DEVELOPMENT 327 



exterior, are the three most characteristic features of the 

 vertebrate phylum. 



The organs of sense are highly developed, and consist of 

 paired olfactory sacs, eyes, and auditory sacs situated in the 

 head, together with an extensive system of integumentary 

 organs. Their sensory cells are in every case ectodermal. 



The eggs are very large, and are impregnated within the 

 body of the female. In the common Dog-fish (Scyllium) 

 they are laid shortly after impregnation, each enclosed in a 

 horny egg-shell : in the Piked Dog-fish (Acanthias] and the 

 Smooth Hound (Mustelus) they are retained in the oviduct 

 until the adult form is assumed. 



