416 



COLLEGE ZOOLOGY 



The Skeleton (Fig. 354). --The notochord of Petromyzon 

 persists as a well-developed structure in the adult (Fig. 355, nc\ 

 Fig. 354, 12). In the trunk region the notochord is supple- 

 mented by small cartilaginous neural arches (Fig. 354, jj). 

 Cartilaginous rays hold the fins upright. The organs in the head 

 are supported by a cartilaginous skull and a cartilaginous bran- 

 chial basket (10). 



The skull is very simple. Its principal parts, as shown in 

 Figure 354, are an annular cartilage (2) which holds the mouth 



FIG. 354. Lateral view of skull of Petromyzon mariniis. i, horny teeth; 

 2, annular cartilage; 3, anterior labial cartilage; 4, posterior labial cartilage; 

 5, nasal capsule; 6, auditory capsule; 7, dorsal portion of trabeculse; 8, lateral 

 distal labial cartilage; Q, lingual cartilage; 10, branchial basket; n, cartilag- 

 inous cup supporting pericardium ; 12, sheath of notochord ; 13, anterior 

 neural arches fused together. (From Shipley and MacBride, after Parker.) 



open, two labial cartilages (3, 4) which form a roof-like support 

 for the buccal funnel, a lingual cartilage (p) supporting the 

 tongue, an olfactory capsule (5), two auditory capsules (6), and 

 a cranial roof (7). The branchial basket is a cartilaginous frame- 

 work (zo) which supports the gill-sacs and the walls of the peri- 

 cardium (n). 



The Muscular System. - - The muscles of the body-wall are 

 zigzag myotomes (Fig. 355, d.m, v.m.). The tongue (/, f 1 ) 

 is moved by large muscles (p.m.t, r.m.tJ), and the buccal funnel 

 is supplied with a number of radiating muscles. 



The Digestive System. - - Petromyzon lives on the blood of 

 other animals. The expansion of the buccal funnel (Fig. 355, 

 o.f.) causes the mouth to act like a sucker and enables the ani- 

 mal to cling to stones or to fasten itself to fishes such as shad, 



