VASCULAR SYSTEM OF MOLLUSCA. 369 



Annulate Vermes. These vessels still retain so much of their 

 primitive character that they are not called auricles, but branchial 

 veins. 



This homology of the two pairs of auricles with two transverse 

 trunks of a dorsal vessel (Fig. 195, A and B) points to a primitive 

 condition, which, while it is characteristic of the Nautilidas, agrees 

 also with the paheontological relations which these Cephalopoda 

 have to the rest of the extant members of their group. The presence 

 of but one pair of auricles appears, on the other hand, to be a degene- 

 ration (Placophora, Dibranchiate Cephalopoda, Larnellibranchiata, 

 and many Gastropoda), which corresponds to the reduction of the 

 gills. The key, therefore, to the comprehension of the mode of 

 formation of the ventricle and auricles in the Mollusca is to com- 

 pare them with more indifferent organs. When part of the dorsal 

 vessel is converted into the ventricle, the parts that are continued 

 from it form arterial trunks, and these, when they retain their 

 primitive course, are distinguished as anterior and posterior aortas 

 (aorta cephalica, et aorta intestinalis sive abdominalis). In some 

 of the Cephalopoda, the Octopoda (D), we meet with an important 

 change in position, for the trunk of the dorsal vessel has made a 

 loop-like turn, so that the two arterial portions (ae and at) run in 

 the same direction for a certain distance. The points at which they 

 arise from the ventricle are thereby approximated. The circu- 

 latory system of those Gastropoda, which are characterised by the 

 fact that only one arterial trunk is given off from the ventricle, 

 may be derived from a similar relation of parts (E). This single 

 arterial trunk divides into two branches, which are distributed in 

 just the same regions as are the two arterial trunks (ac and at), 

 which are given off from either end of the ventricle in the Cepha- 

 lopoda and Larnellibranchiata. It may, therefore, be regarded as 

 developed from the two arterial trunks, which were primitively 

 disposed along a single axis. The final reduction of the auricles 

 to one is due to a reduction in the number of the gills, and 

 is correlated with the union of the anterior and posterior arterial 

 trunks. 



The auricles and ventricle appear, therefore, to have been derived 

 from different portions of a primitive vascular apparatus, and point 

 to a metameric arrangement; taken in conjunction with the signs 

 of metamerism exhibited in the nervous system (p. 344), they lead 

 us to suppose that the ancestors of the Molluscan phylum were 

 segmented organisms. 



§ 285. 



The heart, in the Placophora and Larnellibranchiata (Fig. 196, v), 

 lies in the middle line of the body, just below the back, and is sur- 

 rounded by a pericardium ; it receives blood from two lateral 

 auricles (a) while the above-mentioned vascular trunks arise from it 

 anteriorly and posteriorly. In the Placophora the heart is placed a 

 good way back, so that the anterior arterial trunk is of a considerable 



2 B 



