ON OUK PRESENT KNOWLEDGE OF THE CRUSTACEA. 205 



In this list the earliest or nauplius form belongs to Euphausia, or the 

 lowest stage in the classification of the adult animal ; while the next 

 stage, or zoaea condition, belongs to all the higher forms with the exception 

 of one genus only among the Brachyura, and some of the Macrura. 

 To these belong the phyllosoma and megalopa stages. 



Before we can conclude our report on the development of Crustacea, it is 

 desirable that we should examine the earlier stages of the embryo, as well 

 as the character of the various ova in relation to the adult forms. 



The eggs of Crustacea vary in size in different genera and sometimes in 

 form, but not very much in this latter feature, never more than from 

 round to oval and egg-shaped. But in size the vai'iation is greater, and 

 this not in relation to the proportion of the animal ; for Palinurus, which 

 is two feet long, has the ovum only one-quarter the size of that of Astacus, 

 which is only three inches long. 



Some idea may be gathered by the following list of the diameter of the 

 eggs of the animals that have been examined ; that are from the ovum 

 of the fresh-taken animal and from specimens preserved in spirits. 



Crangon ^ 



Cr. boreas ...£ 

 Homarus. 

 Carcinus , 



V 



20 



The ova are attached to the pleopoda of the mother in all forms of De- 

 capod Crustacea by means of a membranous filament that varies in separate 

 genera. In Palcemon, it is very thin and transparent, and differs from 

 that of the Brachyura and other forms. It is not easy to determine its 

 origin, but there are connected with it, as if incorporated in the structure, 

 certain epithelial- like cells, that in form and appearance resemble those 

 that Mr. Alfred Sanders has figured as living zoosperms belonging to 

 Palcemon Squilla ; they are much larger and appear as if flattened, and 

 absorbed into the surrounding structure, which spreads out to an ex- 

 treme tenuity, and encompasses the entire ovum, which it holds and 

 suspends. In some genera it is exceedingly slender and delicate, and 

 easily ruptured ; in others it is strong, fibrous, and not easily broken. 



The observations that I have made have generally been on the most 

 common forms that I could procure alive, such as Crangon, Palcemon, 

 Homarus, Astacus, Palinurus, Portunus, Garcinus, and Cancer. The two 

 first of these are very suitable for examination from the beautifully trans- 

 parent nature of the vitellus ; while those of Homarus and Astacus afford 

 advantages from their large size. 



The ovum is generally round, but in some species, as in Palcemon, they 

 afterwards become somewhat oval. (PI. VII., fig. 1.) 



The yolk in most instances fills, or nearly fills, the egg : but in some 

 cases, as described by Metschnikoff, there is a tolerable space between the 

 membrane that encloses the vitellus and the chorion. This he states to 

 be the case in the ova of Euphausia and Penceus, and I have observed 

 that the same condition exists in the ovum of the genus Nika. This space 

 is filled by a clear and slightly viscid fluid. At first the yolk consists of 

 numerous minute cells, very uniform in size, that appear to have little or 

 no cohesive property to each other. Taken separately, they appear to 

 be tolerably transparent, but in the aggregate they assume a colour that 

 is peculiar to each genus. In some the colour of the vitellus is grey, in 

 others yellow, orange, brown, green, and purple. Shortly the mass of 



