VERTEBRATE ANIMALS 211 



the spiracles, and are the openings of the endolymphatic ducts leading 

 down to the membranous labyrinth of the ear. ., 



Scattered over the head, particularly in the snout region, are a 

 number of small apertures leading down into tiny tubes, the mucous 

 canals, which are filled with a gelatinous substance that is extruded 

 if the head is squeezed. A similar series of apertures leads into a 

 regular series of canals above and below the orbits, and on the dorsal 

 and ventral sides of the hinder end of the head. These are connected 

 near the spiracle with a pair of canals that run along the whole 

 length of the fish, one on each side, in a shallow groove of lighter 

 colour, and so are easily discernible ; they constitute the so-called 

 lateral line canals and lodge a series of sense organs. 



As we have already noted, the vent or cloaca is situated 

 between the hinder internal margins of the pelvic fins. It is a short 

 slit leading into a shallow cloacal chamber into which open the hinder 

 end of the alimentary canal, the urinary and genital apertures. 

 The anus proper or termination of the gut opens into the front end 

 of the cloacal chamber. Further back are two small cavities, the 

 cloacal pits, partly overhung by a flap of skin, the cloacal papilla. 

 Anteriorly the pit ends blindly, but it is continued back as a minute 

 canal, the abdominal pore, leading into the coelom, which thus com- 

 municates directly with the exterior, a condition not found in the 

 higher craniates. A considerable amount of variation is met with 

 in this pore showing that it is of small functional importance, and 

 probably represents a vestigeal structure. Sometimes it is present 

 on one side only, sometimes on both sides, or again it may be absent 

 altogether. The two sexes have their urinary and genital apertures 

 differently arranged. In the male there is a small projection, the 

 urinogenital papilla, perforated by a single pore. It lies in the middle 

 line just behind the anus and in front of the cloacal pits, and serves 

 for the transmission of excretory and reproductive products. The 

 female possesses a structure similar in appearance, but serving only 

 for the passage of the urine and so constituting a urinary papilla. 

 The oviducal opening in the female is a fairly large longitudinal cleft 

 on the dorsal wall of the cloacal chamber between the papilla and 

 anus. It leads directly to the two oviducts, which run together just 

 at this point. 



Skin and Exoskeleton. 



The. skin of Scyllium, like that of Rana, is composed of 

 two layers, an outer, the epidermis, derived from the ectoderm, and 

 an inner layer, the dermis, of mesodermal origin. The epidermis 

 is a stratified epithelium with a layer of cubical actively dividing 

 cells, the Malpighian layer, at the base. As they pass outwards the 



