SCYLLIUM CANICULA 263 



Sense Organs. 



In addition to the sense organs dealt with in the frog, and 

 which will be referred to again briefly here, namely, the olfactory, 

 the gustatory, the tactile, the optic and the auditory, the dogfish 

 possesses also a series of cutaneous sense organs. These are widely 

 distributed over the body, and are characteristic of fish in general. 

 Of such organs two distinct varieties are present, those termed the 

 taste buds or end buds, and the sensory organs of the lateral line, the 

 neuromas ts. 



The end buds are very similar to the taste corpuscles of the higher 

 Craniates. They consist of a number of long rod-like sense cells 

 aggregated together in a characteristic manner about a central cell, 

 each possessing a hair-like process projecting above the general 

 level of the epidermis. Closely connected with their deeper ends 

 are arborisations of sensory nerve fibres coming entirely from the 

 facial, glosso-pharyngeal and vagus nerves. In the air-breathing 

 Craniates such organs are confined to the buccal cavity, whereas 

 in the dogfish they are spread irregularly over the surface of the 

 head and branchial region, and in the case of some bony fishes they 

 are even to be found on the body and at the bases of the fins. They 

 not only subserve the function of taste, as we understand it in the 

 higher animals, but, when situated outside the buccal cavity, also 

 inform their possessor when its proper food is near at hand. 



The neuromasts are composed of fewer sensory cells, but each 

 individual cell is larger and somewhat pear-shaped. As in the end 

 buds, the cells have a hair-like sensory process. These organs are 

 supplied exclusively by fibres from the lateralis nerves, and are 

 always situated below the external surface of the body and covered 

 by a fluid or semi-gelatinous substance in which their processes lie. 

 The ampullae are arranged in definite groups on the head and 

 consist of fairly deep tubes swelling out into a chamber at their 

 inner end in which the sensory cells are situated. The lateral line 

 organs are to be found in a series of canals of which the most con- 

 spicuous is the lateral line itself. In addition to this, which as 

 noted previously, runs from just behind the spiracle right down the 

 tail, there are also a canal above the eye, the supra-orbital, one below 

 the eye, the infra-orbital, one in the hinder dorsal part of the head, 

 the occipital, and one in the neighbourhood of the hyoid arch and 

 lower jaw, the hyomandibular canal. 



Eye. 



The eye in Scyllium calls only for brief notice, since it is in 

 the main similar to that of the frog, the differences being in detail 



