204 report — 1878. 



A deep-sea genus, closely allied to that of Axius, that I have named 

 Eiconaxius, taken during the Challenger cruise in the Eastern seas, 

 has the same advanced condition of the embryo, and shows that the 

 Megalopa stage exists not unfreqnently in marine Macrura, although we 

 have previously had no evidence of it. 



The fresh- water genus Asia ens (PI. VI., fig. 5) and the land crab Gecar- 

 cinus, long since made known to us, the former by Rathke, and the latter by 

 Westwood, leave the ovum in the last stage, which is that of an approxi- 

 mation of form to the adult animal, while it yet retains many features that 

 exhibit incompleteness of development. This is most apparent in those 

 parts which show a tendency to depart from the characteristics of the order 

 to adapt themselves to constitutional requirements ; as, for instance, the 

 adult Atyheus mostly lives in dark places under stones, groping in mud 

 and in such like spots at the bottom of the sea at a few fathoms deep. 

 To suit this condition of things, it is highly convenient to the animal that 

 the eyes should be protected ; and since the peculiar habitat of the animal 

 is that of dark holes, those in which the eyes are least improved by use 

 are as suitable to its existence as others ; those which are protected and 

 least liable to injury become the kind most adapted to survive. 



Thus it follows, that while the rest of the animal advances in growth 

 the eyes remain in abeyance, and the anterior margin of the carapace 

 extends beyond and overlaps them, thus affording protection, and by its 

 tenuity admitting a sufficient amount of light for the purposes of the 

 animal's requirements. Thus it appears that the development of Alphceus 

 shows a relative retrograde character in the progress of the eyes to that 

 of other parts. So again, in the comparison of the pleopoda in the 

 Brachyura in the adult with those of the megalopa stage of the same 

 animal, we find that of the younger framed upon a simple type adapted 

 for swimming, while in the adult it is altered to suit other purposes — in 

 the female to snpport the gravid ovum, and in the male those of the 

 anterior to assist in copulation, while those of the posterior are more or 

 less rendered obsolete in consequence of the absence of any duty to fulfil. 



The study of the several forms in which the embryo quits the ovum in 

 Crustacea is, I believe, very instructive, as bearing on the tendency of 

 variation of forms in adult animals. 



From the earliest forms to that of the most perfect, in which the 

 brephalus quits the ovum, there is a series of stages in which the embryo 

 appears ready to take upon itself the conditions of a free and independent 

 animal. This capability does not appear to be connected with any par- 

 ticular adult type, or conditions of existence, but exists in closely allied 

 species and genera as well as in those that are extremely distinct : neither 

 does it appear to bear any relation to the more or less advanced character 

 of the several genera. 



The following list is the order of the various stages of development, 

 when the brephalus quits the ovum, together with the adult form from 

 which it is derived. 



1. Nauplius Euphausia (Metschuikoff), Penasus ? (F. Miiller.) 



2. Meta-naupliu9 None. 



3. Proto-zosea None. 



4. Zosea Brachyura, Anomura, and some Macrura. 



5. Phyllosoma Palinurus. 



6. Megalopa Astacus (Rathke), Gecarcinus (Westwood). 



