ECHINODERMATA. 207 



of five parts (Aristotle's lantern). In the asteroids the mouth 

 opens by a short oesophagus into an expanded stomach, which 

 is divided into an oral, or cardiac, and a pyloric portion (Fig. 

 96). From the pyloric part the narrow intestine passes to 

 the anus. Outpocketings (caeca) may occur in any of these 

 divisions. The most important are the hepatic caeca, which 

 are glandular in function. 



240. The body cavity is usually well developed both in the 

 disc and in the arms, is lined with a ciliated epithelium, and 

 contains a fluid with amoeboid corpuscles. It is completely dis- 

 tinct from the digestive cavity. Thin outgrowths of the body- 

 wall (papules or branchice) contain extensions of the ccelom. 

 These assist in respiration. 



241. Ambulacral or Water- vascular System. This sys- 

 tem of tubular organs is peculiar to the echinoderms. It 

 originates (see also 248), in common with the body cavity, as 

 an outgrowth from the archenteron and is to be regarded as 

 a specialized portion of the body cavity. In some cases these 

 two cavities are in communication in the adult. It consists 

 essentially of a ring-vessel about the mouth from which pass 

 radial tubes, one in each arm. From the radial tubes arise 

 lateral channels which communicate directly or through 

 bladder-like ampullae, with distensible feet which reach the 

 exterior by pores in the skeleton (Figs. 97, 98) . The tip of the 

 foot may be provided with a sucking-disc, serving as a means 

 of attachment and of locomotion. Frequently the walls of 

 these feet are thin and apparently serve for respiration, and the 

 terminal " foot " at the end of each radius may be highly 

 modified to form a sense organ (tentacle). The feet, the 

 ampullae, and even the radial vessels may be wanting. The 

 ring-canal, in typical forms, communicates with the surround- 

 ing sea-water by means of a tube (stone canal) which termi- 

 nates in a sieve-like plate, the madreporic body, through which 

 the water enters the water- vascular system. In the majority 

 of the Holothuroids the madreporic tubes open into the body 



