92 BALANOGLOSSIDA. 



thickening of the basement membrane of the walls of the 

 pharynx on each side the gill-slits. It is made up on each side 

 of a series of structures placed vertically in the walls of the 

 pharynx and presenting a resemblance to a three-pronged fork 

 (Fig. 62). The three prongs of each piece are joined together 

 dorsally and end freely ventrally. The central prong is single 

 (though presenting the appearance when closely examined of 

 being composed of two fused rods) and forks ventrally. It is 

 contained in the septum separating two gill-slits. The anterior 

 and posterior prongs lie in the adjacent tongue bars, which also 

 contain a corresponding prong of an adjacent skeletal piece. The 

 anterior and posterior prongs do not fork ventrally. It would 

 thus appear that each tongue bar contains two skeletal rods 

 belonging to adjacent skeletal pieces. The whole skeleton lies 

 close to the wall of the pharynx. Coelom is present in the 

 tongue bars and its epithelium contributes with that of the 

 adjacent endoderm to the formation of this skeletal basement 

 membrane (Fig. 61). In the primary or septal bars there is 

 no coelom in the adult, so that the skeletal rods in them are 

 exclusively formed by the pharyngeal epithelium. 



The vascular system consists, as already explained, of channels 

 hollowed out in the basement membranes of the body. 



It might be said to represent a persistent portion of the spare which 

 theoretically occurs between the two laminse of which all or nearly all 

 the basement membranes consist. But as the basement membranes do 

 not show a composition of two lamella 1 it will be perhaps safer at this 

 point to take the first statement as representing all that we actually k>itr 

 on this subject. 



The blood is usually described as colourless, but it appears to 

 be coloured red in some forms at least, if we may judge from the 

 fact that a red line can be seen through the body wall along the 

 lines of the chief blood vessels ; and it contains a few floating 

 amoeboid cells of the ordinary type. 



An epithelial lining is present in the blood vessels of a few 

 forms (Ptychodcra, etc.), but frequently nothing of the kind can 

 be detected. Some of the larger vessels are provided with 

 muscles (usually circular), which arc furnished by the walls of 

 adjacent portions of the coelom. The principal blood vessels 

 are as follows : (1) A longitudinal dorsal vessel running through 

 the body and collar, and passing into the proboscis to become 



