4 THE VOYAGE OF H.M.S. CHALLENGES. 



natatory exopodites on a certain number of the limbs would seem to remove these 

 Crustacea far from the Isopoda. and to bring them in closer approximation to the 

 Schizopoda, but this character is, perhaps, the only one pointing to a genetic connec- 

 tion between these two groups. In all other respects I find the difference so very- 

 great as cpiite to forbid the adoption of the view of a direct descent of the one from the 

 other. In some characters the Cumacea would seem to occupy a still more primitive 

 position than even the Mysidae. Thus, in the development of the higher Crustacea we 

 find the Cumacean type, as it were, imitated by the early stage preceding the Mysis-stage, 

 and to which the name of Zoea has been applied, the tail being in this stage, as in the 

 Cumacea, very mobile and slender, and at first without any trace of ventral limbs, and the 

 natatory exopodites confined to the anterior limbs only. Moreover, the general, form of 

 the body in the Cumacea, and especially that of the tail, strongly remind us of that 

 ancient group to which Mr. Packard has given the name of Phyllocarida, and of which the 

 recent genus Nebalia is regarded as a direct descendant. True, the limbs in the Cumacea 

 are very different from those in Nebalia, but it is by no means proved that the limbs in 

 all of the ancient Phyllocarida were of exactly the same structure as in the recent genus 

 Nebalia. It has been generally admitted that the phyllopodous form of the limbs is 

 the most primitive one in Crustacea, and that all other forms might be derived from 

 this type. But the legs in the oldest of all known Crustacea, the Trilobita, have 

 been stated by Mr. Walcott to exhibit a totally different form, and this fact does 

 not seem to corroborate the general validity of the above supposition. The structure of 

 the branchial apparatus in the Cumacea is very remarkable and quite unlike all that is 

 observed in other Crustacea. It is true that the part to which the gills are affixed 

 represents the epipodite of the maxillipeds, and that this part is also found in the 

 Mysidaa, but here it always constitutes a simple membranous plate without any trace of 

 gills, and may be assumed only indirectly to subserve a respiratory function. As is well 

 known, we also find the same part peculiarly developed for respiratory purposes in the 

 cheliferous Isopoda, but even here without any trace of gills. The antennae in the 

 Cumacea are totally different in structure from those in the Mysidae or any other form of 

 the Podophthalmia, whereas they exhibit, especially as regards their peculiar modification 

 in the males, a certain similarity to those in Nebalia, as also to those in the 

 Amphipoda. 



In conclusion, I am inclined to regard the Cumacea as representing the descendants 

 of a very ancient form, long ago extinct, which may have combined some characters 

 of both the Phyllocarida and Trilobita. Perhaps even some of the palaeozoic forms 

 placed among the Phyllocarida may have formed a direct transition to the Cumacean 

 type. 



