544 



THE INVERTEBRATA 



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 and the 

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



dig.gl. 



gxpe. pcd. ^„ 



M-c. 8h.p. 



ma.c 



ped.g. p.v.C. an. k.op. A 

 vis.g. 



-pa.g. 



ped-ij. B 



pig. vis.g. 



pcd.g.-l — 



ped.g. 



Fig- 373- 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 ; F. foot ;g.coe. genital coelom ; k. kidney ; k.op. kidney opening; 

 M. mouth; ma. mantle; ma.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 

 (palliovisceral in B) commissure ; sh.p. shell plates ; st. stomach ; ven. ventricle ; 

 vis.g. visceral ganglia. 



contained the ctenidia. In the alimentary canal the fore gut formed a 

 muscular body, the buccal mass, and a radula (p. 557) and the mid gut 

 an oesophagus, stomach and digestive glands and intestine. The heart 

 had a median ventricle and a pair of auricles. The perivisceral coelom 

 reduced by the development of an extensive haemocoele (p. 556) is 

 represented by the pericardium with which communicates in front 



