12 



COMPARATIVE ANATOMY 



and reproductive organs, and in most forms contains gills. The foot is 

 muscular and usually is the chief organ of locomotion. The circulation 

 is not closed, but the blood passes through a system of lacunar spaces. 

 The dorsal heart is enclosed in a pericardium which is a part of the coelom. 

 Arteries and veins afford connexions with the gills. The excretory 

 organs bear a fundamental resemblance to the nephridia of annelids. 

 Respiration is by means of gills, which in air-breathing snails are enclosed 

 in a chamber with a narrow opening to form a sort of lung. 



The alimentary canal has a muscular pharynx and a stomach enlarge- 

 ment. The anus is posterior or is displaced forward in the mantle cavity. 



SALIVARY GL. 



GONAD 



HEART 



FOOT 

 GANGLION 



INT.GLAND INTESTINE 



NEPHRIDIUM MANTLE CAV 



MOLLUSC. 



Fig. 9. — Diagrams of a Mollusc. Figure A shows a longitudinal section, while £ is a 

 cross section. (Redrawn from Stempell, after Kuhn and Sedgwick.) 



The nervous system has a circumesophageal ring and a series of cerebral, 

 pedal and visceral ganglia with their commissures and connectives. 

 Sense organs are varied. Many molluscs are hermaphroditic, the gonads 

 opening into coelomic sacs. Development involves metamorphosis, and 

 the veliger larva resembles the trochophore larva of annelids. 



Sixty-one thousand species are recognized. 



The phylum contains a number of primitive or aberrant forms, among 

 which are the chitons with eight calcareous plates on the back instead 

 of a single shell, dentalium with a nearly straight tapering shell open at 

 both ends, and certain marine forms that are worm-like and without 

 shells. 



Though the molluscs as a group are highly successful in the struggle 

 for existence, as witness the multitude of their species, and the enormous 



