214 



COMPARATIVE ANATOMY 



CHAP. 



\ 19 



therefore no longer covered with endothelium. Nevertheless this canal system of 

 the Octopoda shows the same morphologically important characteristics as the ccelom 

 of the Dccapoda. There are, for instance, on each side three canals which open 

 together, one entering the renal sac, the second widening round the pericardial 

 gland (appendage of the branchial heart) to form a flask-shaped capsule, and the 

 third running to the genital gland to be continued into its wall. [In so far as in the 

 Octopoda the heart is excluded from the coelom, which has been reduced to the ' ' water 

 canal system," the reduction of this cavity has gone further in these Mollusca than 

 in any others, which all retain at least the heart in one portion of the ccelom, the 

 pericardium. 



In the Lamellibranchia and Gastropoda, the only part of the ccelom retained, 

 besides the 'genital glands, is the pericardium. The pericardium and the gonad are, 

 however, entirely separated 



from one another. In Lamelli- 

 branchs, there is in the peri- 

 cardium, besides the heart, a 

 part of the hind - gut which 

 traverses it ; in the Gastropoda 

 (except in those Diotocardia 

 in which the hind-gut pene- 

 trates the heart), only this 

 latter organ. Rarely (<\g. 

 Phyllirhoe) the auricle also 

 is excluded from the peri- 

 cardium. 



The pericardial gland is 

 found in most Mollusca. It 

 is a glandular differentiation 

 of the endothelial wall of the 

 pericardium, and perhaps, as 

 already suggested, shares the 

 excretory functions of the 

 kidney. Its position in the 

 pericardium varies, but it 

 seems in all cases shut off from 

 the blood vascular system, 

 with which it is, however, 

 functionally connected. Its 

 secretions or excretions must be discharged into the pericardium, and thence out- 

 wards through the kidney. 



Among the Prosobranchia, in the Diotocardia, the pericardial gland is found on 

 the auricle, its walls forming dendriform branched outgrowths into the pericardial 

 cavity, these being covered with pericardial endothelium. Where pericardial glands 

 are found in the Monotocardia, they lie on the wall of the pericardium itself. 

 Similar lobate formations occur among the Opisthobranchia, in Aplysia, and 

 Notarchus, on the anterior aorta which runs along the pericardial wall ; in Pleuro- 

 branchus and Plcurobranchcea on the lower, in Doridopsis and Phyllidcea on the dorsal 

 pericardial wall. The lateral furrows of the pericardium of Doris form niches, which 

 may again have accessory niches. These enlargements of the surface of the peri- 

 cardial epithelium have also been considered as pericardial glands. 



Pericardial glands are much more common among the Lamellibranchia than 

 among the Gastropoda, but are wanting in the most primitive forms (Nucula, 

 Anomia). The gland is usually of a rusty red colour, and occurs in two 



FIG. ITS. Eledone moschata. This figure corresponds with 

 Fig. 177 of Sepia (after G-robben). Si, Efferent duct of the 

 digestive gland; 17a, pericardial gland (appendage of the 

 branchial heart) ; 23, water canals. 



