118 CRUSTACEA. 



If we consider the variations found in the different planes of organisation 

 among the Vertebrata, we find a certain element of similarity between the 

 discoidal cleavage of these latter and of the Crustacea in the fact that in both 

 cases a deposit of food-yolk on one side of the principal axis takes place, and 

 determines the peculiar type of development. In those Vertebrates in which 

 discoidal cleavage occurs, the dorsal side of the body takes the lead in develop- 

 ment, while the ventral side is hindered in its development by the accumulation 

 of food -yolk. The blastopore is here shifted to the dorsal side. In those 

 Crustacea in which discoidal segmentation is found, on the contrary, the 

 rudiments of the ventral side appear first, and the blastopore takes up a ventral 

 position corresponding with the plan of organisation of the group. In this 

 case, the dorsal side of the body is influenced in its development by the 

 accumulation of food-yolk. 



In many Crustacea, when the formation of the blastoderm is 

 complete, a cuticle is secreted at the surface of the blastoderm cells. 

 We follow Van Beneden (No. 79) in calling this membrane the 

 blastodermic cuticle (cuticula blastodermica). Its appearance can 

 only be explained by a process of ecdysis shifted back to an early 

 embryonic period. Similar membranes are given off by the eggs 

 of the Arachnids and of Limulu*. 



The formation of a blastodermic cuticle is to be observed specially among 

 the Malacostraca. It has, however, also been seen in the parasitic Copepoda 

 (Van Beneden, No. 17). In the Malacostraca it is very common, being found 

 in Nebalia (Van Beneden, No. 79), the Cumacea (H. Blanc, No. 35), in many 

 Decapoda (Lerebottllet, No. 58, and Reichenbach, Nos. 64 and 65, found 

 it in Astacus ; P. Mayer, No. 59, in Eupagurus; Bobretzky, No. 41, in 

 Palaemon ; Kingsley, No. 53, and Van Beneden, No. 79, in Crangon ; 

 Dohrn, in Portunus), in the Amphipoda (Van Beneden and Bessels, No. 2, 

 in Gammarus locusta; Van Beneden, No. 79, in Caprella; Uljanin, No. 75, 

 Orchestia) ; finally, in the Isopoda (Van Beneden, No. 79, in Asellus ; 

 Bobretzky, No. 80, in Oniscus). Further, it has been observed by Dohrn 

 in Tanais. 



The formation of this blastodermic cuticle is, in many Crustaceans, 

 followed by other ecdyses in later embryonic stages. This is especi- 

 ally the case where the development is abbreviated, i.e., where many 

 of the stages of development are shifted back into embryonic life. 

 The cuticles formed during these ecdyses are usually distinguished 

 by swellings corresponding to the rudiments of the limbs. These 

 membranes are called larval integuments. 



As there is considerable variation in the time of appearance of all these 

 cuticular membranes, it is often difficult, in single cases, to distinguish between 

 the actual egg-integument, the blastodermic cuticle, and the larval integuments 

 which appear later, and to establish with exactitude the homology of the special 

 cuticular structure in each individual case. There can, however, be no doubt 

 that the cuticle very common among the Arthrostraca, and observed in Ligia 

 by F. MtlLLER (No. 4), and further, the integument developed in Myds and 



