DOG-FISH. • 1& 



which its surface is increased in area, all of which are richly 

 supplied by branches of the olfactory nerves. Posteriorly the 

 nasal sacs are closed, there being no communication with the-, 

 pharynx, as there is in Mixine and Lepidosiren. Two processes 

 project from the margin of each nasal opening towards one- 

 another, and divide the original simple aperture into an efferent 

 and afferent orifice. The olfactory sac is connected with the 

 mouth by the nasal groove, which extends from the sac to the 

 angle of the gape. This represents the nasopalatine canal, 

 of man. 



EYE. 



Notice that anterior segment of the bulb is very much flattened- 

 The sclerotic coat is cartilaginous in texture. The ciliary pro- 

 cesses of choroid are rudimentary. 



A highly vascular plexus — the choroid gla/nd—lies on outer 

 side of choroid. The upper and lower lids are represented by 

 two small moveable pieces. 



THE SPINAL COLUMN 



Consists of the persistent notochord, with rudiments of the 

 superior and inferior cartilaginous arches which grow around it,. 

 and so form cartilaginous circular centra. The entire verte- 

 bral column is divisible into not more than two distinct regions : — 

 an abdominal and caudal region. The abdominal vertebrae pos- 

 sess a superior or neural arch, through which the spinal cord' 

 passes, a superior spinous process — the neural spine, and a trans- 

 verse process on either side, to which the ribs are attached. There 

 are no marked transverse processes in the caudal vertebras, but 

 below the body of the vertebras are given off inferior or haemal 

 arches,, which carry inferior spinous processes- — the hasmal spines. 

 The extreme end of the spine is bent up so as to form an obtuse 

 angle with the trunk part of the spine, and the median fin-rays 

 which lie below it are much longer than those which lie above it, 

 causing the upper division of the caudal fin-rays to' be much 

 smaller than the lower. This kind of tail is known as hetero- . 

 cereal. 



The first vertebra articulates with £he skull by a single 

 median facet. 



The ribs are rudimentary, and are buried in the muscles. They 



do not enclose a thoracic cavity as in higher vertebrates, and there 



is no sternum. ^. ... . . ... „_ 



Digitized by Microsoft® 



