6 ZOOLOGY SECT. 



outwards in the series of external prominences already mentioned. 

 Posteriorly it terminates in an anal aperture situated at the 

 posterior extremity of the body. In the posterior part of its 

 extent in some Enteropneusta the intestine presents a ventral 

 median ridge-like outgrowth of its epithelium — the pygochord. 

 Throughout its length the intestine lies between the dorsal and 

 ventral divisions of the vertical partition, which act as mesen- 

 teries. 



As the animal forces its way through the sand, a quantity 

 of the latter enters the digestive canal through the permanently 

 open mouth, and is eventually passed out again by the anus in 

 the shape of castings, which may be thrown out on the surface 

 of the sand in a form resembling that taken by the castings of 

 earthworms. 



A series of pores (gastro-cutaneous 'pores), variously arranged 

 in the different genera, connect the intestine with the surface. 



Notochord or CESophageal diverticulum. — The dorsal wall 

 of the part of the digestive canal immediately following upon 

 the mouth gives off a diverticulum (div.) that runs forwards 

 some distance into the basal part of the proboscis after giving off 

 a short ventral branch. The diverticulum contains a narrow 

 lumen, and its wall is composed of a single layer of long and 

 very narrow cells each of which contains a vacuole. This layer 

 of cells forming the wall of the diverticulum is continuous with 

 the epithelium of the digestive canal itself, the cells being 

 somewhat modified by the presence of the vacuoles. The 

 diverticulum, owing partly to its structure, partly to its relations, 

 is usually regarded as representing the notochord of the 

 typical Chordata. In close relation with this on its ventral 

 surface is the chitinoid prohoscis-skeleton {iiroh. skel.) which consists 

 of a median part of an hour-glass shape, and with a tooth-shaped 

 process, bifurcating behind into two flattened bars which lie 

 in the anterior region of the oesophagus and support the opening 

 into the lumen of the diverticulum. 



There is a blood -vascular system with dorsal and ventral 

 longitudinal trunks. The dorsal vessel {dors, v.) lies above the 

 notochord, and ends in front in a sinus, the dorsal sinus or heart 

 (dors, sin.), situated in the anterior part of the collar and the 

 neck of the proboscis, in close contact with the notochord. From 

 the posterior part of the sinus is given off a vessel which 

 bifurcates to supply the proboscis. In communication with 

 the sinus in front are a number of vessels of a bilateral plexus 

 in the glomerulus, a glandulai- organ, probably excretory, situated 

 at the anterior end of the alimentary diverticulum. From the 

 posterior end of each half of the glomerulus there passes 

 backwards an efferent vessel which breaks up into a plexus ; the 

 two plexuses- unite ventrally to form a median ventral plexus 



