pectoral girdle 



iliac process 



pubic process 



clasper 



^^^^^^^5:^ 



HYDROLAGUS 



preaxiol rodia 



(propterygium) 



anterior margin 



SQUALUS ACANTHIAS 



Figure 6-66. Pectoral and pelvic girdles and fin bases of Hydrolagus, A and C, and Squalus, B 

 and D. 



tons. The branchial arches are formed from ectomesoderm; 

 this is an origin not even suggested for the scapulocoracoid. 

 The resemblance between girdle and arch is superficial and 

 misleading. The crowning difficulty is the position of the 

 pelvic girdle, which only secondarily, as in some teleosts, be- 

 comes closely associated with the head and pectoral girdle. 



RIBS AND STERNUM 



The ribs have already been mentioned in regard to the 

 vertebral column but a summary is desirable. There are 

 two main questions with regard to these structures: (1) what 

 is their relationship to the vertebrae and (2) do all verte- 

 brates have comparable ribs? The answer to the first ques- 

 tion can be derived from the observation of the continuity 



between the connective tissue sheaths of the neural tub)e and 

 notochord, the somatopleure of the body cavity, the dorsal, 

 ventral, and horizontal septa, and the myosepta. This basic 

 interrelationship suggests that, like the neural and ventral 

 arches, the rib is just a part of the stress reaction. Unlike 

 the neural or hemal spines, which secondarily lose their 

 myoseptal relation due to the folding of the myomeres, the 

 rib retains this relationship. 



In some of the fishes, ribs arise at the intersection with 

 the horizontal septum as well as along the somatopleure- 

 myoseptum union. Those of the horizontal septum are the 

 dorsal ribs, those of the somatopleure the ventral or pleural 

 ribs. The points of attachment of these two ribs are not 

 fixed but are related to the form of the body cavity. If this 

 cavity extends up around the ventral half of the centrum, 



RIBS AND STERNUM • 187 



