41 6 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, ij). 

 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 



r 



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

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

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

 distal labial cartilage; 9, lingual cartilage; 10, branchial basket; //, 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 (j, 4) which form a roof-like support 

 for the buccal funnel, a lingual cartilage (9) supporting the 

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

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

 work (10) 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 (/, t 1 ) 

 is moved by large muscles (p.m.t, r.jn.t.} y 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, 

 0./.) 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, 



