104 F. W. GAMBLE, M.SC, AND J. H. ASHWORTH, B.SC. 



Wiren evidently believes that there is no capillary system except 

 in the gills aud the alimentary canal. He suggests that the assimila- 

 tion of food and oxygen by the tissues is effected chiefly through the 

 mediation of the ccelom, which he points out is parcelled off in the 

 intermuscular spaces, by a channelling out of the subepidermic connec- 

 tive tissue, into "periheemal canals." Though this suggestion is a 

 valuable and correct one, we have found a very perfect system of capil- 

 laries in the skin in all parts of the body, and in the nephridia and 

 septa the same is the case. The extension of the ccelom into the inter- 

 muscular and subdermal spaces has, however, all the appearance of 

 acting as the equivalent of lymph-spaces of higher forms. The trans- 

 formation of the constituents of the blood into ccelomic fluid takes place 

 in all probability with especial rapidity in the neighbourhood of the 

 dark chlorogogenous processes of the ventral vessel (cf. Cuenot, 1891). 



7. The Gills (PI. VI., Figs. 2—4). 



The general characters of these organs have been mentioned in the 

 introductory part of this paper, and little remains to be added. 



There are thirteen pairs of gills from the seventh to the nineteenth 

 cheetigerous somites inclusive. The shape varies from the short den- 

 dritic type of the littoral form to the delicate, richly-branched gill of 

 the Laminarian variety. The gills are hollow, being out-growths of the 

 body-wall enclosing an extension of the ccelom, and what little evidence 

 we have of their development (see Benham, 1893) points to their being 

 independent structures, and not modified dorsal cirri. 



The walls of the gills, though thin, are muscular, and there are also 

 muscular bands stretching across the cavity of the gill (Fig. 23) ; and 

 Milne Edwards has pointed out that the contraction of the gills, which 

 often proceeds like a wave from before backwards down the sides of 

 the body, must exert a powerful influence in propelling the blood 

 partly into the efferent vessels, and partly to the parietal capillaries. 



The ventral vessel supplies all the gills w y ith their afferent branches. 

 The first seven pairs return the blood to the sub-intestinal vessels, and 

 so to the heart ; while the efferent branches of the remainder open into 

 the dorsal vessel. 



