Erichthus larva of SquillaFig. 226. Second stage of Erichthus larva of Squilla with five maxillipeds and the first pair of abdominal appendages.(From Claus.)
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. 227g). 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.
Advanced Erichthus larva of SquillaFig. 227. Advanced Erichthus larva of Squilla with five pairs of abdominal appendages.(From Claus.)f.first maxilliped;g.second maxilliped.
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.
Advanced Erichthus larva of SquillaFig. 228. Advanced Erichthus larva of Squilla when the three posterior maxillipeds have become reduced to minute pouches.(From Claus.)
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.
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.
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. Theeyes 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.
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. 229A) 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 posteriorpairs 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.
Stages in the development of Cyclops tenuicornisFig. 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 mandiblemd.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.
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 mandiblemd.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 otherchanges take place. First of all a pair of appendages arises behind the mandibles, which form the maxillæ (fig. 229B); at the same time the basal joint of the maxillæ develops a cutting blade. Three successive pairs of appendages (fig. 229C) 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. 229D) 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 alarge number of secondary characters. So far as is known a more or less modified Nauplius larva is usually preserved.
Stages in the development of Achtheres percarumFig. 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.
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. 230A). 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 fromthis rudimentary Nauplius stage into a stage corresponding with the Cyclops stage of the free forms (fig. 230B). 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 (pm1andpm2) 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.
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. 230C) 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-œsophageal 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 maxillipedsof the male (fig. 230E,pm2) separate again; while in the female (fig. 230D) 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.
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 larvalhistory 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. 231and232A), 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. 231lh), 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.
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,andmd) 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 thebody—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.
Nauplius larva of Lepas fascicularisFig. 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.
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].
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 perhapsequivalent 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. Betweenthe 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].
Larval forms of the ThoracicaFig. 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.
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. 232B), 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 natatoryfeet, 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.
Stages in the development of the RhizocephalaFig. 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.
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. 233A), 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. 233B) 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.
Two stages in the development of CyprisFig. 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.
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. 234A). 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 ofantennæ 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.