ANATOMY OF ARENICOLA ASSIMILIS. 745 



respects the alimeutary canal resembles tliafcof A. marina. 

 The ventral groove^ wliicli is well seen in the intestine^ luay 

 be traced forwards into the stomach to about the level of the 

 eighth or seventh seta. 



Vascular System. — The vascular system closely agrees 

 with that of A. marina (see Gamble and Ash worthy 1898, 

 pi. ii), except in the fifth and seventh cha^tigerous seg- 

 ments, lu each of these there is only one pair of vessels, 

 afferent branches of the ventral vessel, passing to the 

 nephridia. The first pair of efferent branchial vessels is 

 situated in the eighth segment ; this and the four succeeding 

 pairs open into the subintestinal vessels, while the last eight 

 gills, i. e. those of the thirteenth to twentieth segments, 

 return blood to the dorsal vessel. The body-wall is well 

 supplied with blood-vessels, especially in the anterior region ; 

 in sections of the peristomium aud first chastigerous seg- 

 ment there are numerous vessels lying either in the con- 

 nective tissue or in small coelomic canals (see below) just 

 beneath the epidermis (fig. 22) ; in sections of some of the 

 posterior segments the vessels are not so abundant. 



The heart is of moderate size, and has the usual relations. 

 There is a cardiac body formed by ingrowths, chiefly of the 

 posterior wall of the heart, and this is well developed in one 

 of the specimens 120 mm. long. 



Coelom. — The ccelom is spacious, as in A, marina. A 

 remarkable feature noticed at once in sections (fig. 22) of the 

 anterior end of A. assimilis is the large number of ctt3lomic 

 spaces in the body-wall and between the muscles. In this 

 portion of the animal there are exceedingly numerous ccclomic 

 canals lying in the subepidermal tissue of the body-wall, 

 penetrating into the muscle-bands, especially of the buccal 

 musculature, insinuating themselves between the brain-lobes 

 and between the brain and the pi-ostomial epithelium, and 

 often accompanying the blood-vessels which supply the body- 

 thai, ol' A. niai-iiia, but llic presence of multiple oesophageal glands in the 

 former wiiile there is only a single pair in the latter species is a point of 

 differeuce of considerable systematic importance. 



