CHAPTER XVI 



THE PHYLUM MOLLUSCA 



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

 a ventral muscular/oo/ 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 



-^na. 



ped.g. 



Fig. 372. 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. Ali- 

 mentary canal shown by stippling, 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. 



kepatopancreas ; with a blood system consisting of a hearty 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 cir- 

 cumoesophageal ring, often concentrated into cerebral and pleural 

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

 in development, but always represented by the 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 

 possessed the molluscan characters given in the definition above and 



