CHAPTER XVI 



THE PHYLUM MOLLUSCA 



Unsegmented coelomate animals with a head (usually well developed), 

 a ventral muscular foot and a dorsal visceral hump ; with soft skin, that 

 part covering the visceral hump (the mantle), often secreting a shell 

 which is largely calcareous and produced into a free flap or flaps to 

 enclose partially a mantle cavity into which open the anus and the 

 mesoblastic kidneys (usually a single pair) ; a pair of ctenidia (organs 

 composed of an axis with a row of leaf-like branches on each side, 

 contained in the mantle cavity, originally used for breathing) ; having 

 an alimentary canal usually with a buccal mass, radula and salivary 

 glands, and always a stomach into which opens a digestive gland or 



«^:f?- vm. 



pa^ 



pcd.g. 



Fig. 348. Comparison between annelidan and molluscan organization. Side 

 views of A, post-trochosphere larva of Annelida with segmenting trunk; 

 B, veliger larva of Paludina (Mollusca) before torsion. After Naef. Note 

 the general resemblance especially between the ventral nerve cord of A and 

 the ventral ganglia and cords of B. an. anus; brn. brain or suprapharyngeal 

 ganglion of annelid; ce.g. cerebral ganglion of Mollusca; F. foot; M. mouth; 

 ma. mantle; ped.g. pedal, pl.g. pleural, pa.g. parietal, sbg. subpharyngeal, 

 vis.g. visceral ganglia; vm. velum. 



hepatopancreas ; with a blood system consisting of a heart, a median 

 ventricle and two lateral auricles, arterial system and venous system 

 often expanding into a more or less extensive haemocoele, with haemo- 

 cyanin as respiratory pigment; a nervous system consisting of a 

 circumoesophageal ring, often concentrated into cerebral and pleural 

 ganglia, pedal cords or ganglia and visceral loops; coelom, varying 

 in development, but always represented by t\iQ pericardium, the cavity 

 of the kidneys (which communicates with the pericardium), and the 

 cavity of the gonads; often with larvae of the trochosphere type. 



While we do not know exactly what the ancestral molluscs looked 

 like, we can make a very shrewd guess at their structure. They 



