CRUSTACEA. 



9 



N 



tional lungs. In other land crabs no such pulmonary organs are developed, and yet it 

 is possible to drown them by a prolonged submergence in the water. We shall recur 

 again to this subject in treating of the Decapods. 



The nervous system of the Crustacea, like that of all Arthropoda, 

 usually consists of a large anterior ganglion, or " brain " (supra-oesophageal 

 ganglion), from Avhich two nervous cords or commissures arise, and which, 

 passing back, one on either side of the resophagus, connect the brain with 

 a series of similar but smaller ganglia lying on the floor of the body, there 

 being typically a single ganglion for each segment of the body ; but occa- 

 sionally we find two or more of these secondary brains united, and the 

 cords which should connect them obliterated. From the supra-oesophageal 

 ganglion nerves go to the eyes and the antenna, in which the organs of 

 sense are most specialized, and hence, as it is through this portion of the 

 nervous system that the animal receives the larger portion of its knowledge 

 of the external world, the supra-oesophageal ganglion may be dignified 

 with the name of the brain. From the other ganglia nerves arise which pass 

 to the muscles, organs, and limbs of the corresponding portions of the body. 

 The foregoing account applies to the majority of the Crustacea, but various 

 modifications are found, and in the adults of some of the parasitic forms 

 no trace of a nervous system has as yet been found. 



One of the most interesting subjects connected with the Crustacea is 

 their reproduction, a field which has already furnished many valuable re- 

 sults, but which, nevertheless, has scarcely begun to be worked. A few 

 general features only will be mentioned here, the variations in the differ- 

 ent groups being described in their proper place in the succeeding pages. 



The eggs of the Crustacea are almost invariably carried by the mother, 

 either attached to some portion of the body (usually the abdominal legs) 

 or covered in a brood-pouch usually attached to some portion of the thorax. 



The eggs after fertilization segment more or less completely, 

 there usually being a central portion of the yolk which does 

 not divide. A portion of the resulting cells soon invaginate, 

 and are destined to form the lining cells of that portion of 

 the alimentary tract known as the mesenteron. The place 

 where this invagination took place soon closes up, and on 

 either side the appendages are seen to bud out. These 

 appear at first as simple buds, the pair which are to form the 

 antenna first being seen, and very soon after the antennula3 

 and mandibles arise simultaneously, 



and then after them, in varying order, the other appendages. 

 These appendages increase in length and become divided 

 into a series of joints, and each, except the first, acquires a 

 two-branched condition. The mesenteron, formed as we have 

 seen, for a time exists without any connection with the ex- 

 terior, but soon there is a pushing in at each end of the body, 

 and the tubes thus formed unite with that already existing, 

 forming the completed alimentary canal. 



The stage of growth at which the embryo hatches from FIG. 11. Gastruia of crayfish. 



i . -i /. a - Abdomen, m. Mesenteron. 



tne egg varies even among closely allied forms, there being p. Proctodeum. s. stomodeum. 





FIG. 9. Ner- 

 vous system 

 of lobster. 



FIG. 10. Section of segmented 

 egg of shrimp. 



