212 



COMPARATIVE ANATOMY 



CHAP. 



In Proneomenia, the dorsal vessel is wanting in the region of the mid-giit. The 

 ca-lom is much less spacious, and instead of surrounding the intestine lies only on 

 its dorsal side. It is developed merely as a hermaphrodite glandular sac, its endo- 

 thelial wall yielding the genital products. 



In the region of the hind-gnt, the vessel lying in the dorsal mesentery is developed 

 as a heart, the coeloni being here represented by tlie pericardium. 



Fig. 175.— Diagrammatic sections througli an Annelid (A) and a Solenogastrid (B and C), to 



illustrate the relation of the ccelom to the genital glands and nephridia. B, Region of the cloaca ; 

 C, region of the mid-gut ; 1, dorsal mesentery ; 2, dorsal vessel or heart ; 3, genninal epitlieliuni ; 



4, crelom — in B = pericardium, in C = hermaphrodite gland (in the ccelom are genital products); 



5, nephridia ; 6, intestine ; 7, cloaca. 



The pericardium is connected with the cloaca by two canals ; these may be 

 considered as the morphological equivalents of nephridia {cf. Fig. 175). 



As the genital glands have been recognised as part of the ccelom in the Soleno- 

 gastres, Nautilus, and Sepia, they must necessarily fall under the same category in 

 all other Molluscs, even when no longer in direct connection or in open communica- 

 tion with the same. 



In the Chitonidw, tlie ccelom is large, and falls into three distinct divisions. One 

 contains the intestine and digestive gland (liver), whicli are accordingly outwardly 



Fig. 170.— Diagrammatic longitudinal section througli Chiton, to illustrate the relation 

 between the various parts of the ccelom (after Haller). 1-8, Position of the eight dorsal shell- 

 plates ; il7, anterior portion of the dorsal integument; L, snout; m, mouth; I, digestive gland 

 (liver); d, intestine;/, foot; n, kidney; p, pericardium; e, portion of the creloin surrounding the 

 intestine; ft, heart; /p, band connecting pericardium and genital gland ; grt?/-, genital gland; ?«, band 

 connecting the genital gland and the posterior portion of the creloni which surrounds the intestine. 



(i.e. on the side turned to the crelom) covered with an endothelium. The mesen- 

 teries, however, which originally attached tlie intestine to the body wall, and 

 along which the parietal endothelium passed into the visceral endothelium of the 

 intestine and liver, have disajipeared, with the exception of portions retained on 

 the hind-gut. The two other divisions of the cicloin are : (1) the pericardium, and 



