166 COMPARATIVE ANATOMY 



4. Gustatory Organs. 



Folds and prominences found in the mouth in some divisions of 

 the Mollusca have been taken for gustatory organs, although there are 

 no physiological and hardly any histological grounds for this opinion. 

 The existence of so-called gustatory pits on a prominence in the buccal 

 cavity has been proved only in a few Chitonida' and Diotocardia (Haliotis, 

 Fissurella, Trochus, Turho, and Patella). This " gustatory prominence " 

 (which has been best examined in Chiton) lies on the floor of the 

 buccal cavit}^ close behind the lip. A few gustatory pits are found 

 in its epithelium, sunk somewhat below the surrounding epithelium. 

 They consist of sensory cells with freely projecting sensory cones, and 

 of supporting cells. 



On each side of the mouth in the Pulmonata lies an oral lobe, 

 and under its deep epithelium, which is covered by a thick cuticle, 

 lies a ganglion. Smaller ganglia are found in the small lobes at 

 the upper edge of the mouth. All these ganglia receive nerves which 

 radiate from a branch of the anterior tentacle nerve. These oral 

 lobes (Semper's organ) are considered to be gustatory organs. 



5. Subradular Sensory Organ of Chiton. 



In the buccal cavity of Chiton a subradular organ of unknown 

 physiological significance has been found. It is described as " a pro- 

 minence lying below and in front of the radula," and in shape re- 

 sembles two beans with their concave edges turned to one another, 

 the ends touching ; the space between them forms a channel into 

 which a small gland opens. Below this organ lie two ganglia, the 

 subradular or lingual ganglia (cf. section on the nervous system). The 

 epithelium of the subradular organ consists of green pigmented 

 ciliated cells and two kinds of sensory cells. A similar organ occurs 

 in Patella, but has not been thoroughly examined, and at the same 

 part in various Diotocardia there is a prominence, which, however, has 

 no sensory cells. The Scaphopoda also possess a subradular organ. 



6. The Sensory Organs on the Shell of Chiton. 



There are numerous organs definitely arranged on the shell of 

 the Chitonidai which have, no doubt correctly, been considered as 

 sensory, i.e. tactile organs (Fig. 143). They are called sesthetes, and 

 lie in pores on the tegmentum {cf. p. 39) ; they are club-shaped or cylin- 

 drical, and each carries a deep cup-like chitinous cap. Each megal- 

 sesthete gives off all round numerous fine branches or micrsesthetes, 

 each of which ends in a swelling which carries a small chitinous 

 cap. The body of the aesthetes consists principally of long cells like 

 glandular cells ; it is produced into a fibre which runs along the 

 base of the tegmentum, and from here passes together with the 



