368 



COMPARATIVE ANATOMY. 



body by taking' iu the water. By its aid, retracted parts can be 

 protruded, limp parts erected, and the whole musculature of the 

 body-wall, and especially that of the foot, is thus made very 

 important. The power of protruding certain parts, which have 

 been withdrawn into the shell, and especially the foot, depends on 

 these relations, which are definitely known to exist in the case of the 

 Lamellibranchiata and Gastropoda, as also in that of the Pteropoda. 

 As yet it is not definitely known whether water is taken into the 

 haemal system in the Cephalopoda. 



Vascular System. 



§ 284. 



In all the Mollusca, excepting the Scaphopoda, the vascular 

 system is arranged, in almost exactly the same way, so far as all 

 essential points are concerned. These are, first, the presence of a 

 dorsal longitudinal trunk, a portion of which is developed 

 into a central organ (ventricle) ; secondly, transverse 

 vessels are connected, with the longitudinal one, which, 

 when lateral gills are present, carry the blood from them 

 to the heart, and are further differentiated into organs 

 of circulation by becoming- auricles for the ventricle. 

 This dorsal development of the chief portions of the circulatory 

 system is a point in which they resemble the Vermes (cf. p. 108). 



The symmetrical arrangement of the auricles in Mollusca which 

 are in other points markedly divergent, shows us that this is a 

 peculiarity which lies at the very bottom of their organisation; 



Fig. 195. Diagram to show the relations of the Circulatory Centres in the 

 Mollusca. A Part of the dorsal vascular trunk and transverse trunks of a Worm. 

 B Heart and auricles of Nautilus, C of a Lamellibranch or of Loligo, D of 

 Octopus. E Heart and auricle of a Gastropod, v Ventricle, a Auricle, ac Arteria 

 cephalica. ai Arteria ahdominalis. The arrows show the direction of the current 



of blood. 



while the presence of two pairs of auricles, which open behind one 

 another into the ventricle, in the tetrabranchiate Cephalopoda, 

 points to a metameric arrangement of the vascular system, 

 just as much as does the presence of several transverse trunks in the 



