Part 5 (1/2)
Cunningham)]
The Shrimps and Prawns of the tribe Caridea are e comparable to that of the Lobster, in which they swis So to the tribe Penaeidea, however, have a still more remarkable metamorphosis, which is very ies to those of the lower Crustacea Fritz Muller discovered in 1863 that _Penaeus_ is hatched fro 29, A), a for the Copepoda, Branchiopoda, and Cirripedes The nauplius, unlike the larvae which we have been considering, has an unsegmented body, and has only three pairs of limbs The body is pear-shaped in outline, and near the front end is seen the median eye, sometimes called, from its presence in this type of larva, the ”nauplius-eye”; the paired eyes are not yet developed The three pairs of limbs are shown by their later development to be the antennules, antennae, and mandibles; the first pair are unbranched, the second and third divided into exopodite and endopodite It is interesting to notice that the antennae and mandibles, which in the adult animal are so widely different that it is difficult to trace any resemblance between them, are in the nauplius almost identical in for placed in front of the mouth as in the adult, lie on either side of it, and each has at its base a hooked spine which projects inwards and serves for seizing particles of food and passing them into the mouth; the antennae of the nauplius, in fact, serve as jahile it is only later that the mandibles take on this function
[Illustration: FIG 29--LARVAL STAGES OF THE PRAWN--_Penaeus_ (SEE PLATE IV) 45 (After F Muller)
A, Nauplius; B, young zoea; C, older zoea; D, early ”schizopod” stage]
In the further developth and becomes divided into so behind those already ular order frorows out into a carapace, beneath which the paired eyes begin to develop in front Thus after passing through _ 29, B) the larva beco that of the Crab already described in that the swi the uropods well developed and for a tail-fan at the end of the abdoes present as rudiments, and the stalked eyes free fro 29, D), in which the prawn-like shape is assue exopodites used for swih they do not quite disappear in the adult _Penaeus_, and the function of swians is taken over by the abdominal swimmerets
In _Penaeus_ the larvae are of coestes_ the zoea has a very re spines, each bearing two comb-like rows of secondary spines The developrowths of the surface of the body is a very coanisms that, like these larvae, float or swinificance will be discussed in a later chapter
The shrimp-like Euphausiacea have a larval development very like that of _Penaeus_ Most, if not all, of the species are hatched froes very similar to those described above The adult anie, since the exopodites of the thoracic legs re
[Illustration: FIG 30--NEWLY-HATCHED YOUNG OF A CRAYFISH (_Astacus fluviatilis_) ENLARGED]
Even a the Decapoda, however, there arein a form that does not differ essentially from the adult, and are therefore said to have a direct development This is often the case with species which live in fresh water or in the depths of the sea For exa
30), when hatched, possess all the appendages of the adult except the first pair of swimmerets and the uropods, or outer plates of the tail-fan The carapace is ale amount of food-yolk, which supplies the nourishes of its development
The chelae have hooked tips, by s securely to the swimmerets of the mother After a time it moults, and the uropods are set free, the chelae lose their hooked tips, the carapace assuely absorbed), and the young Crayfish leaves the protection of its parent, to shi+ft for itself The essential point of difference between the development of the Crayfish and that of the closely related Lobster (see Fig 8, p 28) is not soare less profound in the fore In the Lobster the earlier stages are capable of independent existence, and they differ frorown ani at the surface instead of creeping at the bottom of the sea
A similar case to that of the Crayfishes is found in the River Crabs of tropical countries, belonging to the family Potamonidae These Crabs are as closely related to some marine Crabs as are the Crayfishes to the Lobsters, yet the difference in their inning life as ic zoeae, they leave the shelter of the31)
[Illustration: FIG 31--YOUNG SPECIMEN OF AN AFRICAN RIVER CRAB (_Potamon johnstoni_), TAKEN FROM THE ABDOMEN OF THE MOTHER MUCH ENLARGED
The adult of an allied species is figured on Plate XXIII]
Ast the Decapoda, instances of direct development like those just described are exceptional, but in some of the other orders of the Malacostraca direct developreat division Peracarida, as we have already seen, the females are provided with a pouch, or marsupium (fros are carried Within this pouch the young undergo the whole of their development, and they only leave it, as a rule, when they have attained the structure of the adults A the more familiar representatives of this division, the Sand-hoppers (Amphipoda), the Woodlice (Isopoda), and the Opossum Shrimps (Mysidacea), may be mentioned as examples of this mode of development The Woodlice and their immediate allies differ a little fro leave the brood-pouch with the last pair of legs still undeveloped, though in other respects they are like miniature adults
In those Crustacea which have a direct developes, it is soes in the early development of the embryo This is shown most clearly, perhaps, in the Opossum Shri-membrane (or e, and lies free within the brood-pouch as a ot-shaped body, on which three pairs of rudimentary limbs can be made out The later development shows that these three rudiments correspond to the antennules, antennae, and ot-shaped euised nauplius without the power of swi an independent existence In other cases--as, for instance, in the Crayfish, where the earlier stages are confined within the egg-h more difficult to exaroups of the Malacostraca, the Syncarida and Leptostraca are hatched in nearly the adult fores These larvae (Fig 32) are all distinguished by the large size of the carapace, which in soreater part of the body Some Storeat size, solass-like transparency gives the appearance
[Illustration: FIG 32--EARLY LARVAL STAGE OF A SPECIES OF SQUILLA, PROBABLY _S dubia_ 10 (After Brooks)]
As we have seen, it is exceptional to find a free-swi the Malacostraca, but it is the coe in the other subclasses of Crustacea Most of the Branchiopoda are hatched in this forradual series of changes in which new soular order from before backwards till the full number is reached The Water-fleas (Cladocera), however, differ fro a direct develops are carried in a brood-pouch under the back of the carapace, and in this the eo their develops or young can generally be seen through the transparent carapace (see Fig 12, p 37)
[Illustration: FIG 33--LARVAL STAGES OF THE BRINE SHRIMP (_Artemia salina_) (After Sars)
A, Nauplius, just hatched; B-E, later stages, showing progressive increase in nues The adult for 55, p 164]
Many of the Ostracoda have a direct develop, has only the first three pairs of appendages, and is therefore regarded as a nauplius, although it possesses a bivalved shell like that of the adult, and is very unlike the nauplius larvae of other Crustacea
[Illustration: FIG 34--EARLY NAUPLIUS LARVA OF A COPEPOD (_Cyclops_)
MUCH ENLARGED (Froy”)
_a'_, Antennule; _a''_, antenna; _gn_, jaw-spine of antenna; _lbr_, upper lip; _ in the nauplius stage; and, indeed, it was to the young of the co