I58 THE PINEAL ORGAN 



organs of Chiton somewhat resemble the sensory organs of the lateral 

 line system of vertebrates, but they differ in certain respects and are 

 considered to have been evolved independently and not to be homologous. 



Fig. 116. — Trochophore Larva of a Chiton, showing Apical Tuft of Hairs 

 and Pair of Ocelli. (After Kowalewsky.) 



The Eyes of Gasteropods 



The gasteropods are univalve shellfish and they include such well- 

 known types as the periwinkles, whelks, snails, and slugs. In most cases 

 the eyes, which are of the simple upright type, are situated on tubercles 

 at the bases of the tentacles or appear as slight projections near the middle 

 of the tentacle (Fig. no), but in the snails and slugs the eyes are borne 

 on the ends of a second pair of tentacles, which are longer than and are 

 placed behind the first pair (Fig. in). The eyes of all are developed 

 from pit-like depressions of the epidermis, and since the developmental 

 stages of the more complex types are indicated by the adult form of the 

 lower and simpler types, we shall consider the latter first. 



1. Patella, the Common Limpet (Fig. 33, Chap. 3, p. 48). — Each eye 

 consists of a pit-like depression of the cuticle. The cells lining the pit 

 are elongated and continuous externally with nerve-fibres, which join 

 to form the optic nerve. The inner ends of the cells which are directed 

 towards the hollow of the pit are clear and refractile ; the central part of 

 each cell is pigmented, while the outer part which contains the nucleus is 

 clear. 



2. Trochus. — The spiral shell of this mollusc is conical in form and 

 since when the outer covering is removed a bright pearly surface is 

 exposed, it is much used for decorative purposes. As in the former type 

 the eyes are formed by a depression of the cuticular epithelium. They 



