GILLS AND HEART 325 



only the oral cavity arises from the stomodaeum and the 

 cloaca from the proctodaeum. 



In the skin covering the jaws dermal ossicles of unusual 

 size are developed and constitute the teeth (T). The chief 

 digestive glands are two in number, an immense liver (Lr) 

 occupying the whole anterior and ventral region of the 

 ccelome, and a small pancreas (Pn), attached to the anterior 

 end of the intestine. The ducts of both glands open into 

 the intestine, and their secreting cells are, as in former cases, 

 endodermal. Gland-cells are also found in the walls of 

 the stomach and intestine. 



The respiratory organs or gills (B) consist of five pairs of 

 pouches opening on the one hand into the pharynx (P/i) 

 and on the other to the exterior by the branchial clefts 

 already noticed : they have their walls raised into ridges, 

 the branchial filaments (Br. fil) which are covered with 

 epithelium and are abundantly supplied with blood-vessels. 

 The gills are developed as offshoots of the pharynx, and the 

 respiratory epithelium is therefore endodermal, not ecto- 

 dermal as in the crayfish and mussel. 



The heart (fit) lies below the pharynx in a separate 

 anterior compartment of the ccelome, the pericardial cavity. 

 It is composed of four chambers arranged in a single longi- 

 tudinal series (sinus venosus, auricle, ventricle, and conus 

 arteriosus), and is to be looked upon as a muscular dilatation 

 of a ventral blood-vessel. The blood is propelled by the 

 heart from the conus arteriosus into a paired series of 

 hoop-like vessels (aortic arches) resembling the transverse 

 commissures of Polygordius (Fig. 69, A, p. 270), which take 

 it through the gills and pour it, in a purified condition, into 

 the dorsal vessel (dorsal aorta, D. Ao) whence it is taken to 

 all parts of the body to be finally returned by thin-walled 

 vessels, called veins, to the sinus venosus. The ventral 



