240 On the Larva 0/ Palsemonetes variant. 



Claus *. There are two main sinuses, which run along its 

 length : a dorsal sinus, in which the intestine lies, and a ven- 

 tral one, which contains the nerve-cord. These two sinuses 

 are generally separated by masses of muscle ; but they 

 communicate at intervals by means of lateral sinuses. 



Theoretical Considerations. 



The state of the body-cavity in the anterior region of the 

 thorax of Palcemonetes may be compared with that of Peri- 

 patus, as described by Sedgwick f, at the time when the dorsal 

 portions of the mesoblastic somites have attained their maxi- 

 mum development. Bearing in mind that the dorsal sac of 

 Palcemonetes has been formed by the 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 repre- 

 sents the two dorsal portions of the mesoblastic somites, whilst 

 the central cavity, the lateral cavities, and the nephridia 

 agree, with the one exception that the two lateral portions of 

 the nerve-cord of Periimtus have united in the middle line in 

 Palcemonetes, and in the process have passed out of the lateral 

 cavities. Tlie agreement is so close that it appears to me to 

 be fully justifiable to homologize the various parts. If this 

 be so, it follows that the dorsal sac of Palcemonetes is homo- 

 logous with the dorsal portions of the mesoblastic somites of 

 Peripatus and that its cavity is a true coelom. The central 

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

 represent the pseudocoele, and, being filled with blood, may 

 be termed, with Lankester, hcemocoele. 



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

 of the heart, we find that the different cavities correspond with 

 those which persist in the adult Peripatus. Heart, peri- 

 cardium, and pericardial septum of Palcemonetes present 

 exactly the same relations as in Peripatus, and 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 



* Claus, 0., " Zur Kenntniss der Ki'eislaufsorgane der Schizopoden und 

 Decapoden," Arb. Zool. Inst. Wien, vol. v., 1884. 



t Sedgwick, A., " The Development of the Cape Species of Peripatus : 

 Parts I.-IV.," Quart. Journ. Micr. Sci. vols, xxv.-xxviii., 1885-88. 



