:288 



Mollusca. 



■of delicate, chitinoid teeth of various forms, with their points 

 projecting backwards. The teeth, in one row may be all similar, but 

 Tery frequently some differ from the others ; each row is definitely 

 .arranged, and a median tooth is usually present, on either side of which 

 the other teeth are symmetrically disposed. The successive rows are 

 generally similar. The anterior end of the radula is continually being 



Fig. 235. 



Fig. 236. 



Fig. 235. Portion of the radula 

 of a Cephalopod. — Orig. 



Fig. 236. Diagrammatic longitudinal 

 section of the mouth of a Gastropod, 

 fc jaw, mouth, r radula, rs radula sac, 

 sp oesophagus. — Orig. 



worn down and rubbed away ; the posterior end lies in a narrow sac, 

 the radula- sac, which is frequently very deep; new teeth are 

 formed at its end, and the radula is gradually pushed out from it. 

 Besides this organ, which is very characteristic of the MoUusca, there 

 •occur also within the cavity of the mouth other hard structures, 

 varying in form (also composed of a substance like chitin), which may 

 be termed jaws. 



The respiratory organs are for the most part gills of 

 various kinds, usually occurring in the mantle cavity, which on this 

 account may also be termed the branchial chamber. In some forms 

 gills are wanting, and the mantle chamber may then (as in the 

 Gastropoda) serve as a pulmonary organ ; from others special 

 respiratory organs are altogether absent. The vascular system 

 is for the most part well developed, although the blood flows partly 

 through spaces between the organs. The heart consists of one or 

 two (in the Nautilus as many as four) auricles into which the 

 blood flows from the gills (or pulmonary chamber), and a thick- 

 walled ventricle, which receives it from the auricles, and drives 

 it into the body. The venous blood collects in one or more large 

 ^spaces which supply the respiratory apparatus. The excretory 

 organs or kidneys are saccular, and have each two openings, 

 of which one lies on the surface of the animal, whilst the other 

 leads into the so-called pericardium, a portion of the body-cavity 

 ;Surrounding the heart. The number of the kidneys varies (one to 



