16 LABORATORY .MANUAL FOR VERTEBRATE ANATOMY 



c) Notochord: The notochord is the broad, brown rod situated just dorsal to 

 the esophagus. It is the chief axial skeleton of the animal, the vertebral column 

 being very embryonic. 



d) Nervous system: Above the notochord is a narrow canal, the neural canal, 

 in which the spinal cord and brain are located. The spinal cord is a slender 

 cord occupying the canal; the brain is an enlarged lobed structure situated just 

 dorsal to the anterior extremity of the notochord. 



e) Olfactory apparatus: The section should bisect the olfactory aperture. 

 Note that a canal extends from this aperture and opens into a sac, the olfactory 

 sac, situated just anterior to the tip of the brain. The folds in the wall of the 

 sac bear the olfactory mucous membrane. From the anterior margin of the 

 olfactory sac a tube arises which bends ventrally and posteriorly and widens 

 into an elongated sac, the pituitary pouch, lying ventral to the anterior end of 

 the notochord. The peculiar dorsal position of the olfactory sac as well as its 

 relation to the pituitary sac (which in other vertebrates has no connection with 

 the olfactory cavities) are due to a shifting of these organs during development 

 from their original ventral position (see P and H, Fig. 799, p. 133). The 

 pituitary sac corresponds to the anterior lobe of the pituitary body or hypophysis 

 of other vertebrates. 



/) Pericardial cavity: Posterior to the last gill pouch is a somewhat conical 

 cavity, the pericardial cavity, within which the heart is situated. 



Make a detailed drawing of the sagittal section. 



For a complete account of the anatomy of cyclostomes consult P and H, 

 pages 123-44; CNH, Vol. VII, pages 150-52, 216-22, 247-48, 279-82, 371-73, 

 385-95; N, pages 87-94. 



E. EXTERNAL ANATOMY OF THE DOGFISH 



The dogfish or dog shark is a true fish, belonging to the lowest group of 

 fishes, the subclass Elasmobranchii. It is a very generalized vertebrate, and 

 hence a knowledge of its structure is indispensable for an understanding of 

 vertebrate anatomy. There are two common dogfishes usually obtainable for 

 laboratories, the smooth dogfish, Mustelus canis, and the spiny dogfish, Acanthias 

 vulgaris. These two species are slightly different in several respects; attention 

 will be called to these differences where necessary. 



^ 1. Body and skin.— The body of the dogfish has the shape and proportions 

 which we recognize as most advantageous for free-swimming animals— fusiform 

 (spindle shaped) and pointed at each end, thus offering little resistance to the 

 water. The body is divided into head, trunk, and tail, which are not, however, 

 distinctly bounded from each other. Trunk and tail are provided with fins for 

 purposes of locomotion. The body is clothed with minute scales, each of which 

 bears a tiny spine. Pass the hand over the skin of the dogfish and note the 



