150 Dr. W. T. Caiman on the 



Euphausiidce. On the assumption that the primitive Malaco- 

 straca possessed at least two epipodial appendages on each 

 thoracic limb (as in Anaspides), the distal series may have 

 become modified as branchiae in the Euphausiidse and the 

 proximal in the Lophogastridse. In any case, the form of 

 the gills differs considerably in the two cases, and the only 

 point which they have in common as against the Decapoda is 

 the arrangement in one instead of several series. 



Among the characters in which the Mysidacea differ from 

 the Euphausiacea and agree with the Edriophthalmate orders 

 the most conspicuous is the possession by the female sex of 

 a brood-pouch or marsupium, in which the eggs and young- 

 are carried. It cannot be doubted that this structure is 

 homologous throughout the whole series which I have name 1, 

 from this feature, the Peracarida, in spite of real or alleged 

 differences in the mode of its development. It is formed by 

 a series of overlapping plates (which Glaus considers, with 

 great probability, to be of the nature of epipodites) attached 

 to the inner side of the coxopodites of some or all of the 

 thoracic limbs. When, as in many Isopoda, the coxopodites 

 are fused with the body, the plates are attached to the sternal 

 surface of the somites. In some cases these plates or oostegites 

 develop as bud-like outgrowths from the bases of the limbs, 

 increasing in size at successive ecdyses as sexual maturity is 

 approached ; but in certain Isopoda it has been shown that the 

 course of development is abbreviated, the oostegites growing in 

 the space between the sternal cuticle and the hypodermis, and 

 being set free, completely formed, at a single moult *. 

 Probably some similar process has given rise to the statement 

 that the oostegites arise by splitting of the ventral cuticle in the 

 Cumacea j" and in the Isopod Gnathia \. At the same time 

 it is certain that the formation of the brood-pouch is profoundly 

 modified in certain parasitic Isopods of the tribe Epiearidea. 

 In many of these the oostegites develop in the typical fashion 

 just described, but in the more specialized forms the structure 

 is very different and hard to understand. In HemionisciiSj 

 where the development has been worked out in detail by 

 Caullery and Mesnil §, the marsupial cavity is hollowed out 



* Cf. Leickmann, " Beitr. z. Naturgesck. d. Isopoden,'' Bibl. Zool. x. 

 (1891). 



t Ox. 0. Sars, (l Beskr. af de paa Freg. Josepkines Exp. f undue 

 Cumaceer," Kongl. Svenska Vet,-Akad. Handl. ix. 13 (1871), p. 19. 



} Dokm, " Entw. nnd Organ, v. Praniza (Anceus) maxillaris,' Zeitsckr. 

 f. wiss. Zool. xx. (1870) p. 70. 



§ " Recherekes sur VHemionucus balani, Buchholz . . . .," Bull. Sci. 

 France et Belgique, xxxiv, pp. 3113-362. pis. xvii. & xviii. -3 iigg. in text 

 (1901). 



