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None of these attempts has ever met with general acceptance; my own investigations lead me to a conception which nearly approaches Van Beneden's. I assume four sections of 5 segments each the primitive body, the fore-body, the hind-body, and the middle-body.
The formation of the abdominal segments has commenced. Paired eyes still wanting. Traces of the outer maxillipedes make their appearance betimes. Then feet appear upon four segments of the middle-body, and these are biramose on the three anterior segments, and simple, the inner branch being deficient, on the fourth segment.
It is to be supposed that, not only originally but even still, in the larvae of the first Malacostraca, the new body-segments and pairs of limbs are formed singly, first of all the segments of the fore-body, then those of the abdomen, and finally those of the middle-body, and, moreover, that in each region of the body the anterior segments were formed earlier than the posterior ones, and therefore last of all the hindermost segment of the middle-body.
The middle-body with its appendages, those five pairs of feet to which these animals owe their name of Decapoda, is either entirely wanting, or scarcely indicated; the abdomen and tail are destitute of appendages, and the latter consists of a single piece. The mandibles, as in the Insecta, have no palpi.
The sequence of the sections of the body in order of time seems originally to have been, that first the fore-body, then the hind-body, and finally the middle-body was formed. This is a circumstance which renders very doubtful the equivalence of the middle-body of the Malacostraca with the section of the body which in the Copepoda bears the swimming feet and in the Cirripedia the cirri.
Embryo of Cryptoniscus planarioides, magnified 90 diam. The last pair of feet on the middle-body of the larva, consequently the penultimate pair in the adult animal, is almost always similar in structure to the preceding pair.
The latter again becomes destitute of appendages, so that in this case at an early period four, and at a later only three, segments of the middle-body bear limbs. The fifth segment is still entirely wanting, whilst all the abdominal segments have also acquired limbs, and this one after the other, from before backwards. Zool.
The young animal, when it begins to take care of itself, resembles the old ones in almost all parts, except one important difference; it possesses only six, instead of seven pairs of ambulatory feet; and the last segment of the middle-body is but slightly developed and destitute of appendages.
Do not Nauplius-like states in this case also precede the Zoea? The last segment and the last two pairs of feet of the middle-body are wanting. The developmental history of Mysis, the near relationship of which with the Shrimps and Prawns has recently again been generally recognised, has been described in detail by Van Beneden. So far as I have tested them I can only confirm his statements.
Thus, that which appears probable even from the comparison of the limbs of the adult animal, finds fresh support in the developmental history, namely, that the lower Crustacea, like the Insects, are entirely destitute of the region of the body corresponding to the middle-body of the Malacostraca.
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