Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/682

048 CEUSTACEA but resemble the young of Bopyrus ; they are, in fact, larval Isopods. The female of Entoniscus (fig. 44) resides within the body of a species of Porcel- lana, lying in a thin- walled sac b3tween the liver, intestine, and heart of its host, the head being destitute of eyes or antennae; the thorax has become an irregular inarticulate sac, beset with enormous brood laminoe ; the long vermiform and ex tremely mobile abdomen has Flo 44 _. E^TJu, cana-orum . fc- s<vord-shapad legs ; and swell- male. Magn. 8 times. (Fritz Mttller.) ing out above it in a globular form, as if in a hernial sac, the haart lies at the base of the first segment. The young of this singular parasite closely resembles that of Bopyms and Cryptothiria. The embyro of the Amphipoda can be distinguished from xhat of the Isopoda at a very early period ; the former being b3iit downwards with tli3 dorsal surface external, whilst tha latter is bant backwards with the ventral surface external. Tii3 embryo in all the genera which have been examined is attached on th3 anterior part of the back to the inner egg membrane by a peculiar structure reminding one of the union of tha young Isopoda with the larval msmbrane, and of tli3 unpiirad " adherent organ" on the nap3 of ths Cladocera, so remark ibly developed in Evadne, and persistent through its life in that genus ; but though present in the young of D.ip iuiji, it disappears in the adult (Friti Muller). The metamorphosis of the young A-nphipad after it quits the eg^ seams greatly reduced and simplified, for before quitting the egg it acquires its full number of seg ments and limbs. 1 In those instances in which certain of the segments are amalgamated together, or where one or more segments are deficient inth3 adult, W3 find the same fusion and the same deficiencies in the young animals taken from the brood-pouch of their in )ther. The dsvelopanat of the IlypariicliTe, an oceanic group of Amphipods found only in the gill-cavities of the M edits :e, is very exception il and ra in ark ible. Thus, in Hi/peria tha youngest larvae, taken by Fritz M.iller from the brool-pju? 1 .! of tii3 mother, already possessed the whole of the thiracic feat; 0:1 the other hand, those of the abdoman wara not as yat developed. All the feet are at first simple, but soon beeonn converted into highly denticulated prehensile feet. In this state they remain for a very long time, the abdominal appendages growing into powerful natatory organs, whilst tli3 eyas, at first want ing or very minute, expand into largi hemispheres occupying the entire lateral, and even encroaching upon the dorsal and frontal walls of the head. Tha femile.s (ffyperia) are dis tinguished by a very broad thorax, and the males (Les(riyouus) by their long antenne. The youngest larvoa o mnot swim, but are provided with ehelate feet (as shown by Spencs Bate) by which they cling firmly to the swim ming-lamina of their host. The feet of the adults are simple, but they are then excellent swimmers, and are not unfre- quently mat with free in the open sea. The diversity in structure of the antennre in the adult male and female Ilyperiidoe is so great as to have led naturalists to place them in separate genera or even families ; but this differ ence is developad only when the animals are full-grown. Up 1 " Even peculiarities in the structure of the limbs, so far as they are common to both sexes, are usually well marked in the newly-hatched young, so that the latter generally differ from their parents only by their stouter form, the smaller number of the antennal joints and olfactory filaments, and also of the sette and teeth with which the body or feet are armed, and perhaps by the comparatively larger size of the secondary flagellum " (Fritz Miiller, Fur Darwin, Engl. trtns. p. 76) to this period the young of both sexes resemble the females. In the male shore-hoppers (Orchestia] the second pair of the anterior feet is provided with a powerful hand (tig. 45), as FIG. 45. Orchei ia Daricinii, n. sp. male. (Fritz Muller.) in the majority of Amphipoda, but quite different from the fern lie ; the young nevertheless resemble the female. This is also the case in the adult male in Lirmtlus, in which the second pair of appendages (antennae) are peculiarly modified (see 3a in fig. 1 2 above) ; but in the young male they exactly resemble those of the adult female. According to Spence Late and Fritz Muller, this second pair of antennse are absent in the females of Brachyscdvs, although the ir.ale possesses them, like other Amphipcds. In the foregoing brief sketch of the evolution cf the young in the Malaccstraca, it will be perceived that certain lines of development are followed, but these are subject to great diversity, and often vary greatly in the same order. 2 Thus we have : I. The larval metcmorphosis undergone within the egg. 3 II. The larval metamorphosis undergone within the incubatory pouch of the mother. 4 III. 1 he young first appearing as free-swimming soar. 5 IV. The young first appearing as navpiii* These four stages in the larval development of the Crustacea, it will be perceived, are not by any means strictly confined to jrart:cular orders of the Jjclacostraca, nor do they hold p>c.d fcr all the members of the group in which they have been observed to occur. There is, in fact, no " hard and fast " rule in the class, tut en the contrary, there would appear to be numerous exceptions and varia tions in every group. In considering the larvi l development of the Entcmcs- traca, we shall f.nd that their early history, vhen c< mpared with that of the Malacostraca, is greatly simplified, end that the first tr rauplius fcim of the ycung, which Fritz Muller except ion oil// met with in Fi^avsj has now become the rule almost without exception. Embryology of Limvfiis.- Starting ith that remark able representative of a most ancient and r.cw almost extinct order, the Merostemata, ve fird in Limiihts a genus in which the young may be said to undergo all their earlier metamorphoses within the egg, thus at once offering an excep tion to the general rule as regards the Enti mostraca. The embryology of Limiting has been investigated Ly Dr .Antcn 2 In the normal Isopoda, as -ve have seen, the development of the young is one of progress to the adult ; but in the parasitic forn-.s the young animal before attaining the adult stale actually l.as to ui.drrgo a retrograde metamorphosis. 3 An instance of this occurs in the re<aroda-Brnchyura, viz., Gecarcinus(J.O. Westwood);in the Anomoura, D iomia (H. Woodward ; in the Macroura, Potamolivs (A stacks) fvriatilis (Rathke). hi the Amphipoda the young appear to have always acquired their full number of segments and appendages before quitting the egg (Fritz Muller, Spence Bate, &c.) 4 In the Stomapoda by Mysis, and Cumaccce (?) ; in the Isopw a by Asi lliis, Liffia, &c. 5 In the Decapoda-Brachyura by Cnrcinus (Fpence Bate), by Cydo- grapsus and many other crabs and lobsters (Fritz Miiller). 6 In the Decapoda-Brachyura by a prawn near to Penocvs (Fritz Miiller). 7 It is very desirable that this remarkable and isolated case in tie

development of the Macrouran Decapod should be confirmed by others.