220 



Give a Description of the Skeleton of Raid and Scyllium. 



The skeleton is mainly cartilaginous. In Raia the 

 vertebral column consists of an unsegmented anterior plate 

 and a segmented posterior portion. The vertebrae (amphi- 

 coelous) have dorsal neural arches, and either lateral trans- 

 verse processes with rib rudiments, or (in the caudal region) 

 ventral haemal arches. 



The skull (hyostylic) has (in Raia} a prolonged rostrum, 

 which (in Scyllium) consists of three bars converging in 

 front. Two knob-like prominences (condyles) below the 

 foramen magnum serve for articulation with the vertebral 

 column. There are seven visceral arches, the posterior 

 five (primarily four-jointed) form the framework of the 

 gill-cleft region. There are also accessory parts (e.g., nasal 

 cartilages about the openings of the olfactory capsules). For 

 description of skull and visceral arches see pages 198, 199. 



The pectoral girdle, behind the last branchial arch, is a 

 hoop of cartilage incomplete dorsally ; its thick ventral 

 (coracoid) portion is (in Raia) attached to the anterior 

 plate, and at each side there are three articular facets for 

 the basal pieces of the fin. The pectoral fin has three 

 basal pieces, namely, the propterygium (large in Raia, very 

 small in Scyllium), the small mesopterygium and the large 

 metapterygium ; these bear jointed radials, which along 

 with horny fibres (the fin-rays) support the fin. The small 

 pelvic girdle consists of a bar of cartilage (pubis), not attached 

 to the vertebral column, and with two facets on each cud. 

 The pelvic fin has only one basal piece, the basi- or meta- 

 pterygium, which articulates with the posterior facet, and 

 which (in the male) is connected with the skeleton of the 

 clasper. 



Write an Account of the Alimentary Si/xlan of Hie ftkute and 



the Dogfish. 



The buccal cavity (stomodaeum) opens into the large 

 pharynx, in the walls of which are the gill-clefts. From the 

 pharynx a short gullet or oesphagus leads into the large 

 U-shaped stomach, which has a marked thickening, the 

 pylorus, at its junction with the intestine. The intestine 

 is short ; it consists of a small proximal portion (small 

 intestine or duodenum), a dilated and spirally-marked 

 colon (with an internal fold, the spiral valve), and a narrow 

 rectum ending in the cloaca. The rectum has a small 

 diverticulum, the rectal gland. The oosophageal a.ud 

 Tccfa,! portions of the gut are attached to the dorsal body 

 wall by mesentery. 



