i 3 4 COMPARATIVE ANATOMY 



ments, is that the green gland excretes, among other sub- 

 stances, a nitrogenous substance, which has been called 

 carcinuric acid. 



It is interesting to note that the larvae of many decapods 

 allied to the crayfish have a pair of provisional excretory 

 organs opening at the bases of the second pair of maxillae, so 

 that in the decapods we have an exact reversal of the conditions 

 which obtain in phyllopod Crustacea. It is possible, though 

 there is no direct evidence on the subject, that the ancestral 

 crayfish had a pair of excretory tubes opening at the bases of 

 each pair of limbs, and we do know that in the very primitive 

 arthropod, Peripatus capensis^ there is a pair of excretory 

 tubes opening at the bases of each of the seventeen pairs of 

 legs. These, excretory tubes are formed as ventral outgrowths 

 of as many pairs of ccelomic sacs, and in the adult condition 

 the originally spacious ccelom is reduced to the cavities of the 

 gonads, and to a series of small vesicular sacs forming the 

 internal ends of the excretory tubules. It has been suggested 

 that the end-sac of the excretory organs of Crustacea are 

 homologous with the internal ccelomic vesicles of the excretory 

 tubules of Peripatus, and if this comparison is just, we have 

 the representatives of a pair of ccelomic sacs in the second 

 and fifth segments of the Crustacea ; the green glands or 

 shell glands being tubular outgrowths of these sacs, are 

 therefore peritoneal funnels, homologous with the genital 

 ducts of Annelida. It should be possible to settle the question 

 by reference to the development of the organs in question, but 

 unfortunately the evidence is contradictory. It is stated that 

 in the lobster the body of the green gland is formed out of a 

 group of mesoderm cells, which acquires a cavity, grows 

 towards the base of the second antenna, and there meets and 

 fuses with an invagination of the ectoderm. This would 

 confirm the supposition that the green glands are peritoneal 

 funnels. But it has been stated again that in the phyllopod, 

 Leptodora hyalina^ both antennary and maxillary glands are 

 formed from the ectoderm, and in this case there can be no 

 question of ccelomic cavities. It is best to await further 

 evidence on the subject, and, meanwhile, to suspend our 

 judgment as to the homologies of the excretory organs of the 

 Crustacea. 



The nervous system of the crayfish is depicted in fig. 31. 



