,342 X<>pJmdia, $c., of Larva of Palsemonetes varians. [Dec. 8, 



union of two lateral cavities, which lay on either side of the aorta, 

 the differences between the two forms are very slight. The dorsal 

 sac represents the two dorsal portions of the mesoblastic somites, 

 whilst the central cavity, the lateral cavities, and the nephridia 

 a^ree, with the one exception that the two lateral portions of the 

 nerve cord of Peripatu* have united in the middle line in Palcemnnetes, 

 and in the process have passed out of the lateral cavities. The agree- 

 ment is so close that it appears to me to be fully justifiable to homo- 

 logise the various parts. If this be so, it follows that the dorsal sac 

 of Palcemonetes is homologous with the dorsal portions of the meso- 

 blastic somites of Peripahis. and that its cavity is a true coelom. The 

 central and lateral cavities, together with the cavities of the legs, will 

 represent the pseudoccele, and being filled with blood may be termed, 

 with Lankester, hcewwccele. 



Passing now to the posterior part of the thorax, the region of the 

 heart, we find that the different cavities correspond with those which 

 per*iLt in the adult Peripaius. Heart, pericardium, and pericardial 

 septum of Palcemonetes present exactly the same relations as in Peri- 

 patus, ard are clearly homologous structures in the two animals. The 

 central and lateral cavities only differ on account of the relative 

 position of the nervous system, and this difference has already been 

 accounted for. It must be noted, however, that in this region no 

 nephridia are found in the cavities of the limbs. Beneath the 

 anterior end of the pericardial septum are found, as has been already 

 stated, the genital organs, and here also the comparison with Peripatus 

 may be instituted. 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. The 

 genital organs of Palcemonetes must then, be regarded as the repre- 

 sentatives, in this region, of the coelom. 



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

 tions of the Crustaceans under consideration may be stated briefly 

 thus : bo*.h enterocoele (true ccelom) and pseudoccele are present, the 

 enteroccele consisting of the dorsal sac, the green gland, and shell 

 gland, or the end sacs of these organs, together with the genital 

 organs and their ducts, whilst the pericardial septum may be regarded 

 as equivalent to portions of the walls of another part of the same 

 structure. 



The pseudoccele 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 pseudoccele is filled with blood, and hence can be termed a 

 haemocoele. 



I hope, shortly, to publish a more detailed account, with figures, 

 of the relations described in this communication. 



