NEPHRIDIA AND BODT-OAVITY OF DECAPOD CRUSTACEA. 419 



The central and lateral cavities only differ with regard to the 

 relative position of the nervous system, and this difference has 

 already been accounted for. In the leg-cavities of Palse- 

 monetes in this region no nephridia have been developed. 



Beneath the anterior end of the pericardial septum are found, 

 as has been already stated, the genital organs (fig. 18, gen.), 

 and here also the comparison with Peripatus may be insti- 

 tuted. We find a similar agreement to that which existed in 

 the other regions compared, and we may with a considerable 

 degree of probability again homologise corresponding parts of 

 the two forms. The genital organs of Palsemonetes must 

 then be regarded as the representatives in this region of the 

 coelom. 



It is worthy also of note that the genital ducts, as seen in 

 young adults of Paleemonetes, agree in all their relations 

 with those of Peripatus, and the probability of their being 

 derived from nephridia, as suggested by Lankester (No. 15), is 

 very great. 



If the homologies here suggested are valid, the body-cavity 

 relations of the Crustaceans under consideration may be stated 

 briefly thus : — Both enterocoele (true coelom) and pseudocoele 

 are present, the enterocoele consisting of the dorsal sac, the 

 green gland and shell-gland, together with the genital organs 

 and their ducts, whilst the pericardial septum may be regarded 

 as equivalent to the fused walls of another portion of the same 

 structure (cf. Korschelt u. Heider, No. 14, p. 901). The 

 pseudocoele consists of the heart and arteries, the pericardial 

 cavity, the central cavity of the thorax, with the lateral 

 cavities and the cavities of the limbs, and the various sinuses 

 of the abdomen. The whole of the pseudocoele is filled with 

 blood, and hence can be termed a hsemocoele. 



A few considerations may now be added with regard to that 

 portion of the coelom which persists as the dorsal sac. The 

 most striking feature of this coelom is the late period of de- 

 velopment at which it opens; but it may be pointed out that 

 the cavity of the green gland (both of its tube and end-sac), 

 which we also regard as coelomic in nature, begins to appear 



