666 



THE INVERTEBRATA 



the hinder part of the proboscis, strengthening the neck of that 

 structure and supporting a group of organs (heart, pericardium, 

 glomerulus), which form with it the proboscis complex: at its root is 

 a skeletal thickening of its basement membrane. Backwards, the 

 buccal cavity leads into the pharynx, from which the gill slits open. 



coei 



Fig. 465. 



Fig. 466. 



Fig. 465. A longitudinal horizontal section through G/owo6a/a«M^. Diagrani- 

 matic. From Shipley and MacBride. al. alimentary canal ; br.s. branchial sac 

 with external opening; col. collar; col.ca. collar cavity; d.b. dorsal blood 

 vessel;^, reproductive organs ; jg'/om. glomerulus; h. hezrt; pent, pericardium; 

 peh. perihaemal cavity ; por' . collar pore ; por. proboscis pore ; pro. proboscis ; 

 pro.ca. proboscis cavity; trk. trunk. 



Fig. 466. A Tornaria larva. From Sedgwick, after Metschnikoff. a, From 

 the left-hand side, b, From the dorsal side. The larva is in the regressive 

 stage, and numerous secondary foldings of its ciliated rings have been lost. 

 An. anus; ap. apical sense plate; coe.' rudiment of anterior (proboscis) 

 coelom ; coe." , coe.'" rudiments of right middle (collar) and hinder (trunk) 

 coeloms; M. mouth ;^cm. pericardium. 



In most species there is a ventral gutter, below the gill slits, leading 

 to the intestine, which lies in the abdominal region. Along this gutter 

 passes the mud which the animal swallows for food, excess of water 

 leaving by the gill slits, which thus act as a straining apparatus. 

 The blood vessels are for the most part mere crevices between the 



