V 



ZOOLOGY SECT. 



structures called the limh-girdles ; they occur in the portions of the 

 trunk adjacent to the appendages and serve for the articulation 

 of the latter. In the embryonic condition they are continuous 

 with the basalia and are probably to be looked upon as ingrowths 

 of the jDrimitive fin-skeleton (Fig. 766). The shoulder-girdle or 

 2)ectoral arch has primarily the form of paired bars, which may 

 unite in the middle ventral line so as to form an inverted arch. 

 Each bar — i.e. each half of the arch — furnishes a concave or convex 

 glenoid surface (Fig. 767, gl.) for the articulation of the pectoral 

 fin or fore-limb, and is thereby divided into two portions — a dorsal^ 

 or scapular region, above the glenoid surface, and a ventral or 

 coracoid region below it. The coracoid region is again divisible, in 

 all classes above Fishes, into two portions : an anterior, the j>ro-corrt- 

 coid (p. cor), and a posterior, the coracoid proper. Each of these 

 regions 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, OYpclcic arch, is very similar. It 

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

 ventral line, and are divided by the ctcctcdmlnm (Fig. 768, acth.), 

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

 or iliac region, and a ventral or puho-iscMal 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 ilitcm (IL) serially homologous with the 

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

 clavicle, and a postero-ventral 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 

 marroiv. 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. 760, A, hue. c), pharynx {ph.), gullet, stomach {st.), and 

 intestine {int.), the latter sometimes communicating with the 

 exterior by a cloaca {el.), which receives the urinary and genital 

 ducts. The buccal cavity is developed from the stomodaaim of 

 the embryo : the proctoda^um 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 



