the abdomen, which is now to some extent constricted off from the
cephalo-thorax. The appendages of the four anterior abdominal somites
have disappeared, and the caudal lobe has become very small. In front
of it are placed two pairs of spinning mammillæ. A delicate cuticle
has become established, which is very soon moulted.
Acarina. The development of the Acarina, which has been mainly
investigated by Claparède (No. 446), is chiefly remarkable from the
frequent occurrence of several larval forms following each other after
successive ecdyses. The segmentation (_vide_ p. 116) ends in the
formation of a blastoderm of a single layer of cells enclosing a
central yolk mass.
A ventral plate is soon formed as a thickening of the blastoderm, in
which an indistinct segmentation becomes early observable. In Myobia,
which is parasitic on the common mouse, the ventral plate becomes
divided by five constrictions into six segments (fig. 202 A), from the
five anterior of which paired appendages very soon grow out (fig. 202
B). The appendages are the cheliceræ (_ch_) and pedipalpi (_pd_) and
the first three pairs of limbs (_p1-p3_). On the dorsal side of the
cheliceræ a thickened prominence of the ventral plate appears to
correspond to the procephalic lobes of other Arachnida. The part of
the body behind the five primitive appendage-bearing segments appears
to become divided into at least two segments. In other mites the same
appendages are formed as in Myobia, but the preceding segmentation of
the ventral plate is not always very obvious.
In Myobia two moultings take place while the embryo is still within
the primitive egg-shell. The first of these is accompanied by the
_apparently total disappearance of the three pediform appendages_, and
the complete coalescence of the two gnathiform appendages into a
proboscis (fig. 202 C). The feet next grow out again, and a second
ecdysis then takes place. The embryo becomes thus inclosed within
three successive membranes, viz. the original egg-shell and two
cuticular membranes (fig. 202 D). After the second ecdysis the
appendages assume their final form, and the embryo leaves the egg as
an hexapodous larva. The fourth pair of appendages is acquired by a
post-embryonic metamorphosis. From the proboscis are formed the
rudimentary palpi of the second pair of appendages, and two elongated
needles representing the cheliceræ.
[FIG. 202. FOUR SUCCESSIVE STAGES IN THE DEVELOPMENT OF MYOBIA
MUSCULI. (After Claparède.)
_s1-s4._ post-oral segments; _ch._ cheliceræ; _pd._ pedipalpi; _pr_
proboscis formed by the coalescence of the cheliceræ and pedipalpi;
_p1, p2_, etc. ambulatory appendages.]
In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are
not accompanied by the peculiar changes observable in Myobia: the
cheliceræ and pedipalpi fuse however to form the proboscis. The first
larval form is hexapodous, and the last pair of appendages is formed
at a subsequent ecdysis.
In Atax Bonzi, a form parasitic on Unio, the development and
metamorphosis are even more complicated than in Myobia. The first
ecdysis occurs before the formation of the limbs, and shortly after
the ventral plate has become divided into segments. Within the
cuticular membrane resulting from the first ecdysis the anterior five
pairs of limbs spring out in the usual fashion. They undergo
considerable differentiation; the cheliceræ and pedipalpi approaching
each other at the anterior extremity of the body, and the three
ambulatory legs becoming segmented and clawed. An oesophagus, a
stomach, and an oesophageal nerve-ring are also formed. When the larva
has attained this stage the original egg-shell is split into two
valves and eventually cast off, but the embryo remains enclosed within
the cuticular membrane shed at the first ecdysis. This cuticular
membrane is spoken of by Claparède as the deutovum. In the deutovum
the embryo undergoes further changes; the cheliceræ and pedipalpi
coalesce and form the proboscis; a spacious body cavity with blood
corpuscles appears; and the alimentary canal enclosing the yolk is
formed.
The larva now begins to move, the cuticular membrane enclosing it is
ruptured, and the larva becomes free. It does not long remain active,
but soon bores its way into the gills of its host, undergoes a fresh
moult, and becomes quiescent. The cuticular membrane of the moult just
effected swells up by the absorption of water and becomes spherical.
Peculiar changes take place in the tissues, and the limbs become, as
in Myobia, nearly absorbed, remaining however as small knobs. The
larva swims about as a spherical body within its shell. The feet next
grow out afresh, and the posterior pair is added. From the proboscis
the palpi (of the pedipalpi) grow out below. The larva again becomes
free, and amongst other changes the cheliceræ grow out from the
proboscis. A further ecdysis, with a period of quiescence, intervenes
between this second larval form and the adult state.
The changes in the appendages which appear common to the Mites
generally are (1) the late development of the fourth pair of
appendages, which results in the constant occurrence of an hexapodous
larva; and (2) the early fusion of the cheliceræ and pedipalpi to form
a proboscis in which no trace of the original appendages can be
discerned. In most instances palpi and stilets of variable form are
subsequently developed in connexion with the proboscis, and, as
indicated in the above descriptions, are assumed to correspond with
the two original embryonic appendages.
_The history of the germinal layers._
It is a somewhat remarkable fact that each of the groups of the
Arachnida so far studied has a different form of segmentation. The
types of Chelifer and the Spiders are simple modifications of the
centrolecithal type, while that of Scorpio, though apparently
meroblastic, is probably to be regarded in the same light (_vide_ p.
120 and p. 434). The early development begins in the Scorpion and
Spiders with the formation of a ventral plate, and there can be but
little doubt that Chelifer is provided with an homologous structure,
though very probably modified, owing to the small amount of food-yolk
and early period of hatching.
The history of the layers and their conversion into the organs has
been studied in the case of the Scorpion (Metschnikoff, No. 434), and
of the Spiders; and a close agreement has been found to obtain between
them.
It will be convenient to take the latter group as type, and simply to
call attention to any points in which the two groups differ.
The epiblast. The epiblast, besides giving rise to the skin
(hypodermis and cuticle), also supplies the elements for the nervous
system and organs of sense, and for the respiratory sacks, the
stomodæum and proctodæum.
At the period when the mesoblast is definitely established, the
epiblast is formed of a single layer of columnar cells in the region
of the ventral plate, and of a layer of flat cells over other parts of
the yolk.
When about six segments are present the first changes take place. The
epiblast of the ventral plate then becomes somewhat thinner in the
median line than at the two sides (fig. 203 B). In succeeding stages
the contrast between the median and the lateral parts becomes still
more marked, so that the epiblast becomes finally constituted of two
lateral thickened bands, which meet in front in the procephalic lobes,
and behind in the caudal lobe, and are elsewhere connected by a very
thin layer (fig. 203 C). Shortly after the appendages begin to be
formed, the first rudiments of the ventral nerve cord become
established as epiblastic thickenings on the inner side of each of the
lateral bands. The thickenings of the epiblast of the two sides are
quite independent, as may be seen in fig. 203 C, _vn_, taken from a
stage somewhat subsequent to their first appearance. They are
developed from before backwards, but either from the first, or in any
case very soon afterwards, cease to form uniform thickenings, but
constitute a linear series of swellings--the future ganglia--connected
by very short less prominent thickenings of the epiblast (fig. 200 C).
The rudiments of the ventral nerve cord are for a long time continuous
with the epiblast, but shortly after the establishment of the dorsal
surface of the embryo they become separated from the epiblast and
constitute two independent cords, the histological structure of which
is the same as in other Tracheata (fig. 206, _vn_).
[FIG. 203. TRANSVERSE SECTIONS THROUGH THE VENTRAL PLATE OF AGELENA
LABYRINTHICA AT THREE STAGES.
A. Stage when about three segments are formed. The mesoblastic plate
is not divided into two bands.
B. Stage when six segments are present (fig. 200 B). The mesoblast
is now divided into two bands.
C. Stage represented in fig. 200 D. The ventral cords have begun to
be formed on thickenings of the epiblast, and the limbs are
established.
_ep._ epiblast; _me._ mesoblast; _me.s._ mesoblastic somite; _vn._
ventral nerve cord; _yk._ yolk.]
The ventral cords are at first composed of as many ganglia as there
are segments. The foremost pair, belonging to the segment of the
cheliceræ, lie immediately behind the stomodæum, and are as
independent of each other as the remaining ganglia. Anteriorly they
border on the supra-oesophageal ganglia. When the yolk-sack is formed
in connection with the ventral flexure of the embryo, the two nerve
cords become very widely separated (fig. 206, _vn_) in their middle
region. At a later period, at the stage represented in fig. 201 B,
they again become approximated in the ventral line, and delicate
commissures are formed uniting the ganglia of the two sides, but there
is no trace at this or any other period of a median invagination of
epiblast between the two cords, such as Hatschek and other observers
have attempted to establish for various Arthropoda and Chætopoda. At
the stage represented in fig. 201 A the nerve ganglia are still
present in the abdomen, though only about four ganglia can be
distinguished. At a later stage these ganglia fuse into two continuous
cords, united however by commissures corresponding with the previous
ganglia.
The ganglia of the cheliceræ have, by the stage represented in fig.
201 B, completely fused with the supra-oesophageal ganglia and form
part of the oesophageal commissure. The oesophageal commissure is
however completed ventrally by the ganglia of the pedipalpi.
The supra-oesophageal ganglia are formed independently of the ventral
cords as two thickenings of the procephalic lobes (fig. 205). The
thickenings of the two lobes are independent, and each of them becomes
early marked out by a semicircular groove (fig. 200 D) running
outwards from the upper lip. Each thickening eventually becomes
detached from the superficial epiblast, but before this takes place
the two grooves become deeper, and on the separation of the ganglia
from the epiblast, the cells lining the grooves become involuted and
detached from the skin, and form an integral part of the
supra-oesophageal ganglia.
At the stage represented in fig. 201 B the supra-oesophageal ganglia
are completely detached from the epiblast, and are constituted of the
following parts: (1) A dorsal section formed of two hemispherical
lobes, mainly formed of the invaginated lining of the semicircular
grooves. The original lumen of the groove is still present on the
outer side of these lobes. (2) Two central masses, one for each
ganglion, formed of punctiform tissue, and connected by a transverse
commissure. (3) A ventral anterior lobe. (4) The original ganglia of
the cheliceræ, which form the ventral parts of the ganglia[183].
[183] For further details _vide_ self, No. 440.
The later stages in the development of the nervous system have not
been worked out.
The development of the nervous system in the Scorpion is almost
identical with that in Spiders, but Metschnikoff believes, though
without adducing satisfactory evidence, that the median integument
between the two nerve cords assists in forming the ventral nerve cord.
Grooves are present in the supra-oesophageal ganglia similar to those
in Spiders.
The mesoblast. The history of the mesoblast, up to the formation of a
ventral plate subjacent to the thickened plate of epiblast, has been
already given. The ventral plate is shewn in fig. 203 A. It is seen to
be formed mainly of small cells, but some large cells are shewn in the
act of passing into it from the yolk. During a considerable section of
the subsequent development the mesoblast is confined to the ventral
plate.
[FIG. 204. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELENA
LABYRINTHICA.
The section is through an embryo of the same age as that represented
in fig. 200 C, and is taken slightly to one side of the middle
line so as to shew the relation of the mesoblastic somites to the
limbs. In the interior are seen the yolk segments and their
nuclei.
1-16. the segments; _pr.l._ procephalic lobe; _do._ dorsal
integument.]
[FIG. 205. SECTION THROUGH THE PROCEPHALIC LOBES OF AN EMBRYO OF
AGELENA LABYRINTHICA.
The section is taken from an embryo of the same age as fig. 200 D.
_st._ stomodæum; _gr._ section through semicircular groove
in procephalic lobe; _ce.s._ cephalic section of body cavity.]
The first important change takes place when about six somites are
established; the mesoblast then becomes divided into two lateral
bands, shewn in section in fig. 203 B, which meet however in front in
the procephalic lobes, and behind in the caudal lobes. Very shortly
afterwards these bands become broken up into a number of parts
corresponding to the segments, each of which soon becomes divided into
two layers, which enclose a cavity between them (_vide_ fig. 204 and
fig. 207). The outer layer (somatic) is thicker and attached to the
epiblast, and the inner layer (splanchnic) is thinner and mainly, if
not entirely, derived (in Agelena) from cells which originate in the
yolk. These structures constitute the mesoblastic somites. In the
appendage-bearing segments the somatic layer of each of them, together
with a prolongation of the cavity, is continued into the appendage
(fig. 203 C). Since the cavity of the mesoblastic somites is part of
the body cavity, all the appendages contain prolongations of the body
cavity. Not only is a pair of mesoblastic somites formed for each
segment of the body, but also for the procephalic lobes (fig. 205).
The mesoblastic somites for these lobes are established somewhat later
than for the true segments, but only differ from them in the fact that
the somites of the two sides are united by a median bridge of
undivided mesoblast. The development of a somite for the procephalic
lobes is similar to what has been described by Kleinenberg for
Lumbricus (p. 339), but must not be necessarily supposed to indicate
that the procephalic lobes form a segment equivalent to the segments
of the trunk. They are rather equivalent to the præ-oral lobe of
Chætopod larvæ. When the dorsal surface of the embryo is established a
thick layer of mesoblast becomes formed below the epiblast. This layer
is not however derived from an upgrowth of the mesoblast of the
somites, but from cells which originate in the yolk. The first traces
of the layer are seen in fig. 204, _do_, and it is fully established
as a layer of large round cells in the stage shewn in fig. 206. This
layer of cells is seen to be quite independent of the mesoblastic
somites (_me.s_). The mesoblast of the dorsal surface becomes at the
stage represented in fig. 201 B divided into splanchnic and somatic
layers, and in the abdomen at any rate into somites continuous with
those of the ventral part of the mesoblast. At the lines of junction
of successive somites the splanchnic layer of mesoblast dips into the
yolk, and forms a number of transverse septa, which do not reach the
middle of the yolk, but leave a central part free, in which the
mesenteron is subsequently formed. At the insertion of these septa
there are developed widish spaces between the layers of somatic and
splanchnic mesoblast, which form transversely directed channels
passing from the heart outwards. They are probably venous. At a later
stage the septa send out lateral offshoots, and divide the peripheral
part of the abdominal cavity into a number of compartments filled with
yolk. It is probable that the hepatic diverticula are eventually
formed in these compartments.
[FIG. 206. TRANSVERSE SECTION THROUGH THE THORACIC REGION OF AN
EMBRYO OF AGELENA LABYRINTHICA.
The section is taken from an embryo of the same age as fig. 201 A,
and passes through the maximum protuberance of the ventral
yolk-sack.
_vn._ ventral nerve cord; _yk._ yolk; _me.s._ mesoblastic somite;
_ao._ aorta.]
The somatic layer of mesoblast is converted into the muscles, both of
the limbs and trunk, the superficial connective tissue, nervous
sheath, etc. It probably also gives rise to the three muscles attached
to the suctorial apparatus of the oesophagus.
The heart and aorta are formed as a solid rod of cells of the dorsal
mesoblast, before it is distinctly divided into splanchnic and somatic
layers. Eventually the central cells of the heart become blood
corpuscles, while its walls are constituted of an outer muscular and
inner epithelioid layer. It becomes functional, and acquires its
valves, arterial branches, etc., by the stage represented in
fig. 201 B.
The history of the mesoblast, more especially of the mesoblastic
somites, of the Scorpion is very similar to that in Spiders: their
cavity is continued in the same way into the limbs. The general
character of the somites in the tail is shewn in fig. 207. The caudal
aorta is stated by Metschnikoff to be formed from part of the
mesenteron, but this is too improbable to be accepted without further
confirmation.
The hypoblast and alimentary tract. It has already been stated that
the yolk is to be regarded as corresponding to the hypoblast of other
types.
For a considerable period it is composed of the polygonal yolk cells
already described and shewn in figs. 203, 204, and 205. The yolk cells
divide and become somewhat smaller as development proceeds; but the
main products of the division of the yolk nuclei and the protoplasm
around them are undoubtedly cells which join the mesoblast (fig. 203
A). The permanent alimentary tract is formed of three sections, viz.
stomodæum, proctodæum, and mesenteron. The stomodæum and proctodæum
are both formed before the mesenteron. The stomodæum is formed as an
epiblastic pit between the two procephalic lobes (figs. 200 and 205,
_st_). It becomes deeper, and by the latest stage figured is a deep
pit lined by a cuticle and ending blindly. To its hinder section,
which forms the suctorial apparatus of the adult, three powerful
muscles (a dorsal and two lateral) are attached.
[FIG. 207. TAIL OF AN ADVANCED EMBRYO OF THE SCORPION TO ILLUSTRATE
THE STRUCTURE OF THE MESOBLASTIC SOMITES. (After Metschnikoff.)
_al._ alimentary tract; _an.i._ anal invagination; _ep._ epiblast;
_me.s._ mesoblastic somite.]
The proctodæum is formed considerably later than the stomodæum. It is
a comparatively shallow involution, which forms the rectum of the
adult. It is dilated at its extremity, and two Malpighian vessels
early grow out from it.
The mesenteron is formed _in the interior of the yolk_. Its walls are
derived from the cellular elements of the yolk, and the first section
to be formed is the hinder extremity, which appears as a short tube
ending blindly behind in contact with the proctodæum, and open to the
yolk in front. The later history of the mesenteron has not been
followed, but it undoubtedly includes the whole of the abdominal
section of the alimentary canal of the adult, except the rectum, and
probably also the thoracic section. The later history of the yolk
which encloses the mesenteron has not been satisfactorily studied,
though it no doubt gives rise to the hepatic tubes, and probably also
to the thoracic diverticula of the alimentary tract.
The general history of the alimentary tract in Scorpio is much the
same as in Spiders. The hypoblast, the origin of which as mentioned
above is somewhat uncertain, first appears on the ventral side and
gradually spreads so as to envelop the yolk, and form the wall of the
mesenteron, from which the liver is formed as a pair of lateral
outgrowths. The proctodæum and stomodæum are both short, especially
the former (_vide_ fig. 207).
_Summary and general conclusions._
The embryonic forms of Scorpio and Spiders are very similar, but in
spite of the general similarity of Chelifer to Scorpio, the embryo of
the former differs far more from that of Scorpio than the latter does
from Spiders. This peculiarity is probably to be explained by the
early period at which Chelifer is hatched; and though a more thorough
investigation of this interesting form is much to be desired, it does
not seem probable that its larva is a primitive type.
The larvæ of the Acarina with their peculiar ecdyses are to be
regarded as much modified larval forms. It is not however easy to
assign a meaning to the hexapodous stage through which they generally
pass.
With reference to the segments and appendages, some interesting points
are brought out by the embryological study of these forms.
The maximum number of segments is present in the Scorpion, in which
nineteen segments (not including the præ-oral lobes, but including the
telson) are developed. Of these the first twelve segments have traces
of appendages, but the appendages of the six last of these (unless the
pecten is an appendage) atrophy. In Spiders there are indications in
the embryo of sixteen segments and in all the Arachnida, except the
Acarina, at the least four segments bear appendages in the embryo
which are without them in the adult. The morphological bearings of
this fact are obvious.
It deserves to be noted that, in both Scorpio and the Spider, the
cheliceræ are borne in the embryo by the first post-oral segment, and
provided with a distinct ganglion, so that they cannot correspond (as
they are usually supposed to do) with the antennæ of Insects, which
are always developed on the præ-oral lobes, and never supplied by an
independent ganglion.
The cheliceræ would seem probably to correspond with the mandibles of
Insects, and the antennæ to be absent. In favour of this view is the
fact that the embryonic ganglion of the mandibles of Insects is stated
(cf. Lepidoptera, _Hatschek_, p. 340) to become, like the ganglion of
the cheliceræ, converted into part of the oesophageal commissure.
If the above considerations are correct, the appendages of the
Arachnida retain in many respects a very much more primitive condition
than those of Insects. In the first place, both the cheliceræ and
pedipalpi are much less differentiated than the mandibles and first
pair of maxillæ with which they correspond. In the second place, the
first pair of ambulatory limbs must be equivalent to the second pair
of maxillæ of Insects, which, for reasons stated above, were probably
originally ambulatory. It seems in fact a necessary deduction from the
arguments stated that the ancestors of the present Insecta and
Arachnida must have diverged from a common stem of the Tracheata at a
time when the second pair of maxillæ were still ambulatory in
function.
With reference to the order of the development of the appendages and
segments, very considerable differences are noticeable in the
different Arachnoid types. This fact alone appears to me to be
sufficient to prove that the order of appearance of the appendages is
often a matter of embryonic convenience, without any deep
morphological significance. In Scorpio the segments develop
successively, except perhaps the first post-oral, which is developed
after some of the succeeded segments have been formed. In Spiders the
segment of the cheliceræ, and probably also of the pedipalpi, appears
later than the next three or four. In both these types the segments
arise before the appendages, but the reverse appears to be the case in
Chelifer. The permanent appendages, except the cheliceræ, appear
simultaneously in Scorpions and Spiders. The second pair appears long
before the others in Chelifer, then the third, next the first, and
finally the three hindermost.
BIBLIOGRAPHY.
_Scorpionidæ._
(434) El. Metschnikoff. "Embryologie des Scorpions." _Zeit. f. wiss.
Zool._ Bd. XXI. 1870.
(435) H. Rathke. _Reisebemerkungen aus Taurien_ (Scorpio). Leipzig,
1837.
_Pseudoscorpionidæ._
(436) El. Metschnikoff. "Entwicklungsgeschichte d. Chelifer." _Zeit.
f. wiss. Zool._, Bd. XXI. 1870.
(437) A. Stecker. "Entwicklung der Chthonius-Eier im Mutterleibe und
die Bildung des Blastoderms." _Sitzung. königl. böhmisch. Gesellschaft
Wissensch._, 1876, 3. Heft, and _Annal. and Mag. Nat. History_, 1876,
XVIII. 197.
_Phalangidæ._
(438) M. Balbiani. "Mémoire sur le développement des Phalangides."
_Ann. Scien. Nat._ Series V. Vol. XVI. 1872.
_Araneina._
(439) M. Balbiani. "Mémoire sur le développement des Aranéides." _Ann.
Scien. Nat_. Series V. Vol. XVII. 1873.
(440) F. M. Balfour. "Notes on the development of the Araneina."
_Quart. Journ. of Micr. Science_, Vol. XX. 1880.
(441) J. Barrois. "Recherches s. l. développement des Araignées."
_Journal de l'Anat. et de la Physiol._ 1878.
(442) E. Claparède. _Recherches s. l'évolution des Araignées._
Utrecht, 1862.
(443) Herold. _De generatione Araneorum in Ovo._ Marburg, 1824.
(444) H. Ludwig. "Ueber die Bildung des Blastoderms bei den Spinnen."
_Zeit. f. wiss. Zool._, Vol. XXVI. 1876.
_Acarina._
(445) P. van Beneden. "Développement de l'Atax ypsilophora." _Acad.
Bruxelles_, t. XXIV.
(446) Ed. Claparède. "Studien über Acarinen." _Zeit. f. wiss. Zool._,
Bd. XVIII. 1868.
_Formation of the layers and the embryonic envelopes in the
Tracheata._
There is a striking constancy in the mode of formation of the layers
throughout the group. In the first place the hypoblast is not formed
by a process which can be reduced to invagination: in other words,
there is no gastrula stage.
Efforts have been made to shew that the mesoblastic groove of Insects
implies a modified gastrula, but since it is the essence of a gastrula
that it should directly or indirectly give rise to the archenteron,
the groove in question cannot fall under this category. Although the
mesoblastic groove of Insects is not a gastrula, it is quite possible
that it is the rudiment of a blastopore, the gastrula corresponding to
which has now vanished from the development. It would thus be
analogous to the primitive streak of Vertebrates[184].
[184] The primitive streak of Vertebrates, as will appear in the
sequel, has no connection with the medullary groove, and is the
rudiment of the blastopore.
The growth of the blastoderm over the yolk in Scorpions admits no
doubt of being regarded as an epibolic gastrula. The blastopore would
however be situated dorsally, a position which it does not occupy in
any gastrula type so far dealt with. This fact, coupled with the
consideration that the partial segmentation of Scorpio can be derived
without difficulty from the ordinary Arachnidan type (_vide_ p. 120),
seems to shew that there is no true epibolic invagination in the
development of Scorpio.
On the formation of the blastoderm traces of two embryonic layers are
established. The blastoderm itself is essentially the epiblast, while
the central yolk is the hypoblast. The formation of the embryo
commences in connection with a thickening of the blastoderm, known as
the ventral plate. The mesoblast is formed as an unpaired plate split
off from the epiblast of the ventral plate. This process takes place
in at any rate two ways. In Insects a groove is formed, which becomes
constricted off to form the mesoblastic plate: in Spiders there is a
keel-like thickening of the blastoderm, which takes the place of the
groove.
The unpaired mesoblastic plate becomes in all forms very soon divided
into two _mesoblastic bands_.
The mesoblastic bands are very similar to, and probably homologous
with, those of Chætopoda; but the different modes by which they arise
in these two groups are very striking, and probably indicate that
profound modifications have taken place in the early development of
the Tracheata. In the Chætopoda the bands are from the first widely
separated, and gradually approach each other ventrally, though without
meeting. In the Tracheata they arise from the division of an unpaired
ventral plate.
The further history of the mesoblastic bands is nearly the same for
all the Tracheata so far investigated, and is also very much the same
as for the Chætopoda. There is a division into somites, each
containing a section of the body cavity. In the cephalic section of
the mesoblastic bands a section of the body cavity is also formed. In
Arachnida, Myriapoda, and probably also Insecta, the body cavity is
primitively prolonged into the limbs.
In Spiders at any rate, and very probably in the other groups of the
Tracheata, a large part of the mesoblast is not derived from the
mesoblastic plate, but is secondarily added from the yolk cells.
In all Tracheata the yolk cells give rise to the mesenteron which, in
opposition, as will hereafter appear, to the mesenteron of the
Crustacea, forms the main section of the permanent alimentary tract.
One of the points which is still most obscure in connection with the
embryology of the Tracheata is the origin of the embryonic membranes.
Amongst Insects, with the exception of the Thysanura, such membranes
are well developed. In the other groups definite membranes like those
of Insects are never found, but in the Scorpion a cellular envelope
appears to be formed round the embryo from the cells of the
blastoderm, and more or less similar structures have been described in
some Myriapods (_vide_ p. 390). These structures no doubt further
require investigation, but may provisionally be regarded as homologous
with the amnion and serous membrane of Insects. In the present state
of our knowledge it does not seem easy to give any explanation of the
origin of these membranes, but they may be in some way derived from an
early ecdysis.
CHAPTER XVIII.
CRUSTACEA[185].
[185] The following is the classification of the Crustacea
employed in the present chapter:
I. Branchiopoda. { Phyllopoda.
{ Cladocera.
II. Malacostraca. { Nebaliadæ.
{ Schizopoda.
{ Decapoda.
{ Stomatopoda.
{ Cumaceæ.
{ Edriophthalmata.
{ _Natantia._
III. Copepoda. { Eucopepoda. { _Parasita._
{ Branchiura.
IV. Cirripedia. { Thoracica.
{ Abdominalia.
{ Apoda.
{ Rhizocephala.
V. Ostracoda.
_History of the larval forms[186]._
[186] The importance of the larval history of the Crustacea,
coupled with our comparative ignorance of the formation of the
layers, has rendered it necessary for me to diverge somewhat from
the general plan of the work, and to defer the account of the
formation of the layers till after that of the larval forms.
The larval forms of the Crustacea appear to have more faithfully
preserved their primitive characters than those of almost any other
group.
BRANCHIOPODA.
The Branchiopoda, comprising under that term the Phyllopoda and
Cladocera, contain the Crustacea with the maximum number of segments
and the least differentiation of the separate appendages. This and
other considerations render it probable that they are to be regarded
as the most central group of the Crustaceans, and as in many respects
least modified from the ancestral type from which all the groups have
originated.
The free larval stages when such exist commence with a larval form
known as the Nauplius.
The term Nauplius was applied by O. F. Müller to certain larval forms
of the Copepoda (fig. 229) in the belief that they were adult.
[FIG. 208. TWO STAGES IN THE DEVELOPMENT OF APUS CANCRIFORMIS.
(After Claus.)
A. Nauplius stage at the time of hatching. B. Stage after first
ecdysis.
_an1._ and _an2._ First and second antennæ; _md._ mandible; _mx._
maxilla; _l._ labrum; _fr._ frontal sense organ; _f._ caudal
fork; _s._ segments.]
The term has now been extended to a very large number of larvæ which
have certain definite characters in common. They are provided (fig.
208 A) with three pairs of appendages, the future two pairs of antennæ
and mandibles. The first pair of antennæ (_an1_) is uniramous and
mainly sensory in function, the second pair of antennæ (_an2_) and
mandibles (_md_) are biramous swimming appendages, and the mandibles
are without the future cutting blade. The Nauplius mandibles represent
in fact the palp. The two posterior appendages are both provided with
hook-like prominences on their basal joints, used in mastication. The
body in most cases is unsegmented, and bears anteriorly a single
median eye. There is a large upper lip, and an alimentary canal formed
of oesophagus, stomach and rectum. The anus opens near the hind end of
the body. On the dorsal surface small folds of skin frequently
represent the commencement of a dorsal shield. One very striking
peculiarity of the Nauplius according to Claus and Dohrn is the fact
that the second pair of antennæ is innervated from _a sub-oesophageal
ganglion_. A larval form with the above characters occurs with more or
less frequency in all the Crustacean groups. In most instances it does
not _exactly_ conform to the above type, and the divergences are more
considerable in the Phyllopods than in most other groups. Its
characters in each case are described in the sequel.
Phyllopoda..For the Phyllopoda the development of Apus cancriformis
may conveniently be taken as type (Claus, No. 454). The embryo at the
time it leaves the egg (fig. 208 A) is somewhat oval in outline, and
narrowed posteriorly. There is a slight V-shaped indentation behind,
at the apex of which is situated the anus. The body, unlike that of
the typical Nauplius, is already divided into two regions, a cephalic
and post-cephalic. On the ventral side of the cephalic region there
are present the three normal pairs of appendages. Foremost there are
the small anterior antennæ (_an1_), which are simple unjointed
rod-like bodies with two moveable hairs at their extremities. They are
inserted at the sides of the large upper lip or labrum (_l_). Behind
these are the posterior antennæ, which are enormously developed and
serve as the most important larval organs of locomotion. They are
biramous, being formed of a basal portion with a strong hook-like
bristle projecting from its inner side, an inner unjointed branch with
three bristles, and an outer large imperfectly five-jointed branch
with five long lateral bristles. The hook-like organ attached to this
pair of appendages would seem to imply that it served in some
ancestral form as jaws (Claus). This character is apparently universal
in the embryos of true Phyllopods, and constantly occurs in the
Copepoda, etc.
The third pair of appendages or mandibles (_md_) is attached close
below the upper lip. They are as yet unprovided with cutting blades,
and terminate in two short branches, the inner with two and the outer
with three bristles.
At the front of the head there is the typical unpaired eye. On the
dorsal surface there is already present a rudiment of the cephalic
shield, continuous anteriorly with the labrum (_l_) or upper lip, the
extraordinary size of which is characteristic of the larvæ of
Phyllopods. The post-cephalic region, which afterwards becomes the
thorax and abdomen, contains underneath the skin rudiments of the five
anterior thoracic segments and their appendages, and presents in this
respect an important variation from the typical Nauplius form. After
the first ecdysis the larva (fig. 208 B) loses its oval form, mainly
owing to the elongation of the hinder part of the body and the lateral
extension of the cephalic shield, which moreover now completely covers
over the head and has begun to grow backwards so as to cover over the
thoracic region. At the second ecdysis there appears at its side a
rudimentary shell-gland. In the cephalic region two small papillæ
(_fr_) are now present at the front of the head close to the unpaired
eye. They are of the nature of sense organs, and may be called the
frontal sense papillæ. They have been shewn by Claus to be of some
phylogenetic importance. The three pairs of Nauplius appendages have
not altered much, but a rudimentary cutting blade has grown out from
the basal joint of the mandible. A gland opening at the base of the
antennæ is now present, which is probably equivalent to the green
gland often present in the Malacostraca. Behind the mandibles a pair
of simple processes has appeared, which forms the rudiment of the
first pair of maxillæ (_mx_).
In the thoracic region more segments have been added posteriorly, and
the appendages of the three anterior segments are very distinctly
formed. The tail is distinctly forked. The heart is formed at the
second ecdysis, and then extends to the sixth thoracic segment: the
posterior chambers are successively added from before backwards.
At the successive ecdyses which the larva undergoes new segments
continue to be formed at the posterior end of the body, and limbs
arise on the segments already formed. These limbs probably represent
the primitive form of an important type of Crustacean appendage, which
is of value for interpreting the parts of the various malacostracan
appendages. They consist (fig. 209) of a basal portion (protopodite of
Huxley) bearing two rami. The basal portion has two projections on the
inner side. To the outer side of the basal portion there is attached a
dorsally directed branchial sack (_br_) (epipodite of Huxley). The
outer ramus (_ex_) (exopodite of Huxley) is formed of a single plate
with marginal setæ. The inner one (_en_) (endopodite of Huxley) is
four-jointed, and a process similar to those of the basal joint is
given off from the inner side of the three proximal joints.
[FIG. 209. TYPICAL PHYLLOPOD APPENDAGE. (Copied from Claus.)
_ex._ exopodite; _en._ endopodite; _br._ branchial appendage
(epipodite). The basal portion bearing the two proximal projections
is not sharply separated from the endopodite.]
At the third ecdysis several new features appear in the cephalic
region, which becomes more prominent in the succeeding stages. In the
first place the paired eyes are formed at each side of and behind the
unpaired eye, secondly the posterior pair of maxillæ is formed though
it always remains very rudimentary. The shell-gland becomes fully
developed opening at the base of the first pair of maxillæ. The dorsal
shield gradually grows backwards till it covers its full complement of
segments.
After the fifth ecdysis the Nauplius appendages undergo a rapid
atrophy. The second pair of antennæ especially becomes reduced in
size, and the mandibular palp--the primitive Nauplius portion of the
mandible--is contracted to a mere rudiment, which eventually
completely disappears, while the blade is correspondingly enlarged and
also becomes toothed. The adult condition is only gradually attained
after a very large number of successive changes of skin.
The chief point of interest in the above development is the fact of
the primitive Nauplius form becoming gradually converted without any
special metamorphosis into the adult condition[187].
[187] Nothing appears to be known with reference to the manner in
which it comes about that more than one appendage is borne on
each of the segments from the eleventh to the twentieth. An
investigation of this point would be of some interest with
reference to the meaning of segmentation.
Branchipus like Apus is hatched as a somewhat modified Nauplius, which
however differs from that of Apus in the hinder region of the body
having no indications of segments. It goes through a very similar
metamorphosis, but is at no period of its metamorphosis provided with
a dorsal shield: the second pair of antennæ does not abort, and in the
male is provided with clasping organs, which are perhaps remnants of
the embryonic hooks so characteristic of this pair of antennæ.
The larva of Estheria when hatched has a Nauplius form, a large upper
lip, caudal fork and single eye. There are two functional pairs of
swimming appendages--the second pair of antennæ and mandibles. The
first pair of antennæ has not been detected, and a dorsal mantle to
form the shell is not developed. At the first moult the anterior pair
of antennæ arises as small stump-like structures, and a small dorsal
shield is also formed. Rudiments of six or seven pairs of appendages
sprout out in the usual way, and continue to increase in number at
successive moults: the shell is rapidly developed. The chief point of
interest in the development of this form is the close resemblance of
the young larva to a typical adult Cladocera (Claus). This is shewn in
the form of the shell, which has not reached its full anterior
extension, the rudimentary anterior antennæ, the large locomotor
second pair of antennæ, which differ however from the corresponding
organs in the Cladocera in the presence of typical larval hooks. Even
the abdomen resembles that of Daphnia. These features perhaps indicate
that the Cladocera are to be derived from some Phyllopod form like
Estheria by a process of retrogressive metamorphosis. The posterior
antennæ in the adult Estheria are large biramous appendages, and are
used for swimming; and though they have lost the embryonic hook, they
still retain to a larger extent than in other Phyllopod families their
Nauplius characteristics.
The Nauplius form of the Phyllopods is marked by several definite
peculiarities. Its body is distinctly divided into a cephalic and
post-cephalic region. The upper lip is extraordinarily large,
relatively very much more so than at the later stages. The first pair
of antennæ is usually rudimentary and sometimes even absent; while the
second pair is exceptionally large, and would seem to be capable of
functioning not only as a swimming organ, but even as a masticating
organ. A dorsal shield is nearly or quite absent.
[FIG. 209 A. NAUPLIUS LARVA OF LEPTODORA HYALINA FROM WINTER EGG.
(Copied from Bronn; after Sars.)
_an1._ antenna of first pair; _an2._ antenna of second pair; _md._
mandible; _f._ caudal fork.]
Cladocera. The probable derivation of the Cladocera from a form
similar to Estheria has already been mentioned, and it might have been
anticipated that the development would be similar to that of the
Phyllopods. The development of the majority of the Cladocera takes
place however in the egg, and the young when hatched closely resembles
their parents, though in the egg they pass through a Nauplius stage
(Dohrn). An exception to the general rule is however offered by the
case of the winter eggs of Leptodora, one of the most primitive of the
Cladoceran families. The summer eggs develop without metamorphosis,
but Sars (No. 461) has discovered that the larva leaves the winter
eggs in the form of a Nauplius (fig. 209). This Nauplius closely
resembles that of the Phyllopods. The body is elongated and in
addition to normal Nauplius appendages is marked by six pairs of
ridges--the indications of the future feet. The anterior antennæ are
as usual small; the second large and biramous, but the masticatory
bristle characteristic of the Phyllopods is not present. The mandibles
are without a cutting blade. A large upper lip and unpaired eye are
present.
The adult form is attained in the same manner as amongst the
Phyllopods after the third moult.
MALACOSTRACA.
Owing to the size and importance of the various forms included in the
Malacostraca, greater attention has been paid to their embryology than
to that of any other division of the Crustacea; and the proper
interpretation of their larval forms involves some of the most
interesting problems in the whole range of Embryology.
The majority of Malacostraca pass through a more or less complicated
metamorphosis, though in the Nebaliadæ, the Cumaceæ, some of the
Schizopoda, a few Decapoda (Astacus, Gecarcinus, etc.), and in the
Edriophthalmata, the larva on leaving the egg has nearly the form of
the adult. In contradistinction to the lower groups of Crustacea the
Nauplius form of larva is rare, though it occurs in the case of one of
the Schizopods (Euphausia, fig. 212), in some of the lower forms of
the Decapods (Penæus, fig. 214), and perhaps also, though this has not
been made out, in some of the Stomatopoda.
[FIG. 210. ZOÆA OF THIA POLITA. (After Claus.) _mxp2._ second
maxillipede.]
In the majority of the Decapoda the larva leaves the egg in a form
known as the Zoæa (fig. 210). This larval form is characterised by the
presence of a large cephalo-thoracic shield usually armed with
lateral, anterior, and dorsal spines. The caudal segments are well
developed, _though without appendages_, and the tail, which functions
in swimming, _is usually forked_. The six posterior thoracic segments
are, on the other hand, _rudimentary or non-existent_. There are seven
anterior pairs of appendages shewn in detail in fig. 211, viz. the two
pairs of antennæ (_At. I._ and _At. II._), neither of them used as
swimming organs, the mandibles without a palp (_md_), well-developed
maxillæ (two pairs, _mx 1_ and _mx 2_), and two or sometimes
(Macrura) three pairs of biramous natatory maxillipeds (_mxp 1_ and
_mxp 2_). Two lateral compound stalked eyes are present, together with
a median Nauplius eye. The heart has in the majority of cases only one
or two (Brachyura) pairs of ostia.
[FIG. 211. THE APPENDAGES OF A CRAB ZOÆA.
_At. I._ first antenna; _At. II._ second antenna; _md._ mandible
(without a palp); _mx. 1._ first maxilla; _mx. 2._ second maxilla;
_mxp. 1._ first maxilliped; _mxp. 2._ second maxilliped.
_ex_. exopodite; _en_. endopodite.]
The Zoæa larva, though typically developed in the Decapoda, is not
always present (_e.g._ Astacus and Homarus), and sometimes occurs in a
very modified form. It makes its appearance in an altered garb in the
ontogeny of some of the other groups.
The two Malacostracan forms, amongst those so far studied, in which
the phylogenetic record is most fully preserved in the ontogeny, are
Euphausia amongst the Schizopods and Penæus amongst the Decapods.
Schizopoda. Euphausia leaves the egg (Metschnikoff, No. 468-9) as a
true Nauplius with only three pairs of appendages, the two hinder
biramous, and an unsegmented body. The second pair of antennæ has not
however the colossal dimensions so common in the lower types. A mouth
is present, but the anus is undeveloped.
[FIG. 212. NAUPLIUS OF EUPHAUSIA. (From Claus; after Metschnikoff.)
The Nauplius is represented shortly before an ecdysis, and in
addition to the proper appendages rudiments of the three
following pairs are present.
_OL._ upper lip; _UL._ lower lip; _Md._ mandible; _Mx´._ and _Mx´´._
two pairs of maxillæ; _mf´._ maxilliped 1.]
After the first moult three pairs of prominences--the rudiments of the
two maxillæ and 1st maxillipeds arise behind the Nauplius appendages
(fig. 212). At the same time an anus appears between the two limbs of
a rudimentary caudal fork; and an unpaired eye and upper lip appear in
front. After another moult (fig. 212) a lower lip is formed (_UL_) as
a pair of prominences very similar to true appendages; and a delicate
cephalo-thoracic shield also becomes developed. Still later the
cutting blade of the mandible is formed, and the palp (Nauplius
appendage) is greatly reduced. The cephalo-thoracic shield grows over
the front part of the embryo, and becomes characteristically toothed
at its edge. There are also two frontal papillæ very similar to those
already described in the Phyllopod larvæ. Rudiments of the compound
eyes make their appearance, and though no new appendages are added,
those already present undergo further differentiations. They remain
however very simple; the maxillipeds especially are very short and
resemble somewhat Phyllopod appendages.
Up to this stage the tail has remained rudimentary and short, but
after a further ecdysis (Claus) it grows greatly in length. At the
same time the cephalo-thoracic shield acquires a short spine directed
backwards. The larva is now in a condition to which Claus has given
the name of Protozoæa (fig. 213 A).
Very shortly afterwards the region immediately following the segments
already formed becomes indistinctly segmented, while the tail is still
without a trace of segmentation. The region of the thorax proper soon
becomes distinctly divided into seven very short segments, while at
the same time the now elongated caudal region has become divided into
its normal number of segments (fig. 213 B). By this stage the larva
has become a true Zoæa--though differing from the normal Zoæa in the
fact that the thoracic region is segmented, and in the absence of a
second pair of maxillipeds.
[FIG. 213. LARVÆ OF EUPHAUSIA. (After Claus.) From the side.
A. Protozoæa larva. B. Zoæa larva.
_mx.´_ and _mx´´._ maxillæ 1 and 2; _mxp1._ maxilliped 1.]
The adult characters are very gradually acquired in a series of
successive moults; the later development of Euphausia resembling in
this respect that of the Phyllopods. On the other hand Euphausia
differs from that group in the fact that the abdominal (caudal) and
thoracic appendages develop as _two independent series_ from before
backwards, of which the abdominal series is the earliest to attain
maturity.
This is shewn in the following table compiled from Claus' observations.
+-------------+---------------------------+--------------------------+
| | APPENDAGES OF THORACIC | |
| LENGTH OF | REGION; VIZ. THE 2ND AND | APPENDAGES OF ABDOMEN. |
| LARVA. | 3rd maxilliped and 5 | |
| | ambulatory appendages. | |
+-------------+---------------------------+--------------------------+
| 3-3-1/2 mm. | 2nd maxilliped, | 1st abdominal appendage. |
| | rudimentary. | |
+-------------+---------------------------+--------------------------+
| 3-1/2-4 mm. | 2nd maxilliped, biramous. | 2nd and 3rd abdominal |
| | 3rd rudimentary. | appendages. |
| | 1st and 2nd ambulatory | 4th and 5th rudimentary. |
| | appendages, rudimentary. | |
+-------------+---------------------------+--------------------------+
| 4-1/2-5 mm. | 3rd maxilliped, biramous. | 4th, 5th, and 6th fully |
| | | developed. |
+-------------+---------------------------+--------------------------+
| 5-5-1/2 mm. | 3rd and 4th ambulatory | |
| | appendages. | |
+-------------+---------------------------+--------------------------+
| 6 mm. | 5th ambulatory appendage. | |
+-------------+---------------------------+--------------------------+
All the appendages following the second pair of maxillæ are biramous,
and the first eight of these bear branched gills as their epipodites.
It is remarkable that the epipodite is developed on all the appendages
anteriorly in point of time to the outer ramus (exopodite).
Although in Mysis there is no free larval stage, and the development
takes place in a maternal incubatory pouch, yet a stage may be
detected which clearly corresponds with the Nauplius stage of
Euphausia (E. van Beneden, No. 465). At this stage, in which only the
three Nauplius appendages are developed, the Mysis embryo is hatched.
An ecdysis takes place, but the Nauplius skin is not completely thrown
off, and remains as an envelope surrounding the larva during its later
development.
Decapoda. Amongst the Decapoda the larva usually leaves the egg in the
Zoæa form, but a remarkable exception to this general rule is afforded
by the case of one or more species of Penæus. Fritz Müller was the
first to shew that the larva of these forms leaves the egg as a
_typical Nauplius_, and it is probable that in the successive larval
stages of these forms the ancestral history of the Decapoda is most
fully preserved[188].
[188] The doubts which have been thrown upon Müller's
observations appear to be quite unfounded.
The youngest known larva of Penæus (fig. 214) has a somewhat oval
unsegmented body. There spring from it the three typical pairs of
Nauplius appendages. The first is uniramous, the second and third are
biramous, and both of them adapted for swimming, and the third of them
(mandibles) is without a trace of the future blade. The body has no
carapace, and bears anteriorly a single median simple eye. Posteriorly
it is produced into two bristles.
[FIG. 214. NAUPLIUS STAGE OF PENÆUS. (After Fritz Müller.)]
After the first moult the larva has a rudiment of a forked tail, while
a dorsal fold of skin indicates the commencement of the
cephalo-thoracic shield. A large provisional helmet-shaped upper lip
like that in Phyllopods has also appeared. Behind the appendages
already formed there are stump-like rudiments of the four succeeding
pairs (two pairs of maxillæ and two pairs of maxillipeds); and in a
slightly older larva the formation of the mandibular blade has
commenced, together with the atrophy of the palp or Nauplius
appendage.
[FIG. 215. PROTOZOÆA STAGE OF PENÆUS. (After Fritz Müller.)]
Between this and the next observed stage there is possibly a slight
lacuna. The next stage (fig. 215) at any rate represents the
commencement of the Zoæa series. The cephalo-thoracic shield has
greatly grown, and eventually acquires the usual dorsal spine. The
posterior region of the body is prolonged into a tail, which is quite
as long as the whole of the remainder of the body. The four appendages
which were quite functionless at the last stage have now sprouted into
full activity. The region immediately behind them is divided (fig.
215) into six segments (the six thoracic segments) without appendages,
while somewhat later the five anterior abdominal segments become
indicated, but are equally with the thoracic segments without feet.
The mode of appearance of these segments shews that the thoracic and
abdominal segments develop in regular succession from before backwards
(Claus). Of the palp of the mandibles, as is usual amongst Zoæa forms,
not a trace remains, though in the youngest Zoæa caught by Fritz
Müller a very small rudiment of the palp was present. The first pair
of antennæ is unusually long, and the second pair continues to
function as a biramous swimming organ; the outer ramus is
multiarticulate. The other appendages are fully jointed, and the two
maxillipeds biramous. On the dorsal surface of the body the unpaired
eye is still present, but on each side of it traces of the stalked
eyes have appeared. Frontal sense organs like those of Phyllopods are
also present.
From the Protozoæa form the larva passes into that of a true Zoæa with
the usual appendages and spines, characterised however by certain
remarkable peculiarities. Of these the most important are (1) the
large size of the two pairs of antennæ and the retention of its
Nauplius function by the second of them; (2) the fact that the
appendages of the six thoracic segments appear as small biramous
Schizopod legs, while the abdominal appendages, with the exception of
the sixth, are still without their swimming feet. The early appearance
of the appendages of the sixth abdominal segment is probably
correlated with their natatory function in connection with the tail.
As a point of smaller importance which may be mentioned is the fact
that both pairs of maxillæ are provided with small respiratory plates
(exopodites) for regulating the flow of water under the dorsal shield.
From the Zoæa form the larva passes into a Mysis or Schizopod stage
(fig. 216), characterised by the thoracic feet and maxillipeds
resembling in form and function the biramous feet of Mysis, the outer
ramus being at first in many cases much larger than the inner. The
gill pouches appear at the base of these feet nearly at the same time
as the endopodites become functional. At the same time the antennæ
become profoundly modified. The anterior antennæ shed their long
hairs, and from the inner side of the fourth joint there springs a new
process, which eventually elongates and becomes the inner flagellum.
The outer ramus of the posterior antennæ is reduced to a scale, while
the flagellum is developed from a stump-like rudiment of the inner
ramus (Claus). A palp sprouts on the mandible and the median eye
disappears.
[FIG. 216. PENÆUS LARVA IN THE MYSIS STAGE. (After Claus.)]
The abdominal feet do not appear till the commencement of the Mysis
stage, and hardly become functional till its close.
From the Mysis stage the larva passes quite simply into the adult
form. The outer ramus of the thoracic feet is more or less completely
lost. The maxillipeds, or the two anterior pairs at any rate, lose
their ambulatory function, cutting plates develop on the inner side of
their basal joints, and the two rami persist as small appendages on
their outer side. Gill pouches also sprout from their outer side.
The respiratory plate of the second maxilla attains its full
development and that on the first maxilla disappears[189]. The
Nauplius, so far as is known, does not occur in any other Decapod form
except Penæus.
[189] From Claus' observations (No. 448) it would appear that the
respiratory plate is only the exopodite and not, as is usually
stated, the coalesced exopodite and epipodite. Huxley in his
_Comparative Anatomy_ reserves this point for embryological
elucidation.
[FIG. 217. LATEST PROTOZOÆA STAGE OF SERGESTES LARVA (ELAPHOCARIS).
(After Claus.)
_mxp´´´._ third pair of maxillipeds.]
The next most primitive larval history known is that which appears in
the Sergestidæ. The larval history, which has been fully elucidated by
Claus, commences with a Protozoæa form (fig. 217), which develops into
a remarkable Zoæa first described by Dohrn as Elaphocaris. This
develops into a form originally described by Claus as Acanthosoma, and
this into a form known as Mastigopus (fig. 218) from which it is easy
to pass to the adult.
The remarkable Protozoæa (fig. 217) is characterised by the presence
on the dorsal shield of a frontal, dorsal and two lateral spikes, each
richly armed with long side spines. The normal Zoæa appendages are
present, and in addition to them a small third pair of maxillipeds.
The thoracic region is divided into five short rings, but the abdomen
is unsegmented. The tail is forked and provided with long spines. The
antennæ, like those of Penæus, are long--the second pair biramous; the
mandibles unpalped. Both pairs of maxillæ are provided with
respiratory plates; the second pair is footlike, and has at its base a
glandular mass believed by Claus to be the equivalent of the
Entomostracan shell-gland. The maxillipeds have the usual biramous
characters. A helmet-shaped upper lip like that of a typical Nauplius
is present, and the eyes are situated on very long stalks.
[FIG. 218. MASTIGOPUS STAGE OF SERGESTES. (From Claus.)
_Mf´´´._ maxilliped 3.]
In the true Zoæa stage there appear on the five thoracic segments
pouch-like biramous rudiments of the limbs. The tail becomes
segmented; but the segments, with the exception of the sixth, remain
without appendages. On the sixth a very long bilobed pouch appears as
the commencement of the swimming feet of this segment. The segments of
the abdomen are armed with lateral spines.
From the Zoæa stage the larva passes into the form known as
Acanthosoma, which represents the Mysis stage of Penæus. The complex
spikes on the dorsal shield of the Zoæa stage are reduced to simple
spines, but the spines of the tail still retain their full size. In
the appendages the chief changes consist (1) in the reduction of the
jointed outer ramus of the second pair of antennæ to a stump
representing the scale, and the elongation of the inner one to the
flagellum; (2) in the elongation of the five ambulatory thoracic
appendages into biramous feet, like the maxillipeds, and in the
sprouting forth of rudimentary abdominal feet.
The most obvious external indications of the passage from the
Acanthosoma to the Mastigopus stage (fig. 218) are to be found in the
elongation of the abdomen, the reduction and flattening of the
cephalo-thoracic shield, and the nearly complete obliteration of all
the spines but the anterior. The eyes on their elongated stalks are
still very characteristic, and the elongation of the flagellum of the
second pair of antennæ is very striking.
[FIG. 219. LARVA OF HIPPOLYTE IN ZOÆA STAGE. (From Claus.)
_Mx´._ and _Mx´´._ maxillæ 1 and 2; _Mf´._ _Mf´´._ _Mf´´´._
maxillipeds.]
[FIG. 220. OLDER LARVA OF HIPPOLYTE AFTER THE THORACIC APPENDAGES
HAVE BECOME FORMED. (From Claus.)]
The maxillæ and maxillipeds undergo considerable metamorphosis, the
abdominal feet attain their adult form, and the three anterior
thoracic ambulatory legs lose their outer rami. The most remarkable
change of all concerns the two last pairs of thoracic appendages,
which, instead of being metamorphosed like the preceding ones, are
completely or nearly completely thrown off in the moult which
inaugurates the Mastigopus stage, and are subsequently redeveloped.
With the reappearance of these appendages, and the changes in the
other appendages already indicated, the adult form is practically
attained.
With reference to the development of the majority of the Carabidæ,
Penæinæ, Palæmoninæ, Crangoninæ, it may be stated generally that they
leave the egg in the Zoæa stage (fig. 219) with anterior appendages up
to the third pair of maxillipeds. The thorax is unsegmented and indeed
almost unrepresented, but the abdomen is long and divided into
distinct segments. Both thoracic and abdominal appendages are absent,
and the tail is formed by a simple plate with numerous bristles, not
forked, as in the case of the Zoæa of Fritz Müller's Penæus and
Sergestes. A dorsal spine is frequently found on the second abdominal
segment. From the Zoæa form the embryo passes into a Mysis stage (fig.
220), during which the thoracic appendages gradually appear as
biramous swimming feet; they are all developed before any of the
abdominal appendages, except the last. In some cases the development
is still further abbreviated. Thus the larvæ of Crangon and
Palæmonetes (Faxon, No. 476) possess at hatching the rudiments of the
two anterior pairs of thoracic feet, and Palæmon of three pairs[190].
[190] Fritz Müller has recently (_Zoologischer Anzeiger_, No. 52)
described a still more abbreviated development of a Palæmon
living in brooks near Blumenau.
[FIG. 221. NEWLY-HATCHED LARVA OF THE AMERICAN LOBSTER. (After
Smith.)]
Amongst the other Macrura the larva generally leaves the egg as a Zoæa
similar to that of the prawns. In the case of the Thalassinidæ and
Paguridæ a Mysis stage has disappeared. The most remarkable
abbreviations of the typical development are presented on the one hand
by Homarus and Astacus, and on the other by the Loricata.
The development of Homarus has been fully worked out by S. J. Smith
(No. 491) for the American lobster (Homarus americanus). The larva
(fig. 221) leaves the egg in an advanced Mysis stage. The
cephalo-thoracic shield is fully developed, and armed with a rostrum
in front. The first pair of antennæ is unjointed but the second is
biramous, the outer ramus forming a large Mysis-like scale. The
mandibles, which are palped, the maxillæ, and the two anterior
maxillipeds differ only in minor details from the same appendages of
the adult. The third pair of maxillipeds is Mysis-like and biramous,
and the five ambulatory legs closely resemble them, the endopodite of
the first being imperfectly chelate. The abdomen is well developed but
without appendages. The second, third, fourth and fifth segments are
armed with dorsal and lateral spines.
In the next stage swimming feet have appeared on the second, third,
fourth and fifth abdominal segments, and the appendages already
present have approached their adult form. Still later, when the larva
is about half an inch in length, the approach to the adult form is
more marked, and the exopodites of the ambulatory legs though present
are relatively much reduced in size. The swimmerets of the sixth
abdominal segment are formed. In the next stage observed the larva has
entirely lost its Schizopod characters, and though still retaining its
free-swimming habits differs from the adult form only in generic
characters.
As has been already stated, no free larval stages occur in the
development of Astacus, but the young is hatched in a form in which it
differs only in unimportant details from the adult.
The peculiar larval form of the Loricata (Scyllarus, Palinurus) has
long been known under the name Phyllosoma (fig. 222 C), but its true
nature was first shewn by Couch (No. 474) [Couch did not however
recognise the identity of his larva with Phyllosoma; this was first
done by Gerstäcker] and shortly afterwards by Gerbe and Coste. These
observations were however for a long time not generally accepted, till
Dohrn (No. 477) published his valuable memoir giving an account of how
he succeeded in actually rearing Phyllosoma from the eggs of Scyllarus
and Palinurus, and shewing that some of the most remarkable features
of the metamorphosis of the Loricata occur before the larva is
hatched.
The embryo of Scyllarus in the egg first of all passes through the
usual Nauplius stage, and then after the formation of a cuticle
develops an elongated thoracico-abdominal region bent completely over
the anterior part of the body. There appear moreover a number of
appendages and the rudiments of various organs; and the embryo passes
into a form which may be described as the embryonic Phyllosoma stage.
In this stage there are present on the anterior part of the body, in
front of the ventral flexure, two pairs of antennæ, mandibles, two
pairs of maxillæ, the second commencing to be biramous, _and a small
stump representing the first pair of maxillipeds_. The part of the
body bent over consists of a small quadrate caudal plate, and an
appendage-bearing region to which are attached anteriorly three pairs
of biramous appendages--the second and third maxillipeds, and the
anterior pair of ambulatory legs--and two pairs of undivided
appendages--the second and third pairs of ambulatory legs. In a
slightly later stage the first pair of maxillæ becomes biramous, as
also does the first pair of maxillipeds in a very rudimentary fashion.
The second and third pairs of ambulatory legs become biramous, while
the second and third maxilliped nearly completely lose their outer
ramus. Very small rudiments of the two hinder ambulatory legs become
formed. If the embryo is taken at this stage (_vide_ fig. 222 A, which
represents a nearly similar larva of Palinurus) out of the egg, it is
seen to consist of (1) an anterior enlargement with a vaulted dorsal
shield enclosing the yolk, two stalked eyes, and a median eye; (2) a
thoracic region in which the indications of segmentation are visible
with the two posterior pairs of maxillipeds (_mxp2_ and _mxp3_) and
the ambulatory legs (_p1_); (3) an abdominal region distinctly divided
into segments and ending in a fork.
[FIG. 222. LARVÆ OF THE LORICATA. (After Claus.)
A. Embryo of Palinurus shortly before hatching.
B. Young Phyllosoma larva of Scyllarus, without the first
maxilliped, the two last thoracic appendages, or the abdominal
appendages.
C. Fully-grown Phyllosoma with all the Decapod appendages.
_at1._ antenna of first pair; _at2._ antenna of second
pair; _md._ mandible; _mx1._ first maxilla; _mx2._
second maxilla; _mxp1-mxp3._ maxillipeds; _p1-p3._
thoracic appendages.]
Before the embryo becomes hatched _the first pair of maxillipeds
becomes reduced in size and finally vanishes_. The second pair of
maxillæ becomes reduced to simple stumps with a few bristles, the
second pair of antennæ also appears to undergo a retrogressive change,
while the two last thoracic segments cease to be distinguishable. It
thus appears that during embryonic life the second pair of antennæ,
the second pair of maxillæ, and the second and third pair of
maxillipeds and the two hinder ambulatory appendages undergo
retrogressive changes, while the first pair of maxillipeds is
completely obliterated!
The general form of the larva when hatched (fig. 222 B) is not very
different from that which it has during the later stages within the
egg. The body is divided into three regions: (1) an anterior cephalic,
(2) a middle thoracic, and (3) a small posterior abdominal portion;
and all of them are characterised by their extreme dorso-ventral
compression, so that the whole animal has the form of a three-lobed
disc, the strange appearance of which is much increased by its
glass-like transparency.
The cephalic portion is oval and projects slightly behind so as to
overlap the thorax. Its upper surface constitutes the dorsal shield,
from which there spring anteriorly the two compound eyes on long
stalks, between which is a median Nauplius eye. The mouth is situated
about the middle of the under surface of the anterior disc. It leads
into a stomach from which an anterior and a lateral hepatic
diverticulum springs out on each side. The former remains as a simple
diverticulum through larval life, but the latter becomes an extremely
complicated glandular structure.
At the front border of the disc is placed the unjointed but elongated
first pair of antennæ (_at1_). Externally to and behind these there
spring the short posterior antennæ (_at2_), at the base of which the
green gland is already formed. Surrounding the mouth are the mandibles
(_md_) and anterior pair of maxillæ (_mx1_), and some distance behind
the second pair of maxillæ (_mx2_), consisting of a cylindrical basal
joint and short terminal joint armed with bristles. The first pair of
maxillipeds is absent.
The thoracic region is formed of an oval segmented disc attached to
the under surface of the cephalic disc. From its front segment arises
the second pair of maxillipeds (_mxp2_) as single five-jointed
appendages, and from the next segment springs the five-jointed
elongated but uniramous third pair of maxillipeds (_mxp3_), and behind
this there arise three pairs of six-jointed ambulatory appendages
(_p1_, _p2_, _p3_, of which only the basal joint is represented in the
figure) with an exopodite springing from their second joint. The two
posterior thoracic rings and their appendages cannot be made out.
The abdomen is reduced to a short imperfectly segmented stump, ending
in a fork, between the prongs of which the anus opens. Even the
youngest larval Phyllosoma, such as has just been described, cannot be
compared with a Zoæa, but belongs rather, in the possession of
biramous thoracic feet, to a Mysis stage. In the forked tail and
Nauplius eye there appear however to be certain very primitive
characters carried on to this stage.
The passage of this young larva to the fully formed Phyllosoma (fig.
222 C) is very simple. It consists essentially in the fresh
development of the first pair of maxillipeds and the two last
ambulatory appendages, the growth and segmentation of the abdomen, and
the sprouting on it of biramous swimming feet. In the course of these
changes the larva becomes a true Decapod in the arrangement and number
of its appendages; and indeed it was united with this group before its
larval character was made out. In addition to the appearance of new
appendages certain changes take place in those already present. The
two posterior maxillipeds, in the Palinurus Phyllosoma at any rate,
acquire again an exopodite, and together with the biramous ambulatory
feet develop epipodites in the form of gill pouches.
The mode of passage of the Phyllosoma to the adult is not known, but
it can easily be seen from the oldest Phyllosoma forms that the dorsal
cephalic plate grows over the thorax, and gives rise to the
cephalo-thoracic shield of the adult.
There are slight structural differences, especially in the antennæ,
between the Phyllosoma of Scyllarus and that of Palinurus, but the
chief difference in development is that the first pair of maxillipeds
of the Palinurus embryo, though reduced in the embryonic state, does
not completely vanish, at any rate till after the free larval state
has commenced; and it is doubtful if it does so even then. The freshly
hatched Palinurus Phyllosoma is very considerably more developed than
that of Scyllarus.
[FIG. 223. THE APPENDAGES OF A CRAB ZOÆA.
_At. I._ first antenna; _At. II._ second antenna; _md._ mandible
(without a palp); _mx. 1._ first maxilla; _mx. 2._ second
maxilla; _mx. 3._ third maxilla; _mxp. 1._ first maxilliped;
_mxp. 2._ second maxilliped.
_ex._ exopodite; _en._ endopodite.]
_Brachyura._ All the Brachyura, with the exception of one or more
species of land crabs[191], leave the egg in the Zoæa condition, and
though there are slight variations of structure, yet on the whole the
Crab Zoæa is a very well marked form. Immediately after leaving the
egg (fig. 210) it has a somewhat oval shape with a long distinctly
segmented abdomen bent underneath the thorax. The cephalo-thoracic
shield covers over the front part of the body, and is prolonged into a
long frontal spine pointing forwards, and springing from the region
between the two eyes; a long dorsal spine pointing backwards; and two
lateral spines.
[191] It has been clearly demonstrated that the majority of land
crabs leave the egg in the Zoæa form.
To the under surface of the body are attached the anterior appendages
up to the second maxilliped, while the six following pairs of thoracic
appendages are either absent or represented only in a very rudimentary
form. The abdomen is without appendages.
[FIG. 224. CRAB ZOÆA AFTER THE THIRD PAIR OF MAXILLIPEDS AND THE
THORACIC AND ABDOMINAL APPENDAGES HAVE BECOME DEVELOPED.
_at1._ antenna of first pair; _at2._ antenna of second pair; _mx1._
first maxilla; _mx2._ second maxilla; _mxp1._ first maxilliped;
_mxp2._ second maxilliped; _mxp3._ third maxilliped; _oc._ eye;
_ht._ heart.]
The anterior antennæ are single and unjointed, but provided at their
extremity with a few olfactory hairs (only two in Carcinus Moenas) and
one or two bristles. The rudiment of the secondary flagellum appears
in very young Zoææ on the inner side of the antennules (fig. 223 _At.
I._). The posterior antennæ are without the flagellum, but are
provided with a scale representing the exopodite (fig. 223 _At. II.
ex_) and usually a spinous process. The flagellum is very early
developed and is represented in fig. 223, _At. II. en_. The mandibles
(_md_) are large but without a palp. The anterior maxillæ (_mx 1_)
have a short two-jointed endopodite (palp) with a few hairs, and a
basal portion with two blades, of which the distal is the largest,
both armed with stiff bristles. The posterior maxillæ have a small
respiratory plate (exopodite), an endopodite (palp) shaped like a
double blade, and two basal joints each continued into a double blade.
The two maxillipeds (_mxp 1_ and _mxp 2_) have the form and function
of biramous swimming feet. The exopodite of both is two-jointed and
bears long bristles at its extremity; the endopodite of the anterior
is five-jointed and long, that of the second is three-jointed and
comparatively short.
In the six-jointed tail the second segment has usually two dorsally
directed spines, and the three succeeding segments each of them two
posteriorly directed. The telson or swimming plate is not at first
separated from the sixth segment; on each side it is prolonged into
two well-marked prongs; and to each prong three bristles are usually
attached (fig. 224). The heart (fig. 224 _ht_) lies under the dorsal
spine and is prolonged into an anterior, posterior, and dorsal aorta.
It has only two pairs of venous ostia.
During the Zoæa stage the larva rapidly grows in size, and undergoes
considerable changes in its appendages which reach the full Decapod
number (fig. 224). On both pairs of antennæ a flagellum becomes
developed and grows considerably in length. Before the close of the
Zoæa condition a small and unjointed palp appears on the mandible.
Behind the second maxilliped the third maxilliped (_mxp3_) early
appears as a small biramous appendage, and the five ambulatory feet
become distinctly formed as uniramous appendages--the exopodites not
being present. The third pair of maxillipeds and three following
ambulatory appendages develop gill pouches. The abdominal feet are
formed on the second to the sixth segments of the tail as simple
pouches.
The oldest Zoæa is transmuted at its moult into a form known as
Megalopa, which is really almost identical with an anomurous Decapod.
No Schizopod stage is intercalated, which shews that the development
is in many respects greatly abbreviated. The essential characters of
the Megalopa are to be found in (1) the reduction of the two anterior
maxillipeds, which cease to function as swimming feet, and together
with the appendages in front of them assume the adult form; (2) the
full functional development of the five ambulatory appendages; (3) the
reduction of the forked telson to an oval swimming plate, and the
growth in size of the abdominal feet, which become large swimming
plates and are at the same time provided with short endopodites which
serve to lock the feet of the two sides.
With these essential characters the form of the Megalopa differs
considerably in different cases. In some instances (_e.g._ Carcinus
moenas) the Zoæa spines of the youngest Megalopa are so large that the
larva appears almost more like a Zoæa than a Megalopa (Spence Bate,
No. 470). In other cases, _e.g._ that represented on fig. 225, the
Zoæa spines are still present but much reduced; and the
cephalo-thoracic shield has very much the adult form. In other cases
again (_e.g._ Portunus) the Zoæa spines are completely thrown off at
the youngest Megalopa stage.
There is a gradual passage from the youngest Megalopa to the adult
form by a series of moults.
[FIG. 225. MEGALOPA STAGE OF CRAB LARVA.]
Some of the brachyurous Zoæa forms exhibit considerable divergences
from the described type, more especially in the armature of the
shield. In some forms the spines are altogether absent, _e.g._ Maja
(Couch, No. 474) and Eurynome. In other forms the frontal spine may be
much reduced or absent (Inachus and Achæus). The dorsal spine may also
be absent, and in one form described by Dohrn (No. 478) there is a
long frontal spine and two pairs of lateral spines, but no dorsal
spine. Both dorsal and frontal spines may attain enormous dimensions
and be swollen at their extremities (Dohrn). A form has been described
by Claus as Pterocaris in which the cephalo-thoracic shield is
laterally expanded into two wing-like processes.
The Zoæa of Porcellana presents on the whole the most remarkable
peculiarities and, as might be anticipated from the systematic
position of the adult, is in some respects intermediate between the
macrurous Zoæa and that of the Brachyura. It is characterized by the
oval form of the body, and by the presence of one enormously long
frontal spine and two posterior spines. The usual dorsal spine is
absent. The tail plate is rounded and has the character of the tail of
a macrurous Zoæa, but in the young Zoæa the third pair of maxillipeds
is absent and the appendages generally have a brachyurous character. A
Megalopa stage is hardly represented, since the adult may almost be
regarded as a permanent Megalopa.
Stomatopoda. The history of the larval forms of the Stomatopoda
(Squilla etc.) has not unfortunately been thoroughly worked out, but
what is known from the researches of Fritz Müller (No. 495) and Claus
(No. 494) is of very great importance. There are it appears two types,
both of which used to be described as adult forms under the respective
names Erichthus and Alima.
[FIG. 226. SECOND STAGE OF ERICHTHUS LARVA OF SQUILLA WITH FIVE
MAXILLIPEDS AND THE FIRST PAIR OF ABDOMINAL APPENDAGES. (From
Claus.)]
The youngest known Erichthus form is about two millimetres in length,
and has the characters of a modified Zoæa (fig. 226). The body is
divided into three regions, an anterior unsegmented region to which
are attached two pairs of antennæ, mandibles, and maxillæ (two pairs).
This portion has a dorsal shield covering the next or middle region,
which consists of five segments each with a pair of biramous
appendages. These appendages represent the five maxillipeds of the
adult[192]. The portion of the body behind this is without appendages.
It consists of three short anterior segments,--the three posterior
thoracic segments of the adult,--and a long unsegmented tail. The
three footless thoracic segments are covered by the dorsal shield.
Both pairs of antennæ are uniramous and comparatively short. The
mandibles, like those of Phyllopods, are without palps, and the two
following pairs of maxillæ are small. The five maxillipeds have the
characters of normal biramous Zoæa feet. From the front of the head
spring a pair of compound eyes with short stalks, which grow longer in
the succeeding stages; between them is a median eye. The dorsal shield
is attached just behind this eye, and is provided, as in the typical
Zoæa, with a frontal spike--while its hinder border is produced into
two lateral spikes and one median. In a larva of about three
millimetres a pair of biramous appendages arises behind the three
footless thoracic segments. It is the anterior pair of abdominal feet
(fig. 226). The inner ramus of the second pair of maxillipeds soon
grows greatly in length, indicating its subsequent larger size and
prehensile form (fig. 227 _g_). When the larva after one or two moults
attains a length of six millimetres (fig. 227) the abdomen has six
segments (the sixth hardly differentiated), each with a pair of
appendages (the two hindermost still rudimentary) which have become
gradually developed from before backwards. The three hindermost
thoracic segments are still without appendages.
[192] These five maxillipeds correspond with the three
maxillipeds and two anterior ambulatory appendages of the
Decapoda.
[FIG. 227. ADVANCED ERICHTHUS LARVA OF SQUILLA WITH FIVE PAIRS OF
ABDOMINAL APPENDAGES. (From Claus.)
_f._ first maxilliped; _g._ second maxilliped.]
Some changes of importance have occurred in the other parts. Both
antennæ have acquired a second flagellum, but the mandible is still
without a palp. The first and second pair of maxillipeds have both
undergone important modifications. Their outer ramus (exopodite) has
been thrown off, and a gill plate (epipodite) has appeared as an
outgrowth from their basal joint. Each of them is composed of six
joints. The three following biramous appendages have retained their
earlier characters but have become much reduced in size. In the
subsequent moults the most remarkable new features concern the three
posterior maxillipeds, _which undergo atrophy, and are either
completely lost or reduced to mere unjointed sacks_ (fig. 228). In the
stage where the complete Erichthus type has been reached, these three
appendages have again sprouted forth in their permanent form and each
of them is provided with a gill sack on its coxal joint. Behind them
the three ambulatory appendages of the thorax have also appeared,
first as simple buds, which subsequently however become biramous. On
their development the full number of adult appendages is acquired.
[FIG. 228. ADVANCED ERICHTHUS LARVA OF SQUILLA WHEN THE THREE
POSTERIOR MAXILLIPEDS HAVE BECOME REDUCED TO MINUTE POUCHES.
(From Claus.)]
The most noteworthy points in the developmental history detailed above
are the following:
(1) The thoracic and abdominal segments (apart from their appendages)
develop successively from before backwards. (2) The three last
maxillipeds develop before the abdominal feet, as biramous appendages,
but subsequently completely atrophy, and then sprout out again in
their permanent form.
(3) The abdominal feet develop in succession from before backwards,
and the whole series of them is fully formed before a trace of the
appendages of the three hindermost thoracic segments has appeared. It
may be mentioned as a point of some importance that the Zoæa of
Squilla has an elongated many-chambered heart, and not the short
compact heart usually found in the Zoæa.
The younger stages of the Alima larva are not known[193], but the
earliest stage observed is remarkable for presenting no trace of the
three posterior pairs of maxillipeds, or of the three following pairs
of thoracic appendages. The segments belonging to these appendages are
however well developed. The tail has its full complement of segments
with the normal number of well-developed swimming feet. The larva
represents in fact the stage of the Erichthus larva when the three
posterior pairs of maxillipeds have undergone atrophy; but it is
probable that these appendages never become developed in this form of
larva.
[193] The observations of Brooks (No. 493) render it probable
that the Alima larva leaves the egg in a form not very dissimilar
to the youngest known larva.
Apart from the above peculiarities the Alima form of larva closely
resembles the Erichthus form.
Nebaliadæ. The development of Nebalia is abbreviated, but from
Metschnikoff's figures[194] may be seen to resemble closely that of
Mysis. The abdomen has comparatively little yolk, and is bent over the
ventral surface of the thorax. There is in the egg a Nauplius stage
with three appendages, and subsequently a stage with the Zoæa
appendages.
[194] His paper is unfortunately in Russian.
The larva when it leaves the egg has the majority of its appendages
formed, but is still enveloped in a larval skin, and like Mysis bends
its abdomen towards the dorsal side. When the larva is finally hatched
it does not differ greatly from the adult.
Cumaceæ. The development of the Cumaceæ takes place for the most part
within the egg, and has been shewn by Dohrn (No. 496) to resemble in
many points that of the Isopods. A dorsal organ is present, and a fold
is formed immediately behind this which gives to the embryo a dorsal
flexure. Both of these features are eminently characteristic of the
Isopoda.
The formation of the two pairs of antennæ, mandibles, and two pairs of
maxillæ and the following seven pairs of appendages takes place very
early. The pair of appendages behind the second maxillæ assumes an
ambulatory form, and exhibits a Schizopod character very early,
differing in both these respects from the homologous appendages in the
Isopoda. The cephalo-thoracic shield commences to be formed when the
appendages are still quite rudimentary as a pair of folds in the
maxillary region. The eyes are formed slightly later on each side of
the head, and only coalesce at a subsequent period to form the
peculiar median sessile eye of the adult.
The two pairs of appendages behind the second maxillæ become converted
into maxillipeds, and the exopodite of the first of them becomes the
main ramus, while in the externally similar second maxilliped the
exopodite atrophies and the endopodite alone remains.
The larva is hatched without the last pair of thoracic limbs or the
abdominal appendages (which are never developed in the female), but in
other respects closely resembles the adult. Before hatching the dorsal
flexure is exchanged for a ventral one, and the larva acquires a
character more like that of a Decapod.
COPEPODA.
Natantia. The free Copepoda are undoubtedly amongst the lowest forms
of those Crustacea which are free or do not lead a parasitic
existence. Although some features of their anatomy, such for instance
as the frequent absence of a heart, may be put down to a retrogressive
development, yet, from their retention of the median frontal eye of
the Nauplius as the sole organ of vision[195], their simple biramous
swimming legs, and other characters, they may claim to be very
primitive forms, which have diverged to no great extent from the main
line of Crustacean development. They supply a long series of
transitional steps from the Nauplius stage to the adult condition.
[195] The Pontellidæ form an exception to this statement, in that
they are provided with paired lateral eyes in addition to the
median one.
While still within the egg-shell the embryo is divided by two
transverse constrictions into three segments, on which the three
Nauplius appendages are developed, viz. the two pairs of antennæ and
the mandibles. When the embryo is hatched the indication of a division
into segments has vanished, but the larva is in the fullest sense a
typical Nauplius[196]. There are slight variations in the shape of the
Nauplius in different genera, but its general form and character are
very constant. It has (fig. 229 A) an oval unsegmented body with three
pairs of appendages springing from the ventral surface. The anterior
of these (_at 1_) is uniramous, and usually formed of three joints
which bear bristles on their under surface. The two posterior pairs of
appendages are both biramous. The second pair of antennæ (_at 2_) is
the largest. Its basal portion (protopodite) bears on its inner side a
powerful hook-like bristle. The outer ramus is the longest and
many-jointed; the inner ramus has only two joints. The mandibles
(_md_), though smaller than the second pair of antennæ, have a nearly
identical structure. No blade-like projection is as yet developed on
their protopodite. Between the points of insertion of the first pair
of antennæ is the median eye (_oc_), which originates by the
coalescence of two distinct parts. The mouth is ventral, and placed in
the middle line between the second pair of antennæ and the mandibles:
it is provided with an unpaired upper lip. There are two bristles at
the hind end of the embryo between which the anus is placed; and in
some cases there is at this part a slight indication of the future
caudal fork.
[196] The term Nauplius was applied to the larva of Cyclops and
allied organisms by O. F. Müller under the impression that they
were adult forms.
[FIG. 229. SUCCESSIVE STAGES IN THE DEVELOPMENT OF CYCLOPS
TENUICORNIS. (Copied from Bronn; after Claus.)
A. B. and C. Nauplius stages. D. Youngest Copepod stage. In this
figure maxillæ and the two rami of the maxilliped are seen
immediately behind the mandible _md._
_oc._ eye; _at1._ first pair of antennæ; _at2._ second pair of
antennæ; _md._ mandible; _p1._ first pair of feet; _p2._ second
pair of feet; _p3._ third pair of feet; _u._ excretory
concretions in the intestine.]
The larva undergoes a number of successive ecdyses, at each of which
the body becomes more elongated, and certain other changes take place.
First of all a pair of appendages arises behind the mandibles, which
form the maxillæ (fig. 229 B); at the same time the basal joint of the
maxillæ develops a cutting blade. Three successive pairs of appendages
(fig. 229 C) next become formed--the so-called maxillipeds (the
homologues of the second pair of maxillæ), and the two first thoracic
limbs. Each of these though very rudimentary is nevertheless bifid.
The body becomes greatly elongated, and the caudal fork more
developed.
Up to this stage of development the Nauplius appendages have retained
their primitive character almost unaltered; but after a few more
ecdyses a sudden change takes place; a cephalo-thoracic shield becomes
fully developed, and the larva comes to resemble in character an adult
Copepod, from which it mainly differs in the smaller number of
segments and appendages. In the earliest 'Cyclops' stage the same
number of appendages are present as in the last Nauplius stage. There
(fig. 229 D) is a well-developed cephalo-thorax, and four free
segments behind it. To the cephalo-thoracic region the antennæ,
mandibles, maxillæ, the now double pair of maxillipeds (derived from
the original single pair of appendages), and first pair of thoracic
appendages (_p1_) are attached. The second pair of thoracic appendages
(_p2_) is fixed to the first free segment, and the rudiment of a third
pair (_p3_) projects from the second free segment. The first pair of
antennæ has grown longer by the addition of new joints, and continues
to increase in length in the following ecdyses till it attains its
full adult development, and then forms the chief organ of locomotion.
The second pair of antennæ is much reduced and has lost one of its
rami. The two rami of the mandibles are reduced to a simple palp,
while the blade has assumed its full importance. The maxillæ and
following appendages have greatly increased in size. They are all
biramous, though the two rami are not as yet jointed. The adult state
is gradually attained after a number of successive ecdyses, at which
new segments and appendages are added, while new joints are formed for
those already present.
Parasita. The earliest developmental stages of the parasitic types of
Copepoda closely resemble those of the free forms, but, as might be
expected from the peculiarly modified forms of the adult, they present
a large number of secondary characters. So far as is known a more or
less modified Nauplius larva is usually preserved.
[FIG. 230. SUCCESSIVE STAGES IN THE DEVELOPMENT OF ACHTHERES
PERCARUM. (Copied from Bronn; after Claus.)
A. Modified Nauplius stage. B. Cyclops stage. C. Late stage of male
embryo. D. Sexually mature female. E. Sexually mature male.
_at1._ first pair of antennæ; _at2._ second pair of antennæ; _md._
mandible; _mx._ maxillæ; _pm1._ outer pair of maxillipeds; _pm2._
inner pair of maxillipeds; _p1._ first pair of legs; _p2._ second
pair of legs; _z._ frontal organ; _i._ intestine; _o._ larval
eye; _b._ glandular body; _t._ organ of touch; _ov._ ovary; _f._
rod projecting from coalesced maxillipeds; _g._ cement gland;
_rs._ receptaculum seminis; _n._ nervous system; _te._ testis;
_v._ vas deferens.]
The development of Achtheres percarum, one of the Lernæopoda parasitic
in the mouth, etc. of the common Perch, may be selected to illustrate
the mode of development of these forms. The larva leaves the egg as a
much simplified Nauplius (fig. 230 A). It has an oval body with only
the two anterior pairs of Nauplius appendages; both of them in the
rudimentary condition of unjointed rods. The usual median eye is
present, and there is also found a peculiar sternal papilla, on which
opens a spiral canal filled with a glutinous material, which is
probably derived from a gland which disappears on the completion of
the duct. The probable function of this organ is to assist at a later
period in the attachment of the parasite to its host. Underneath the
Nauplius skin a number of appendages are visible, which become
functional after the first ecdysis. This takes place within a few
hours after the hatching of the Nauplius, and the larva then passes
from this rudimentary Nauplius stage into a stage corresponding with
the Cyclops stage of the free forms (fig. 230 B). In the Cyclops stage
the larva has an elongated body with a large cephalo-thoracic shield,
and four free posterior segments, the last of which bears a forked
tail.
There are now present eight pairs of appendages, viz. antennæ (two
pairs), mandibles, maxillæ, maxillipeds, and three pairs of swimming
feet. The Nauplius appendages are greatly modified. The first pair of
antennæ is three-jointed, and the second biramous. The outer ramus is
the longest, and bears a claw-like bristle at its extremity. This pair
of appendages is used by the larva for fixing itself. The mandibles
are small and connected with the proboscidiform mouth; and the single
pair of maxillæ is small and palped. The maxillipeds (_pm1_ and _pm2_)
are believed by Claus to be primitively a single biramous appendage,
but early appear as two distinct structures[197], the outer and larger
of which becomes the main organ by which the larva is fixed. Both are
at this stage simple two-jointed appendages. The two anterior pairs of
swimming feet have the typical structure, and consist of a protopodite
bearing an unjointed exopodite and endopodite. The first pair is
attached to the cephalo-thorax and the second (_p2_) to the first free
thoracic segment. The third pair is very small and attached to the
second free segment. The mouth is situated at the end of a kind of
proboscis formed by prolongations of the upper and lower lips. The
alimentary tract is fairly simple, and the anus opens between the
caudal forks.
[197] Van Beneden (No. 506) in the genera investigated by him
finds that the two maxillipeds are really distinct pairs of
appendages.
Between this and the next known stage it is quite possible that one or
more may intervene. However this may be the larva in the next stage
observed (fig. 230 C) has already become parasitic in the mouth of the
Perch, and has acquired an elongated vermiform aspect. The body is
divided into two sections, an anterior unsegmented, and a posterior
formed of five segments, of which the foremost is the first thoracic
segment which in the earlier stage was fused with the cephalo-thorax.
The tail bears a rudimentary fork between the prongs of which the anus
opens. The swimming feet have disappeared, so also has the eye and the
spiral duct of the embryonic frontal organ. The outer of the two
divisions of the maxilliped have undergone the most important
modification, in that they have become united at their ends, where
they form an organ from which an elongated rod (_f_) projects, and
attaches the larva to the mouth or gills of its host. The antennæ and
jaws have nearly acquired their adult form. The nervous system
consists of supra- and infra-oesophageal ganglia and two lateral
trunks given off from the latter. At this stage the males and females
can already be distinguished, not only by certain differences in the
rudimentary generative organs, but also by the fact that the outer
branch of the maxillipeds is much longer in the female than in the
male, and projects beyond the head.
In the next ecdysis the adult condition is reached. The outer
maxillipeds of the male (fig. 230 E, _pm2_) separate again; while in
the female (fig. 230 D) they remain fused and develop a sucker. The
male is only about one-fifth the length of the female. In both sexes
the abdomen is much reduced.
In the genera Anchorella, Lernæopoda, Brachiella and Hessia, _Ed. van
Beneden_ (No. 506) has shewn that the embryo, although it passes
through a crypto-Nauplius stage in the egg, is when hatched already in
the Cyclops stage.
Branchiura. The peculiar parasite Argulus, the affinities of which
with the Copepoda have been demonstrated by Claus (No. 511), is
hatched in a Cyclops stage, and has no Nauplius stage. At the time of
hatching it closely resembles the adult in general form. Its
appendages are however very nearly those of a typical larval Copepod.
The body is composed of a cephalo-thorax and free region behind this.
The cephalo-thorax bears on its under surface antennæ (two pairs),
mandibles, maxillipeds, and the first pair of thoracic feet.
The first pair of antennæ is three-jointed, but the basal joint bears
a hook. The second pair is biramous, the inner ramus terminating in a
hook. The mandible is palped, but the palp is completely separated
from the cutting blade[198]. The maxilla would, according to Claus,
appear to be absent.
[198] It seems not impossible that the appendage regarded by
Claus as the mandibular palp may really represent the maxilla,
which would otherwise seem to be absent. This mode of
interpretation would bring the appendages of Argulus into a much
closer agreement with those of the parasitic Copepoda. It does
not seem incompatible with the existence of the stylet-like
maxillæ detected by Claus in the adult.
The two typical divisions of the Copepod maxillipeds are present, viz.
an outer and anterior larger division, and an inner and posterior
smaller one. The first pair of thoracic feet, as is usual amongst
Copepoda, is attached to the cephalo-thorax. It has not the typical
biramous Copepod character. There are four free segments behind the
cephalo-thorax, the last of which ends in a fork. Three of them bear
appendages, which are rudimentary in this early larval stage. On the
dorsal surface are present paired eyes as well as an unpaired median
eye.
Between the larval condition and that of the adult a number of ecdyses
intervene.
CIRRIPEDIA.
The larvæ of all the Cirripedia, with one or two exceptions, leave the
egg in the Nauplius condition. The Nauplii differ somewhat in the
separate groups, and the post-nauplial stages vary not inconsiderably.
It will be most convenient to treat successively the larval history of
the four sub-orders, viz. Thoracica, Abdominalia, Apoda, and
Rhizocephala.
Thoracica. The just hatched larvæ at once leave the egg lamellæ of
their parent. They pass out through an opening in the mantle near the
mouth, and during this passage the shell of the parent is opened and
the movements of the cirriform feet cease.
The larval stages commence with a Nauplius[199] which, though regarded
by Claus as closely resembling the Copepod Nauplius (figs. 231 and 232
A), certainly has very marked peculiarities of its own, and in some
respects approaches the Phyllopod Nauplius. It is in the youngest
stage somewhat triangular in form, and covered on the dorsal side by a
very delicate and hardly perceptible dorsal shield, which is prolonged
laterally into two very peculiar conical horns (fig. 231 _lh_), which
are the most characteristic structures of the Cirriped Nauplius. They
are connected with a glandular mass, the secretion from which passes
out at their apex. Anteriorly the dorsal shield has the same extension
as the body, but posteriorly it projects slightly.
[199] Alepas squalicola is stated by Koren and Danielssen to form
an exception to this rule, and to leave the egg with six pairs of
appendages.
An unpaired eye is situated on the ventral surface of the head, and
immediately behind it there springs a more or less considerable upper
lip (_lb_), which resembles the Phyllopod labrum rather than that of
the Copepoda. Both mouth and anus are present, and the hind end of the
body is slightly forked in some forms, but ends in others, _e.g._
Lepas fascicularis, in an elongated spine. The anterior of the three
pairs of Nauplius appendages (_At1_) is uniramous, and the two
posterior (_At2_ and _md_) are biramous. From the protopodites of both
the latter spring strong hooks like those of the Copepod and Phyllopod
Nauplii. In some Nauplii, _e.g._ that of Balanus, the appendages are
at first not jointed, but in other Nauplii, _e.g._ that of Lepas
fascicularis, the jointing is well marked. In Lepas fascicularis the
earliest free Nauplius is enveloped in a larval skin, which is thrown
off after a few hours. The Nauplii of all the Thoracica undergo a
considerable number of moults before their appendages increase in
number or segmentation of the body appears. During these moults they
grow larger, and the posterior part of the body--the future thoracic
and abdominal region--grows relatively in length. There also appear
close to the sides of the unpaired eye two conical bodies, which
correspond with the frontal sense organs of the Phyllopods. During
their growth the different larvæ undergo changes varying greatly in
degree.
In Balanus the changes consist for the most part in the full
segmentation of the appendages and the growth and distinctness of the
dorsal shield, which forms a somewhat blunt triangular plate, broadest
in front, with the anterior horns very long, and two short posterior
spines. The tail also becomes produced into a long spine.
[FIG. 231. NAUPLIUS LARVA OF LEPAS FASCICULARIS VIEWED FROM THE
SIDE.
_oc._ eye; _At. 1._ antenna of first pair; _At. 2._ antenna of
second pair; _md._ mandible; _lb._ labrum; _an._ anus; _me._
mesenteron; _d.sp._ dorsal spine; _c.sp._ caudal spine; _Vp._
ventral spine; _lh._ lateral horns.]
In Lepas fascicularis the changes in appearance of the Nauplius, owing
to a great spinous development on its shield, are very considerable;
and, together with its enormous size, render it a very remarkable
form. Dohrn (No. 520), who was the first to describe it, named it
Archizoæa gigas.
The dorsal shield of the Nauplius of Lepas fascicularis (fig. 231)
becomes somewhat hexagonal, and there springs from the middle of the
dorsal surface an enormously long spine (_d.sp_), like the dorsal
spine of a Zoæa. The hind end of the shield is also produced into a
long caudal spine (_c.sp_) between which and the dorsal spine are some
feather-like processes. From its edge there spring in addition to the
primitive frontal horns three main pairs of horns, one pair anterior,
one lateral, and one posterior, and smaller ones in addition. All
these processes (with the exception of the dorsal and posterior
spines) are hollow and open at their extremities, and like the
primitive frontal horns contain the ducts of glands situated under the
shield. On the under surface of the larva is situated the unpaired eye
(_oc_) on each side of which spring the two-jointed frontal sense
organs. Immediately behind these is the enormous upper lip (_lb_)
which covers the mouth[200]. At the sides of the lip lie the three
pairs of Nauplius appendages, which are very characteristic but
present no special peculiarities. Posteriorly the body is produced
into a long ventral spine-like process (_Vp_) homologous with that of
other more normal Nauplii. At the base of this process large moveable
paired spines appear at successive moults, six pairs being eventually
formed. These spines give to the region in which they are situated a
segmented appearance, and perhaps similar structures have given rise
to the appearance of segmentation in Spence Bate's figures. The anus
is situated on the dorsal side of this ventral process, and between it
and the caudal spine of the shield above. The fact that the anus
occupies this position appears to indicate that the ventral process is
homologous with the caudal fork of the Copepoda, on the dorsal side of
which the anus so often opens[201].
[200] Willemoes Suhm (No. 530) states that the mouth is situated
at the free end of the upper lip, and that the oesophagus passes
through it. From an examination of some specimens of this
Nauplius, for which I am indebted to Moseley, I am inclined to
think that this is a mistake, and that a groove on the surface of
the upper lip has been taken by Suhm for the oesophagus.
[201] The enormous spinous development of the larva of Lepas
fascicularis is probably to be explained as a secondary
protective adaptation, and has no genetic connection with the
somewhat similar spinous armature of the Zoæa.
From the Nauplius condition the larvæ pass at a single moult into an
entirely different condition known as the Cypris stage. In preparation
for this condition there appear, during the last Nauplius moults, the
rudiments of several fresh organs, which are more or less developed in
different types. In the first place a compound eye is formed on each
side of the median eye. Secondly there appears behind the mandibles a
fourth pair of appendages--the first pair of maxillæ--and internal to
these a pair of small prominences, which are perhaps equivalent to the
second pair of maxillæ, and give rise to the third pair of jaws in the
adult (sometimes spoken of as the lower lip).
Behind these appendages there are moreover formed the rudiments of six
pairs of feet. Under the cuticle of the first pair of antennæ there
may be seen just before the final moult the four-jointed antennæ of
the Cypris stage with the rudiment of a disc on the second joint by
which the larvæ eventually become attached.
By the free Cypris stage, into which the larva next passes, a very
complete metamorphosis has been effected. The median and paired eyes
are present as before, but the dorsal shield has become a bivalve
shell, the two valves of which are united along their dorsal,
anterior, and posterior margins. The two valves are further kept in
place by an adductor muscle situated close below the mouth. Remains of
the lateral horns still persist. The anterior antennæ have undergone
the metamorphosis already indicated. They are four-jointed, the two
basal joints being long, and the second provided with a suctorial
disc, in the centre of which is the opening of the duct of the
so-called antennary or cement gland, which is a granular mass lying on
the ventral side of the anterior region of the body. The gland arises
(Willemoes Suhm) during the Nauplius stage in the large upper lip. The
two distal joints of the antennæ are short, and the last of them is
provided with olfactory hairs. The great upper lip and second pair of
antennæ and mandibles have disappeared, but a small papilla, forming
the commencement of the adult mandibles, is perhaps developed in the
base of the Nauplius mandibles. The first pair of maxillæ have become
small papillæ and the second pair probably remain. The six posterior
pairs of appendages have grown out as functional biramous swimming
feet, which can project beyond the shell and are used in the
locomotion of the larva. They are composed of two basal joints, and
two rami with swimming hairs, each two-jointed. These feet resemble
Copepod feet, and form the main ground for the views of Claus and
others that the Copepoda and Cirripedia are closely related. They are
regarded by Claus as representing the five pairs of natatory feet of
Copepoda, and the generative appendages of the segment behind these.
Between the natatory feet are delicate chitinous lamellæ, in the
spaces between which the cirriform feet of the adult become developed.
The ventral spinous process of the Nauplius stage is much reduced,
though usually three-jointed. It becomes completely aborted after the
larva is fixed.
In addition to the antennary gland there is present, near the dorsal
side of the body above the natatory feet, a peculiar paired glandular
mass, the origin of which has not been clearly made out, but which is
perhaps equivalent to the entomostracan shell-gland. It probably
supplies the material for the shell in succeeding stages[202].
[202] There is considerable confusion about the shell-gland and
antennary gland. In my account Willemoes Suhm has been followed.
Claus however regards what I have called the antennary gland as
the shell-gland, and states that it does not open into the
antennæ till a later period. He does not clearly describe its
opening, nor the organ which I have called the shell-gland.
[FIG. 232. LARVAL FORMS OF THE THORACICA. (From Huxley.)
A. Nauplius of Balanus balanoides. (After Sp. Bate.) B. Pupa stage
of Lepas australis. (After Darwin.)
_n._ antennary apodemes; _t._ cement gland with duct to antenna.]
The free Cypris stage is not of long duration; and during it the larva
does not take food. It is succeeded by a stage known as the pupa stage
(fig. 232 B), in which the larva becomes fixed, while underneath the
larval skin the adult structures are developed. This stage fully
deserves its name, since it is a quiescent stage during which no
nutriment is taken. The attachment takes place by the sucker of the
antennæ, and the cement gland (_t_) supplies the cementing material
for effecting it. A retrogressive metamorphosis of a large number of
the organs sets in, while at the same time the formation of new adult
structures is proceeded with. The eyes become gradually lost, but the
Nauplius eye is retained, though in a rudimentary state, and the
terminal joints of the antennæ with their olfactory hairs are thrown
off. The bivalve shell is moulted about the same time as the eyes, the
skin below it remaining as the mantle. The caudal process becomes
aborted. Underneath the natatory feet, and between the above-mentioned
chitinous lamellæ, the cirriform feet are formed; and on their
completion the natatory feet become thrown off and replaced by the
permanent feet. In the Lepadidæ, in which the metamorphosis of the
pupa stages has been most fully studied, the anterior part of the body
with the antennæ gradually grows out into an elongated stalk, into
which pass the ovaries, which are formed during the Cypris stage. At
the base of the stalk is the protuberant mouth, the appendages of
which soon attain a higher development than in the Cypris stage. At
the front part of it a large upper lip becomes formed. Above the
mantle and between it and the shell there are formed in the Lepadidæ
the provisional valves of the shell. These valves are chitinous, and
have a fenestrated structure, owing to the chitin being deposited
round the margin of the separate epidermis (hypodermis) cells. These
valves in the Lepadidæ "prefigure in shape, size, and direction of
growth, the shelly valves to be formed under and around them" (Darwin,
No. 519, p. 129).
Whatever may be the number of valves in the adult the provisional
valves never exceed five, viz. the two scuta, the two terga and the
carina. They are relatively far smaller than the permanent valves and
are therefore separated by considerable membranous intervals. They are
often preserved for a long time on the permanent calcareous valves. In
the Balanidæ the embryonic valves are membranous and do not overlap,
but do not present the peculiar fenestrated structure of the
primordial valves of the Lepadidæ.
In connection with the moult of the pupa skin, and the conversion of
the pupa into the adult form, a remarkable change in the position
takes place. The pupa lies with the ventral side parallel to and
adjoining the surface of attachment, while the long axis of the body
of the young Cirriped is placed nearly at right angles to the surface
of attachment. This change is connected with the ecdyses of the
antennary apodemes (_n_), which leave a deep bay on the ventral
surface behind the peduncle. The chitinous skin of the Cirriped passes
round the head of this bay, but on the moult of the pupa skin taking
place becomes stretched out, owing to the posterior part of the larva
bending dorsalwards. It is this flexure which causes the change in the
position of the larva.
In addition to the remarkable external metamorphosis undergone during
the pupa stage, a series of hardly less considerable internal changes
take place, such as the atrophy of the muscles of the antennæ, a
change in the position of the stomach, etc.
Abdominalia. In the Alcippidæ the larva leaves the egg as a Nauplius,
and this stage is eventually followed by a pupa stage closely
resembling that of the Thoracica. There are six pairs of thoracic
natatory legs (Darwin, No. 519). Of these only the first and the last
three are preserved in the adult, the first being bent forward in
connection with the mouth. The body moreover partially preserves its
segmentation, and the mantle does not secrete calcareous valves.
[FIG. 233. STAGES IN THE DEVELOPMENT OF THE RHIZOCEPHALA. (From
Huxley, after Fritz Müller.)
A. Nauplius of Sacculina purpurea.
B. Cypris stage of Lernæodiscus porcellanæ.
C. Adult of Peltogaster paguri.
_II._ _III._ _IV._ Two pairs of antennæ and mandibles; _cp._
carapace; _a._ anterior end of body; _b._ generative aperture;
_c._ root-like processes.]
The very remarkable genus Cryptophialus, the development of which is
described by Darwin (No. 519) in his classical memoir, is without a
free Nauplius stage. The embryo is at first oval but soon acquires two
anterior processes, apparently the first pair of antennæ, and a
posterior prominence, the abdomen. In a later stage the abdominal
prominence disappears, and the antennary processes, within which the
true antennæ are now visible, are carried more towards the ventral
surface. The larva next passes into the free Cypris stage, during
which it creeps about the mantle cavity of its parent. It is enveloped
in a bivalve shell, and the antennæ have the normal cirriped
structure. There are no other true appendages, but posteriorly three
pairs of bristles are attached to a rudimentary abdomen. Paired
compound eyes are present. During the succeeding pupa stage the
metamorphosis into the adult form takes place, but this has not been
followed out in detail.
In Kochlorine, a form discovered by Noll (No. 526) and closely related
to Cryptophialus, the larvæ found within the mantle represent
apparently two larval stages, similar to two of the larval stages
described by Darwin.
Rhizocephala. The Rhizocephala, as might have been anticipated from
their close relationship to Anelasma squalicola amongst the Thoracica,
undergo a development differing much less from the type of the
Thoracica than that of Cryptophialus and Kochlorine.
Sacculina leaves the egg as a Nauplius (fig. 233 A), which differs
from the ordinary type mainly (1) in the large development of an oval
dorsal shield (_cp_) which projects far beyond the edge of the body,
but is provided with the typical sternal horns, etc.; and (2) in the
absence of a mouth. The Cypris and pupa stages of Sacculina and other
Rhizocephala (fig. 233 B) are closely similar to those of the
Thoracica, but the paired eye is absent. The attachment takes place in
the usual way, but the subsequent metamorphosis leads to the loss of
the thoracic feet and generally to retrogressive changes.
OSTRACODA.
Our knowledge of the development of this remarkable group is entirely
due to the investigations of Claus.
Some forms of Cythere are viviparous, and in the marine form Cypridina
the embryo develops within the valves of the shell. Cypris attaches
its eggs to water plants. The larvæ of Cypris are free, and their
development is somewhat complicated. The whole development is
completed in nine ecdyses, each of them accompanied by more or less
important changes in the constitution of the larva.
[FIG. 234. TWO STAGES IN THE DEVELOPMENT OF CYPRIS. (From Claus.)
A. Earliest (Nauplius) stage. B. Second stage.
_A´._ _A´´._ First and second pairs of antennæ; _Md._ mandibles;
_OL._ labrum; _Mx´._ first pair of maxillæ; _f´´_. first pair of
feet.]
In the earliest free stage the larva has the characters of a true
Nauplius with three pairs of appendages (fig. 234 A). The Nauplius
presents however one or two very marked secondary characters. In the
first place it is completely enveloped in a fully formed bivalve
shell, differing in unessential points from the shell of the adult. An
adductor muscle (SM) for the shell is present. Again the second and
third appendages, though locomotive in function are neither of them
biramous, and the third one already contains a rudiment of the future
mandibular blade, and terminates in an anteriorly directed hook-like
bristle. The first pair of antennæ is moreover very similar to the
second and is used in progression. Neither of the pairs of antennæ
become much modified in the subsequent metamorphosis. The Nauplius has
a single median eye, as in the adult Cypris, and a fully developed
alimentary tract.
[FIG. 235. STAGES IN THE DEVELOPMENT OF CYPRIS. (From Claus.)
A. Fourth stage. B. Fifth stage.
_Mx´._ first maxilla; _Mx´´._ and _f´._ second maxilla; _f´´._ first
pair of feet; _L._ liver.]
The second stage (fig. 234 B), inaugurated by the first moult, is
mainly characterized by the appearance of two fresh pairs of
appendages, viz. the first pair of maxillæ and the first pair of feet;
the second pair of maxillæ not being developed till later. The first
pair appear as leaf-like curved plates (_Mx´_) more or less like
Phyllopod appendages (Claus) though at this stage without an
exopodite. The first pair of feet (_f´´_) terminates in a curved claw
and is used for adhering. The mandibles have by this stage fully
developed blades, and have practically attained their adult form,
consisting of a powerful toothed blade and a four-jointed palp.