82 ZOOLOGY SEC*. 



commonly ossifies a replacing bone, the scapula (SCP), appearing 

 in the scapular region, another, the coracoid (COR), in the coracoid 

 region, while in relation with the pro-coracoid is formed a bone, 

 the clavicle (CL), largely or entirely developed independently of 

 pre-existing cartilage. 



The constitution of the hip-girdle, or pelvic arch, is very similar. 

 It consists originally of paired bars, which may unite in the middle 

 ventral line, and are divided by the acetabulum (Fig. 777, acib.), 

 the articular surface for the pelvic fin or hind-limb, into a dorsal 

 or iliac region, and a ventral or pubo-ischial region, the latter being 

 again divisible, in all classes above Fishes, into an anterior portion, 

 or pubis, and a posterior portion, or ischium. Each region is 

 replaced in the higher forms by a bone, the pelvic girdle thus 

 consisting of a dorsal ilium (IL) serially homologous with the 

 scapula, an antero-ventral pubis (PU) with the pro-coracoid and 

 clavicle, and a postero-ve'ntral ischium (IS) with the coracoid. 

 The long bones of the limbs are divisible each into a shaft, and 

 proximal and distal extremities. When ossification takes place the 

 shaft is converted into a tubular bone, the cartilaginous axis of 

 which is absorbed and replaced by a vascular fatty tissue -called 

 marrow. The extremities become simply calcified in the lower 

 forms, but in the higher a distinct centre of ossification may appear 

 in each, forming the epiphysis, which finally becomes ankylosed to 

 the shaft. 



Digestive Organs. The enteric canal is divisible into buccal 

 cavity (Fig. 769, A, buc. c.), pharynx (ph.), gullet, stomach (st.), 

 and intestine (int.), the latter often communicating with the 

 exterior by a cloaca (cl.), which receives the urinary and genital 

 ducts. The buccal cavity is developed from the stomodseum of 

 the embryo : the proctodseum gives rise to a very small area in 

 the neighbourhood of the anus, or, when a cloaca is present, to the 

 external portion of the latter ; all the rest of the canal is formed 

 from the mesenteron, and is therefore lined by an epithelium of 

 endodermal origin. The pharynx communicates with the exterior, 

 in Fishes and in the embryos of the higher forms, by the gill-slits 

 (i. br. a, 1-7) ; it communicates with the stomach by the gullet. 

 The stomach (st.) is usually bent upon itself in the form of a U ; the 

 intestine (int.) is generally more or less convoluted ; hence the 

 stomach and intestine are together considerably longer than the 

 enclosing abdominal cavity. In the embryo the intestine is some- 

 times continued backwards into the haemal canal by an extension 

 called the post-anal gut (p. a. g.), which may perhaps indicate that 

 the anus has shifted forwards in the course of evolution. 



The epithelium of the buccal cavity is usually many-layered, 

 like that of the skin, of which it is developmentally an in-turned 

 portion ; the pharynx and gullet have also a laminated epithelium, 



