328 GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



tissue of the primary body-cavity, a condition which is still passed 

 through by the embryos of the Mollusca, in which the heart arises 

 independently of the vessels (pp. 77 and 216). 



The blood-vessels must have first arisen when the gills developed. 

 These later, in any case, arose as very simple leaf -like or tubular out- 

 growths of the body-wall, such as are found as the most primitive 

 gills in the Annelida, the Arthropoda and the Echinoderma. An 

 increase of surface soon took place and led to the bipectinate gill, 

 the so-called ctenidium so characteristic of the Mollusca. This gill 

 was paired, i.e., a ctenidium was found on either side of the body 

 concealed in a cavity formed by a fold of the integument. The fold 

 is the mantle which grew out on each side from the back, and de- 

 veloped simultaneously with the shell and the gills. As the primitive 

 Mollusc we must thus imagine an animal somewhat flattened dorso- 

 ventrally, whose back was covered by a bowl -shaped shell while its 

 ventral side formed a muscular and slightly projecting creeping sole. 

 Beneath the lateral parts of the shell lay the mantle enclosing the 

 pallial cavity and within this the gills. On the pre-oral part of the 

 body (the head) there were perhaps also the eyes and two tentacles, 

 corresponding to the cephalic feelers of the Archi- Annelida. In the 

 oesophagus, the radular sac with the radula became differentiated as 

 outgrowths, these occurring even in the most primitive of known 

 Molluscs (Amphineura, Solenogastres). The anus lay at the posterior 

 end, the nephridia opening out at either side of it. These latter 

 opened inwardly into the coelomic sacs with which the genital glands 

 were connected. The two coelomic sacs, dorsally to the intestine, held 

 the heart between them. The primary body-cavity was traversed 

 by mesodermal tissue, which became differentiated into connective 

 tissue and muscles. 



From such a simple form of Mollusc can be derived the types 

 represented in the different divisions known to us. Nearest to it 

 stand the Chitones, the characteristics of which have been drawn upon 

 for the above description. The somewhat aberrant conditions of the 

 Amphineura have already been described above (p. 326). After the 

 Chitones come the most primitive Gastropods (Diotocardia), the 

 Chitones themselves being for a long time regarded as Gastropods. 



The foot of the Gastropoda, apart from exceptions to be mentioned 

 presently, has the primitive form of the creeping sole. The marked 

 development of the head which carries the tentacles and eyes is 

 characteristic of most Gastropoda. The shell also has attained higher 

 development and is a constant feature of the Gastropoda ; where it 



