418 EDGAE J. ALLEN. 



leg-cavities, which contain the nephridia and communicate 

 with (4).'^ 



We are now in a position to compare the body-cavities of 

 Palsemonetes and Peripatus. Considering first the ante- 

 rior region of the thorax of the former animal, as shown 

 in figs. 6 and 9, we may compare it with the condition of 

 things in Peripatus at the time when the dorsal portions of 

 the mesoblastic somites have attained their maximum develop- 

 ment. Bearing in mind that the dorsal sac of Paljemonetes 

 has been formed by the union of two lateral cavities, which 

 lay on either side of the aorta (cf. figs. 14, d, and 16, b), the 

 difi'erences 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 agree, with the one exception that the two lateral 

 portions of the nerve-cord of Peripatus have united in the 

 middle line in Palsemonetes, and in the process have passed 

 out of the lateral cavities. The fact that the internal end of 

 the nephridium lies in the lateral cavity, and not in the cavity 

 of the leg, which, as already stated, contains the greater part 

 of its tube, appears to agree with the condition of things 

 figured by Sedgwick for certain parts of Peripatus (No. 19, 

 pt. 4, pi. xxvii, fig. 11). 



The agreement is so close that it appears to me to be fully 

 justifiable to homologise the various parts. If this be so, it 

 follows that the dorsal sac of Palsemonetes is homologous 

 with the dorsal portions of the mesoblastic somites of Peri- 

 patus, and that its cavity is a^true coelom (enterocoele). 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 haemoccele. 



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

 of the heart (figs. 18 and 19), we find that the different cavi- 

 ties correspond with those which persist in the adult Peri- 

 patus. Heart, pericardium, and pericardial septum of Palse- 

 monetes present exactly the same relations as in Peripatus, 

 and are clearly homologous structures in the two animals. 



