664 THE INVERTEBRATA 



numerous small gill openings; these lead into deep pouches which 

 communicate with the pharynx each by a tall opening virtually 

 divided into two by a tongue bar, which hangs from the dorsal side 

 but does not quite join the ventral side as do the tongue bars in 

 Amphioxus. In the region of the gills, and a little way behind it, the 

 trunk is somewhat flattened above or has a pair of lateral ridges or 

 folds, known as tht genital pleurae because when they are present the 

 gonads lie in them. Behind this hranchiogenital region the trunk 

 becomes more cylindrical and tapers gradually, as the abdominal 

 region, to the anus, which is terminal. 



The proboscis and collar are used in burrowing. They are distended 

 by the taking in of water by the action of cilia in the tubes leading to 

 the pores, and contracted by muscles in the body wall, the water being 

 thus driven out. The proboscis first enters the mud and the collar 

 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 net of nerve fibrils to which processes of epithelial 

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

 median lines of the trunk in the form of 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 somewhat thicker and 

 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 

 net on that organ. There are no special sense organs. No dermis 

 interposes between the epithelium and the muscles of the body wall. 



The alimentary canal (Fig. 464) is straight. From the buccal cavity 

 in the collar, the hollow notochord (see p. 661) projects forward into 



Fig. 464. A diagram of a median longitudinal vertical section of a typical 

 member of the Enteropneusta. 



With the exception of the branchial blood vessels, structures which are 

 not median are shown by interrupted lines. 



aff.br., aff.i., aff.ph., off. pro. afferent vessels to gill clefts, intestine, pharynx, 

 and proboscis; ce.n.sy. central nervous system in collar; cm. coecum of noto- 

 chord; col. collar; col.ca. collar cavity (mesentery lacking here) ; d.b.v. dorsal 

 blood vessel; d.b.zv.v. vessel from dorsal body wall; d.rt. "dorsal root"; 

 eff.br., eff.glo., eff.i., eff.pro. efferent vessels from gill clefts, glomerulus (cut 

 short), intestine, and proboscis; glo. glomerulus; h. heart; lat.ph.v. lateral 

 pharyngeal vessel; w. muscles of proboscis; M. mouth; mes.d.coL, mes.d.trk., 

 mes.v.coL, mes.v.trk. mesenteries, dorsal and ventral, of collar and trunk; 

 nch. notochord; /)./). proboscis pore;/)cw. pericardium; pro. proboscis ; /)ro. 

 ca. proboscis cavity; sep. proboscis septum ; sk. notochord skeleton; sk' . pro- 

 cess of sk. at side of mouth; si. gill slit; trk. trunk; trk.ca. trunk cavity (at 

 point where mesentery is lacking) ; v.b.v. ventral blood vessel; v.b.zc.v. vessel 

 to ventral body wall. 



