MOLLUSCA 



471 



possessed the molluscan characters given in the definition above and 

 they resembled the diagrammatic creature shown in side view in 

 Fig. 349 A. They had a head with tentacles, a flat creeping foot, a 

 conical visceral hump covered by a mantle which possibly contained 

 numerous calcareous spicules and not a complete shell, and a posterior 

 mantle cavity into which opened the median terminal anus, the common 

 apertures of the kidneys and the gonads, and which also contained the 



gxpe. pcd. ,.g„ 

 dig.gl '- ' 



ce.g. 



Fig. 349. Types of Mollusca. Side view. Partly after Naef. A, Ancestral 

 mollusc. B, Amphineura. C, Gasteropoda. D, Lamellibranchiata (Nucula, 

 a primitive type). The head-foot is stippled to contrast with the visceral 

 hump and mantle. The course of the alimentary canal is indicated by double 

 dotted lines. In A the mantle cavity has its original posterior position, in C 

 it has become anterior, while in B and D it has extended forward on both 

 sides of the body, becoming very spacious in D. a.a. anterior adductor 

 muscle; an. anus; au. auricle; ce.g. cerebral ganglion; ct. ctenidium; dig.gl. 

 digestive gland; g.coe. genital coelom; k. kidney; k.op. kidney opening; 

 M. mouth; ma. mantle; 77ia.c. mantle cavity; op. operculum; p.a. posterior 

 adductor muscle; pa.g., ped.g., pl.g. parietal, pedal, pleural ganglia; plm. 

 palp-lamella; p.pr. palp-proboscis; pcd. pericardium; p.v.c. pleurovisceral 

 (pallio visceral in B) commissure 'ysh.p. shell plates ; st. stomach ; ven. ventricle ; 

 vis.g. visceral ganglia. 



ctenidia. Internally there was a gut with a radula, a heart with a 

 median ventricle, two auricles and a pericardial cavity into which the 

 two kidneys opened. From this beginning diverged the different 

 groups which we know to-day. The chitons (Amphineura), which 

 have departed least from the ancestral structure, became elongated but 

 limpet-like forms (Fig. 349 B), their visceral hump being protected by 



