INCISUEA (sCISSURELLA) LYTTELTONENSIS. 29 



The otocysts occupy the usual position on the dorsal surfaces 

 of the pedal ganglia and present no unusual features (fig. 9), 



The osphradia are strips of modified epithelium running for 

 some little distance along the lower side of the gill-axes in 

 front of theosphradial ganglia and just ventral to the osphra- 

 dial or branchial nerve (fig. 16). They are very similar in 

 structure and position to the osphradia ofFissurellagrseca. 



Sense-papillffi occur not only on the cephalic tentacles but 

 also on the epipodial tentacles^ all round the margins of the 

 mantle and on the cirrhi bordering the mantle-slit. Those on 

 the cephalic tentacles are by far the largest, those on the 

 margins of the mantle are very minute, but all have essenti- 

 ally the same structure. Fig. 28 represents a longitudinal 

 section through three of the papillae of the cephalic tentacles. 

 Each papilla is a conical projection of the integument of the 

 tentacle and is composed of a number of elongated cells of 

 two kinds, closely packed together like the cells in a taste- 

 bud from the human tongue. The larger cells with larger, 

 pale nuclei are evidently supporting cells, their characters 

 being similar to the adjoining epithelial cells. The more 

 slender, finely granular cells with smaller, deeply staining 

 nuclei are the sense-cells, and each ends in a short stiff 

 cilium projecting from a small cup-shaped depression at the 

 end of the cone. According to Vayssiere these cilia are in 

 constant movement in the living animal. The tentacles of 

 Fissurella are clothed with a vast number of minute papillas 

 giving a velvety texture to the surface. These papillae, 

 though not so highly specialised, have each a single apical 

 sense-bulb, the structure of which is similar to that of the 

 sense-papillte of Incisura. 



Finally, mention may be made of the pedal glands. The 

 anterior pedal gland consists of a mass of unicellular glands 

 lying in the h^mocoele below the buccal bulb (fig. 7, p. gl.). 

 It extends back nearly as far as the pedal ganglia. Ante- 

 riorly these glands become more deeply seated and pass into 

 the muscular mass of the foot, where they debouch into a 

 median ciliated duct (fig. 6) which runs forward and opens 



