ENTEROPNEUSTA 587 



follows and, by distending, gives a purchase for drawing forward the 

 trunk. 



The body is covered by a ciliated epithelium, with gland cells and 

 at its base a plexus of nerve fibrils to which processes of epithelial 

 cells contribute. This plexus is thickened along the dorsal and ventral 

 median lines of the trunk to form nerve cords, which are united by 

 a ring thickening immediately behind the collar. The dorsal cord 

 alone is continued into the collar, and here it is invaginated to form 

 a tube, by which arrangement it is protected during the movement of 

 this prominent part of the body. On the stalk of the proboscis the cord 

 communicates with the general plexus on that organ. There are no 

 special sense organs. No dermis interposes between the epithelium 

 and the muscles of the body wall. 



col.ca. 



nch. 



Fig- 439- A longitudinal vertical section through the middle line of Glosso- 

 balanus. Diagrammatic. From Shipley and MacBride. hr. branchial region 

 of alimentary canal; ce.n.sy. central nervous system; col. collar; col.ca. 

 collar cavity; d.b. dorsal blood vessel; d.rt. dorsal roots of nervous system; 

 h. heart; glom. glomerulus; M. mouth; nch. notochord; oe. "oesophageal" 

 gutter of pharynx; pcm. pericardium; por. proboscis pore; pro. proboscis; 

 pro.ca. proboscis cavity; sit. gill pouches showing internal slits and external 

 openings: the outlines of the external openings are dotted; trk. trunk; v.b. 

 ventral blood vessel; v.pk. ventral pocket of proboscis cavity. 



The alimentary canal (Fig. 439) is straight. From the buccal 

 cavity in the collar, the hollow notochord (see p. 584) projects forward 

 into 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. Backwards, 

 the buccal cavity leads into t\vQ pharynx, from which the gill slits open 

 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 

 basement membranes of the ectoderm, endoderm, and mesoderm, 

 which otherwise are everywhere in contact, having no mesenchy- 



