430 INTKODUCTION TO CRANIATA [CH. 



these are absorbed along with the glycerine, a reconstruction into 

 fat taking place in the intestinal epithelium. 



Intermixed with the tubules of the pancreas are groups of cells 

 known as the islets of Langerhans. These pour some substance 

 into the blood which enables the sugar, which is one of the principal 

 products of digestion, to be combined with other substances so as to 

 build up protoplasm. When the pancreas is attacked by disease, 

 and the "islets of Langerhans" destroyed, a malady known as 

 diabetes sets in. This is a discharge of large quantities of sugar 

 through the excretory organ accompanied by a wasting of the 

 tissues which leads to death. 



The intestine is always somewhat longer than the body. Hence 

 it must be to some extent looped or twisted (Fig. 226), though this 

 may express itself only in a slight curvature. A fold projecting into 

 it in some forms represents the typhlosole of the Urochorda and 

 is known as the spiral valve, since it shares in the twisting. In 

 the intestine the digested food is absorbed and transferred to the 

 blood-vessels and lymph-canals. The last portion of the intestine 

 is usually of larger diameter than the rest and is called, when thus 

 distinguishable, the large intestine. In it the indigestible 

 material is elaborated into faeces for expulsion by the anus. 



The blood system of the Craniate is distinguished by the posses- 

 sion of a large and well-developed heart, which, like 

 system. lat ry the heart of the Urochorda, is an enlargement and 

 specialisation of part of the ventral vessel. The space 

 in which it apparently lies really, into which it protrudes, is called 

 the pericardium, and is only an anterior part of the coelom shut 

 off from the rest by the development of a transverse septum. The 

 heart is constricted into four chambers, becoming successively more 

 thick-walled as we proceed forwards, and named, beginning from 

 behind, the sinus venosus, the atrium, the ventricle and the 

 conus arteriosus (Fig. 214). It is bent into an S-shape, so 

 that the sinus venosus is dorsal and posterior, the atrium dorsal and 

 anterior, the ventricle ventral and posterior, and the conus arteriosus 

 ventral and anterior. The conus arteriosus leads into the ventral 

 aorta, which gives off the arterial arches ; these are branches 

 which in the lower Craniates ascend between the gill-sacs and ramify 

 on their walls. From the gills the blood collects into epibranchial 

 vessels which join to form two longitudinal vessels on the dorsal 

 wall of the pharynx, the roots of the dorsal aorta. These unite 

 behind the pharynx into a single dor.sal aorta, giving blood to all 



