228 ZOOLOGY 



number of vessels. They constitute the blood system according 

 to French authors. The radial one has already been mentioned ; 

 the five radials unite with a circum-oesophageal ring, which is 

 stated to open into the body-cavity by five interradial pores. 

 Inside this is another ring-shaped vessel, into which a large 

 sinus surrounding the stone canal — the axial sinus — opens. 

 Besides this there is an aboral pentagon which sends off inter- 

 radially pairs of vessels which dilate and surround the genital 

 organs. 



The mouth is situated centrally on the ventral surface, 

 surrounded by a ring of nervous matter. The mouth leads by 

 a short oesophagus into a large stomach, the walls of which are 

 folded in many sacculi. When the starfish attempts to devour 

 young molluscs or shellfish which are too large to be taken in 

 at the mouth, these sacculi are protruded and enclose the prey. 

 They are retracted by special muscles. The walls of both the 

 oesophagus and the stomach are ciliated, and the eversible 

 portions contain many glands, the secretions of which possibly 

 exercise a paralysing effect on the prey. The stomach is 

 followed by a pentagonal pyloric portion with its angles 

 situated radially. From each angle a short duct passes to 

 the base of each arm, and here opens into two large hepatic 

 caeca, which occupy a large portion of the space in each arm 

 and extend to its tip. Each caecum is supported by two dorsal 

 mesenteries. From the pyloric portion a short rectum passes 

 to the anus, which is in the next interradius to that bearing 

 the madreporic plate, and is almost central. The rectum gives 

 off two short caeca, which lie in two neighbouring interradii 

 — that between the left and central arm of the trivium, and 

 between the left arm of the trivium and bivium. 



The water-vascular system consists of a circumoral ring 

 which gives off five radial vessels, one running along each arm, 

 and a single interradial stone canal, which passes from the 

 circumoral ring, and opens to the exterior at the madreporic 

 plate, which is calcified. 



The madreporic plate is marked externally by a number 

 of radial grooves ; at the bottom of each of these is situated a 

 row of pores ; these open into a series of tubules, which collect 

 into an ampulla, and this in its turn opens into the lumen of the 



