SENSE ORGANS. SKIN. 



121 



Abdominal pores vary considerably.* In some species they are absent 

 altogether, in others they are present in the adult, while in yet others 

 they are present in some individuals and not in others. Their external 

 opening is always on an ectodermal surface, either just outside the 

 cloacal boundary, or into a cloacal pouch, which is a diverticulum of 

 the proctodeal part of the cloaca. 



The eyes are usually provided with upper and lower cutaneous 

 folds which represent eyelids, and in some forms there is a third 

 inner eyelid or nictitating membrane which can be dra^vn over 

 the eye. 



The otocysts retain their communication with the exterior 

 by means of a canal, the 

 aqueductiLS vestibnli, which 

 opens on the dorsal surface 

 throughout life (p. 77). The 

 lateral line is a canal which 

 extends in the skin from the 

 very hind end of the body to 

 the head, where it branches 

 out to different parts m the 

 usual piscine manner (p. 80). 

 It opens to the exterior at 

 intervals. In addition to the 

 system of the lateral line, 

 there are the openings of the 

 so-called ampullary canals. 

 These are placed in groups in 

 the head (Lorenzini's ampul- 

 lae, p. 81). Luminous or- 

 gans t irregularly scattered 



over the body are found in many pelagic members of the 

 Spinacidae (e.g., Spinax, Laemargus, Isistius) in the form 

 of minute cutaneous patches which probably secrete a 

 luminous mucus. 



The skin is tough and rough owing to the presence of a vast 

 number of placoid scales. These are rhombic bony plates 

 embedded in the cutis and carrying a small spine, which 



* Bles, " Correlated Distribution of Abdominal Pores and Nephrostomes," 

 Journ. Anat. and Phys., 32, 1838, p. 484, 



t R. Burckhardt, "Luminous Organs of Selachians, Anyi. and Mag. Nat. 

 Uist. (7) 6, 1900, p. 558-568. 



Fig. 65. — Placoid scales of an adult SeyUium 

 in surface view (after Klaatsch). The 

 anterior end of the figure is uppermost. 

 The spines are omitted from some of the 

 scales. Ck the central canal (pulp cavity) 

 of the spine as it perforates the basa! 

 plate Sb of the scale ; Sa spine of the 

 scale. 



