6 ZOOLOGY SECT. 



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 mesenteries. 



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 oesophageal 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 proboscis-skeleton (prob. 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 glandular organ, probably excretory, situated at the anterior end 

 of the cesophageal 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 continuous behind with the ventral 

 vessel. The dorsal sinus, having no definite walls, is not contractile ; 

 but a closed sac, the cardiac sac (card, s.), situated on the dorsal 

 side of the sinus, has a muscular ventral wall, by the contractions 

 of which the blood may be propelled. 



The nervous system consists of dorsal and ventral strands 



