G8 



MOLLUSCA. 



eye and (c) twelve internal tentacles, the four ventral of which 

 on the left side are in the male modified to form the spadix, an 

 organ analogous to the hectocotylised arm. Finally, in the female 

 there are on each side, within the latter, fourteen or fifteen ventrally- 

 placed labial tentacles. (Fig. 542.) 



The cephalic cartilage, instead of forming a complete ring, con- 

 sists of two horse-shoe-shaped limbs on which the central parts of 

 the nervous system lie. The eyes are stalked, and are without a 

 lens or other refractile media. The funnel has the form of a lamina 

 rolled upon itself, but the edges are free and not fused. There is no 

 ink-sac. The branchiae are four in number as are also the branchial 

 vessels and the kidneys. 



K 



FIG. 542. Nvut,ilus(regne aniirml). T, tentacles ; P, pupil of the eye ; Ek, terminal 

 chamber ; Tr, funnel ; A", chambers of the shell ; S, siphon ; Ma, mantle ; M, 

 muscle. 



The hinder part of the thick external shell of the Tetrabranchiata 

 is divided by cross partitions into numerous chambers, which are 

 filled with ail 1 and are traversed by a siphon. The shell consists of 

 an external, frequently coloured calcareous layer, and an internal 

 mother-of-pearl layer. The similar structure of many fossil shells 

 allows us to infer a similar organisation for their unknown inhab'- 

 tants. The position and structure of the siphon, as well as the form 

 of the septa, and the lines of fusion of the latter with the shell 

 are important characters for the classification of the fossil Tetra- 

 branchiata. The small number of living species of the genus 

 Nautilus are found in the Indian and Pacific Oceans. 



