SCYLLIUM CANICULA 265 



join up on the dorsal side, and open by a common aperture, while 

 the posterior canal takes the form of an almost complete ring com- 

 municating with the utriculus by a single opening. The ductus 

 endolymphaticus, coming off from the sacculus, runs dorso-mesially 

 to open on the dorsal surface of the skull by an aperture situated in a 

 depression at its hinder end. When treating of the cranial nerves 

 we saw that the enervation of the ear and lateralis system was 

 closely connected. Furthermore, the actual structure of the 

 sensory cells in the ear is practically identical with those of the 

 neuromasts, and they function in the same way, for in each the actual 

 receptive process is surrounded by liquid, the vibrations of which 

 bring about its stimulation. Lastly, when we come to study the 

 development of the embryo, it is found that the ear and the lateral 

 line system all start from a peculiarly modified patch of thickened 

 ectoderm in the hind brain region, so that we consider the ear as 

 simply a highly-specialised portion of the lateralis system. 



This, then, completes our survey of the dogfish, a simple 

 vertebrate animal eminently adapted for life in the water, and one 

 that exhibits, in spite of certain modifications, the main plan of a 

 Chordate in a little specialised form. It is further of interest, since 

 the anatomical relations of certain of its organs, blood-vessels, etc., 

 are closely approached in the developing embryo of the higher 

 animals, including man himself. 



