APPENDICULAR SKELETON FISHES 23 1 



it an anterior os pubis (often containing the obturator foramen) and 

 posterior to this an ischium, all three bones usually meeting in 

 the acetabulum. The ventral bones often inclose a large ischio-pubic 

 fenestra with which the obturator foramen may be united. Pubes 

 and ischia of the two sides may meet by symphysis in the mid- ventral 

 line, a symphysis of pubes being more common than one of ischia. 

 More or less cartilage may persist in the symphysis, and in this a 

 median epipubis may ossify in front, a h5rpoischiuin behind. Some- 

 times an acetabular bone occurs between ilium and pubis, this entering 

 the acetabulum and occasionally excluding the pubis from articula- 

 tion with the free appendage; usually acetabular bone and pubis 

 fuse. There is no distinct ventral bone corresponding to the epister- 

 num and no membrane bones are known in the pelvis. 



There is a close parallel between pectoral and pelvic girdles, especially in 

 Tetrapoda. Each has three rami in either half, one dorsal and two ventral to 

 the articulation of the limb, all preformed in cartilage, and each girdle with a 

 nerve foramen in either half. The resemblance is strengthened if presternum 

 be compared with either epipubis or hypoischium, accordingly as pubis or ischium 

 be regarded as equivalent to coracoid. 



Free Appendages 



The skeletons of the free appendages of fishes and Tetrapoda 

 differ more than do their girdles. The paddle-like paired fins (ptery- 

 gia) of fishes do not support the body, but act more like rudders, 

 while the podia support the weight and are also the chief locomotor 

 organs. The pterygia of fishes have numerous rods (radialia and 

 actinotrichia) to support a broad flexible membrane which does not 

 bend sharply. Hence the radialia are usually jointed, with limited 

 motion between the articles; the actinotrichia are flexible, and the 

 intrinsic muscles of the fin are scanty. Podia, on the other hand, 

 must be more slender, with fewer and firmer parts, the joints between 

 them allowing great flexion and extension, and there must be more 

 and stronger intrinsic muscles to cause motion of part on part and 

 to hold parts firmly when necessary. 



FISHES. — As the extent of differentiation of girdles and append- 

 ages of fishes is slight, and as pectoral and pelvic members differ but 

 little, all are treated here together. Usually the pelvic fin is smaller 

 than the pectoral, in correlation with its lesser use. Both girdles arise 

 from paired cartilages and the individuaHty of the halves persists 

 more often in the pelvis of Teleostomes than in Elasmobranchs, 



