402 Mr. L. A. Borvadaile on the 



that the stock from which the latter two groups have 

 sprung lost a portion of their heritage in this respect after 

 the differentiation of the former. For, not only have some of 

 the lower Eeptants kept the podobranchs on the legs of the 

 fourth pair wliich all the Penseidea* and Caridea have lost, 

 but on several segments in the Potamobiidse we find the full 

 possible branchial equipment. Couti^re (' Comptes Rendus,' 

 1905, p. 64) has elaborated an extiemely ingenious theory of 

 the homologies of the several kinds of epipodial structures of 

 the Decapoda with one another and with those of the lower 

 Crustacea. Shortly put, this theory is as follows : — The 

 primitive number of epipodial outgrowths of the thoracic 

 limb of the Crustacea is two — a distal, the epipodite, belonging 

 to the coxopodite, and a proximal, the proepipodite^ belonging 

 to the true basal joint of the limb, which in the Decapoda is 

 taken into the body during development. Both these struc- 

 tures are found in Brajichijms SLud in Anaspides. In Scliizo- 

 poda and Decapoda both proepipodite and epipodite divide 

 into two parts. The epipodite forms in the Lopiiogastridge 

 (a) the oostegite and (/3) a setiferous tubercle which I shall 

 call the setolranch. In the Caridea the epipodite forms, when 

 present, (a) the " epipodite " [mastigohranch) and (/8) on the 

 legs a setobranch of the same form as in the Lophogastridte, 

 and on maxillipeds 2 and 3 a podobranch and an arthrobranch 

 respectively ; in the Pena^idea it forms (a) the " epipodite " 

 and (/9) the (anterior) arthrobranch, wanting in Caridea and 

 supposed to be there represented by the setobranch. The 

 proepipodite forms in the Lophogastridse a divided gill. In 

 the Decapoda it forms (a) the pleurobranch and [l>) the 

 (posterior) arthrobranch. In the development of Penceus 

 tliis subdivision can actually be seen to take place. The 

 Euphausiacea have lost their proepipodite. 



Now, valuable and suggestive as this theory is, it is to 

 some extent invalidated by the fact that, in the case of 

 section /3 of the epipodite, structures which it regards as 

 alternative developments of the same rudiment can be found 

 coexisting. For it supposes that one arthrobranch (presumably 

 the anterior) and the podobranch and the setobranch are 

 equivalent and alternative structures. But in the Potamo- 

 biidse all these are present together on several segments of 

 the body. In Dromia Bohn has discovered what is un- 

 doubtedly a setobranch on the third maxilliped, where, 

 though the podobranch is wanting, both arthrobranchs are 

 present. On the first leg the setobranch is found on the 



* It is only in certain of the primitive deep-sea Penseids that the first 

 two or three pairs of legs bear podobranchs. 



