xxvii GILLS AND HEART 329 



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 (Ph} 

 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 (Resp. Epthni) and are abundantly supplied 

 with blood-vessels. The gills are developed as offshoots of 

 the pharynx, and the respiratory epithelium is therefore 

 endodermai, not ectodermal as in the crayfish and mussel. 



The heart (Hi] 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. 282), 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 

 position of the heart and the fact that the blood is sent 

 directly from the heart to the respiratory organs are 

 characteristic vertebrate features : so also is the circumstance 

 that the blood from the stomach, intestine, &c., is taken by 

 a specially modified portion of the ventral vessel (portal 

 vein) through the liver on its way to the heart. The blood 

 is red, containing, in addition to leucocytes, oval corpuscles 

 coloured by haemoglobin (see p. 58). 



