120 SUB-CLASS ELASMOBRANCHII. 



vided with a number of lamellate folds of the mucous mem- 

 brane, which are placed on their anterior and posterior walls 

 (except in the last tube, which has no branchial lamellae 

 on its posterior wall), and are attached through their whole 

 length (Fig. 118), not projecting freely as do the pectinate gill 

 processes of Teleosiei. In addition there is usually an anterior 

 tube leading outwards from the pharynx and opening exter- 

 nally on the dorsal surface close behind the eye. This opening 

 is called the spiracle and the tube itself must be regarded as 

 belonging to the series of brancliial tubes of which it is the first. 

 It differs from these, however, in never possessing branchial 

 lamellae, though it often has traces of these as a few small folds 

 of the lining of its anterior wall, which constitute the fseudo- 

 hranch or mandibular gill of these fishes. In the embryo long 

 filaments — the so-called external gills — project from all these 

 openings including the spiracle ; they are in reality externally 

 projecting internal gills. 



In Raji the spiracle is much larger than in Squali and it doubtless allows 

 of the entrance of water into the pharynx when the animal is lying flat upon 

 the ground or partly buried in sand. In Squali, in which it is very variable, 

 being sometimes absent and nearly always small, its function is not clear. 

 In some species in which it is very small it may be present or absent in 

 different individuals. It is sometimes present in embryos of forms in which 

 it is absent in the adult (Carcharias), but whether this is always the case 

 is not known. From the fact that it is smaller than the posterior branchial 

 apertures even at its first appearance (which is subsequent to that of the 

 others) it may be presumed that it is usually absent in such cases, but the 

 matter wants looking into. When it is absent in the adult and present in 

 the embryo, it is without projecting gill filaments in the embryo (Miiller); 

 In Scyllium, Pristiurus, Mustelus, etc., the spiracle gives off a diverticulum 

 to the auditory cartilage of the skull. 



The nasal apertures and mouth are almost always placed 

 on the ventral surface of the head (in Chlamydoselachus the mouth 

 is anterior and the nasal apertures are dorsal), usually at a 

 considerable distance from the front end. The nasal apertures 

 are frequently connected with the mouth by a groove, the oro- 

 nasal groove, and sometimes they are so close that their open- 

 ings are confluent with the mouth. In other cases they are at 

 some distance from the mouth and there is no oro-nasal groove. 

 The anus (cloacal opening) is placed between the pelvic fins, 

 and there are two abdominal pores, one on either side of the 

 anal opening, which lead into the body-cavity. 



