[1] Like Claus I do not regard the eyes of the Crustacea as limbs, and therefore admit no ocular segment; on the other hand I count in the median piece of the tail, to which the character of a segment is often denied. In opposition to its interpretation as a segment of the body, only the want of limbs can be cited; in its favour we have the relation of the intestine, which usually opens in this piece, and sometimes even traverses its whole length, as in _Microdeutopus_ and some other Amphipoda. In _Microdeutopus,_ as Spence Bate has already pointed out, one is even led to regard small processes of this tubular caudal piece as rudimentary members. Bell also (‘British Stalk-eyed Crustacea’ p. xx), states that he observed limbs of the last segment in _Palæmon serratus_ in the form of small moveable points. The attempt has often been made to divide the body of the higher Crustacea into small sections composed of equal numbers of segments, these sections consisting of 3, 5 or 7 segments. 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 primitive body includes the segments which the naupliiform larva brings with it out of the egg; it is afterwards divided, by the younger sections which become developed in its middle, into the head and tail. To this primitive body belong the two pairs of antennæ, the mandibles and the caudal feet (“posterior pair of pleopoda,” Sp. B.). Even in the mature animal the fact that these terminal sections belong to one another is sometimes betrayed by the resemblance of their appendages, especially that of the outer branch of the caudal feet, with the outer branch (the so-called scale) of the second pair of antennæ. Like the antennæ, the caudal feet may also become the bearers of high sensorial apparatus, as is shown by the ear of _Mysis._ 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. The fore-body appears, in the adult animal, to be entirely or partially amalgamated with the head; its appendages (_siagonopoda_ Westw.) are all or in part serviceable for the reception of food, and generally sharply distinguished from those of the following group. The segments of the middle-body seem always to put forth limbs immediately after their own appearance, whilst the segments of the hind-body often remain destitute of feet through long portions of the larval life or even throughout life (as in many female Diastylidæ), a reason, among many others, for not, as is usual, regarding the middle-body of the Crustacea as equivalent to the