SCYLLIUM 53 



to the intestine. In the U-shaped bend which the stomach 

 makes with the intestine lies the pancreas, the duct from which 

 enters the intestine close to the bile-duct. The intestine bends 

 backwards and runs straight to the rectum, which has a small 

 diverticulum (the rectal gland), and leads to the cloaca. The 

 gut is considerably longer than the distance from the mouth 

 to the cloaca, and the " slack " is accounted for by the 

 asymmetry of the stomach. This asymmetry persists through 

 all the higher vertebrates. The internal surface of the 

 intestine is increased by a fold forming the spiral valve. 



Coelom and Myotomes. — The gut is suspended in the 

 splanchnocoel by a dorsal mesentery. Anteriorly the splanch- 

 nocoel is almost completely cut off from the cavity of the 

 pericardium by the transverse septum, which leaves only small 

 pericardio-peritoneal canals. Posteriorly, the splanchnocoel 

 is in communication with the exterior by the pair of abdominal 

 pores. 



The series of somites is not complete. The first three 

 give rise to the eye-muscles, but the myotomes of the fourth 

 and fifth segments disappear during development, leaving the 

 sixth to form the first complete myotome. Each myotome is 

 divided into a dorsal and a ventral portion by a horizontal 

 septum. Into this septum the ribs (known as dorsal ribs) 

 extend. The ventral portions of the most anterior myotomes 

 send muscles forwards beneath the pharynx, in the midventral 

 line. These hypoglossal muscles lose connexion with their 

 original myotomes, and connect the ventral ends of the skeleton 

 of the visceral arches with the coracoid region of the pectoral 

 girdle. The fins contain muscles attached to the radials. 

 These muscles are derived from the myotomes. All muscles 



Fig. 27. — Scyllium : ventral view of dissection showing the alimentary 

 system and the afferent branchial vessels (male). 



ab 1 and 5, first and fifth afferent branchial artery ; bd, bile-duct ; cl t 

 clasper (present in the male) ; /, fold overlying groove running from the 

 nasal sac to the mouth ; gi, first gill-slit ; 1, intestine ; /, liver ; m, mouth ; 

 n, nasal sac ; p y pericardium ; pg, pectoral girdle cut ; pn, pancreas ; pv, 

 hepatic portal vein ; r, rectum ; rg, rectal gland ; s, stomach ; sp, spleen ; 

 sv, sinus venosus ; t, testis ; th, thyroid gland ; ug, urinogenital papilla ; 

 v, ventricle of heart ; va, ventral aorta. 



