THE ECHrNOIDEA 291 



phageal nerve ring, which can be seen in the figure, connecting the 

 five ambulacral nerve cords (am.n). 



The walls of the podia are strengthened by calcareous spicules, 

 and expand at the end into a sucker. The function of the tube- 

 feet is to help in locomotion. The sucker is pressed against a 

 smooth surface ; water from the reservoir or ampulla is driven 

 into the podion, and the tube-foot is thus rendered tense and rigid. 

 The rosette of plates in the sucker is pulled backward, when a 

 vacuum is left between the sucker and the surface against which it 

 is pressed. Firm attachment is thus secured, and the animal can 

 drag itself along (for action, see Fig. XLIL). So powerful are 

 these suckers that, by their means, the Echinoids of the genus Colobo- 

 centrotus (Fig. XXXIV.) can cling to exposed rock surfaces, fully 

 exposed to the surf of a coral reef. 



The supply of water to the water- vascular system is introduced 

 by a membranous vessel the stone-canal (s.e) which rises from 

 the circumoesophageal ring, and is attached to the plate of the 

 apical system which bears the madreporite (TO). As AVC have seen, 

 this plate is perforated by many small pores, through which water 

 can pass into the stone -canal. Owing to the small size of the 

 pores, the water is filtered as it enters. In Echinus esculentus the 

 stone-canal is membranous, and the name therefore appears inap- 

 propriate. The name was first applied to this canal in the genus 

 Cidaris, in which it is hard and calcareous. The flow of water in 

 the interior is regulated by five "Polian vesicles," situated on 

 the circumoesophageal ring, and acting as reservoirs. The latest 

 account of the function of these vessels is given by Uexkiill (82). 



The main points in the distribution of the water- vascular vessel 

 can be easily verified ; but the Blood- vascular or haemal System 

 is more obscure. The most important structure is an ovoid body 

 (Fig. VII. d.o), situated beside the stone-canal ; it is known as 

 the " dorsal or axial organ," and by other names (see Chapter VIII. 

 pp. 23, 25). Its canal joins above with the stone-canal; it is said 

 to open to the exterior through fhe madreporite, but this is denied 

 by some authorities (as Hamann). Round the upper end of the 

 " canal of the dorsal organ " is a circular canal known as the 

 "genital ring," which appears to be connected with a series of 

 haemal vessels or lacunae which surround the dorsal organ. In 

 this case it must be also connected with a ring round the oesophagus, 

 from which five branches pass downward beside the pharynx, and 

 then run up along the test below the ambulacra. The "circum- 

 oesophageal haemal vessel " is connected with a haemal vessel which 

 runs along the inner side of the stomach. 



A third group of canals or vessels consists of a circular sinus 

 round the oesophagus, from which five branches run up the ambu- 

 lacra between the radial water- vascular vessels and the radial nerve 



