150 Dr. W. T. Caiman on tU 



Eupliauslidse. Ou the assumption that the primitive Malaco- 

 straea possessed at least two epipodial appendages on each 

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

 become modified as branchise in the Euphausiidae and the 

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

 the gills diflers 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 tlie 

 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 

 Oumaceaf 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 Bpicaridea. 

 In many of these the oostegites develop in the typical fashion 

 just described, but in the more specialized forms tiie structure 

 is very different and hard to understand. In Hemioniscus, 

 where the development has been worked out in detail by 

 Caullery and Mesnil §, the marsupial cavity is hollowed out 



* Cf. Leicbmann, " Beitr. z. Naturgesch. d. Isopoden/' Bibl. Zool. x. 

 (1891). 



t G, O. Sars, ^' Beskr. af de paa Freg. Josephines Exp. f iindne 

 Cumaceer," Kougl. Svenska Vet.-Akad. Handl. ix. 13 (1871J, p. 19. 



I Dohrn, " Eutw. imd Organ, v. Prauiza [Aiiceus) vuixUluris,'^ Zeitschr. 

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



§ " Eecherclies siir V Hemionuct(s balani, Buchholz . . . .," Bull. Sei. 

 France et Belgique, xxxiv. pp. 31(3-362, pis. xvii. & xviii. 5 ligg. in te.xt 

 (1001). 



