212 



COMPARATIVE ANATOMY 



In Proneomenia, the dorsal vessel is wanting in the region of the mid-gut. Tlie 

 co'lom is much less siiacious, and instead of surrounding tlie intestine lies only on 

 its dorsal side. It is developed merely as a herma^jhrodite glandular sac, its endo- 

 thelial wall yielding the genital products. 



In the region of the hind-gut, the vessel lying in the dorsal mesentery is develojjcd 

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



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



illustrate the relation of the cceloia 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, germinal epitlieliuin ; 



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



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



The pericardium is connected with the cloaca by two canals ; these may l)e 

 considered as the morphological equivalents of nephridia (t/. 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 ChitonidK, the ccelom is large, and falls into three distinct divisions. One 

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



Fig. 170.— Diagrammatic longitudinal section through Chiton, ti' illustrate the relation 

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

 plates; M, anterior portion of the dorsal integument; /,, snout; m, mouth; I, digestive gland 

 (liver) ; d, intestine ; /, foot ; n, kidney ; j), pericardimn ; c, portion of the ccelom surrounding the 

 intestine ; h, heart ; Ip, band connecting pericardium and genital gland ; gdr, genital gland ; la, band 

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



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

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

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

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

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



