266 THE ELASMOBRANCH FISHES 



dividual organ is well illustrated in Heterodontus, in figure 34d, where it is 

 seen as a papilla arising from the floor of the mouth. These organs often are 

 surrounded by a more or less circular group of stomodeal denticles. 



A section through such a taste bud shows it to be placed over a cup-shaped 

 base which looks something like the base of a placoid scale. The organ itself is 

 made up of cells elongated in a vertical direction. These cells are of two sorts. 

 One is a supporting cell and the other is a sense cell. 



Elasmobranch Eye 



Extremes to which the Elasmobranch eye may be developed may be exempli- 

 fied in two t^'pes like Isistius and Sqnatina. In the former, which is a deep sea 

 shark, the eye reaches a size which makes it a conspicuous structure standing 

 out from the sides of the head. In the latter, which is slow moving and noc- 



A B C D 



Fig. 232. Types of Elasmobranch eyes. (From Garman.) A. Parmaturus pilosiis. B. Triakis 

 henlei. C. Scoliodon. D. Carcharias milherti. 

 n., nictitating membrane. 



turnal, there is but little exact vision and the eye is therefore more or less 

 rudimentary. Between the two extremes are multitudes of types. 



In external view (fig. 232) the eye varies greatly in the type of its pupil 

 and the nature of its lids. The pupil may be large, denoting a type of eye un- 

 used to a great deal of light. Figure 233c of Spinax, a deep sea form, shows 

 such a condition. In Triakis henlei the pupil is smaller and assimies a hori- 

 zontal position, while in Scoliodon and CarcJiarias (fig. 232c and d) the pupil 

 is vertical. 



The eyelids, especially the third or nictitating membrane (/?., fig. 232), are 

 also marked characters in external view. In fact on this character alone the 

 Elasmobranehs were separated by J. Miiller into two groups: (1) those hav- 

 ing a nictitating membrane and (2) those devoid of it. To the former group 

 belong Galeus, Mustelus, etc., and to the latter Heptanchus, Acanthias, etc. 

 The nictitating membrane varies greatly in the degree of its development. In 

 a type like Carcharias (fig. 232d) it reaches an optimum development where it 

 can be drawn entirely over the eye. 



There has been considerable interest over the relation of the nictitating 

 membrane to the lower membranous lid. The question is : Is the nictitating 

 membrane the made-over lower lid with the present lower lid formed anew, or 

 does it arise from the lower lid .^ In Mustelus, at least, it has been shown by 

 Harman (1899) to arise as a ridge on the inner (ocular) side of the lower lid. 



