ii8 THE APODID^: PARTI 



the development of the exoskelcton, give place to a 

 simple lacunar system as sufficient for the needs of 

 the body, the only vessel retained being the contrac- 

 tile dorsal heart, necessary for propelling the blood 

 through this lacunar system. It may be objected 

 that the trunk of Apus under the shield has a soft 

 skin, and nevertheless the blood system in this part is 

 entirely lacunar. The trunk is, however, not very con- 

 tractile, and although the blood flows through lacunse, 

 there are definite methods of propelling it through 

 the intestinal sinus, which will be mentioned later on. 



It is clear, then, that the absence of nearly all true 

 blood-vessels from the circulatory system of Apus 

 makes any comparison of it with that of an Annelid 

 impossible. Apart, however, from the dorsal vessel 

 or heart, which is generally recognised as an organ 

 derived from Annelidan ancestors, there is, in Apus, 

 a slight trace of" a pair of typical Annelidan vessels ; 

 these are the short vessels which supply the shell 

 glands. (See Fig. 30, p. 125.) They branch off 

 from the dorsal vessel on each side, and descend 

 towards the dorsal parapodia of the second pair 

 of maxillae, to dip under the shell gland. They 

 are necessary for conducting the blood into the 

 shield in which the coils of the gland lie. They may 

 be homologous with a pair of lateral arteries from the 

 dorsal vessel of the fifth segment, such as typically 

 supply the parapodia and sides of the body in a 

 carnivorous Annelid. 



Attention must be drawn to the tapering away of 

 the heart to a point towards the posterior end of the 



