222 COMPAEATIYE ANATOIVIY. 



therefore to know tlie earliest rudiments of this an-angement. The 

 stone-canal always runs close to the heart-like tube. Calcareous 

 bodies are deposited in it and form a fine network ; they do not 

 differ from those found in the perisome. They are arranged _ in 

 rings ; internally to them there is a longitudinal ridge from which 

 arise two coiled and thinner lamellae, similarly calcified. The cavities 

 which commence at the fine pores of the madreporic plate pass 

 between these lamellae. The ambulacral canals (Fig. 100, A a) extend 

 along the skeleton of the armSj embedded in the ambulacral groove^ 

 and give off branches to the feet which arise between the lateral 

 processes of the segments of the ambulacral skeleton; the ampullae of 

 the feet pass inwards through the clefts between the calcified 

 segments, and so come to lie within the arms (ap). At the points 

 where the ampullae are connected with the ambulacral feet there 

 are valves, which shut when the ampullte contract (Asteracanthion 

 rubens). The number of Polian vesicles varies ; they are sometimes 

 increased in number, and form racemose tufts (Astropecten auran- 

 tiacus), or they may be altogether wanting. 



In the Ophiurida the stone-canal is inserted into a plate sur- 

 rounding the mouth ; but this plate is not formed in the same way 

 as the madreporic plate, but so that the stone-canal takes up fluid from 

 the body-cavity only. At the circular canal the stone-canal widens 

 out into an ampulla^ and is attached to an interradial portion. 

 Pohan vessels are not always present. The suckers have no ampullae. 



In the Crinoida the ambulacral water-vascular trunk runs below 

 the radial blood-vessel, and sends branches into the tentacles of the 

 arms, as well as of the pinnulae (Fig. 115, w). The radial trunks 

 meet in a circular oral canal, which sends off short canaliculi, with 

 open mouths, into the coelom. They take the place of the stone- 

 canal, which is not present. As there are no ampullae or Polian 

 vesicles either, the water- vascular system is in the Crinoi'da of a. lower 

 grade than in the other divisions. 



The Echinoida are allied to the Astero'ida. The madreporic 

 plate always hes at the aboral pole ; it is either formed by one of the 

 genital plates (Fig. 103, m), or by several of these, or an interradial 

 plate is converted into the madreporic plate, or it is formed by a 

 special plate (Clypeastridae). The stone-canal is sometimes soft 

 (Echinus) and sometimes provided with firm walls (Cidaris). The 

 circular canal, provided with five Polian vesicles (these are absent in 

 the Spatangidae), lies in the Echinida at the base of the masticatory 

 apparatus, and gives off its ambulacral canals downwards, whence 

 they radiate out to the ambulacra. On the inner side of the shell, 

 and running along each of the ambulacral areae, are the branches of 

 the ambulacral canals, which are distributed to the pores of the 

 calcareous plates, and supply the suckers or their equivalents which 

 arise at this point ; and give origin to transversely-placed ampullar 

 enlargements (Fig. 116, a). 



In the Holothuroida, owing to the separation of the connecting 

 piece, which later on functions as the stone-canal, from the perisome 

 of the larva, which passes into the substance of the Echinoderm, 



