U PHYLUM ECHINODERMATA 425 



a series of radiating arms, which usually branch dichotomously. 

 In the stalked forms (Fig. 342) a stalk, consisting of a row of 

 elongated ossicles connected together by bundles of ligamentous 

 fibres, attaches the animal to the sea-bottom. Along some of 

 the joints of the stalk are usually arranged a number of slender, 

 many-jointed appendages — the cirri. At its base the stalk 

 usually breaks up into a number of root-like processes ; distally 

 it becomes continuous with the central disc. The ossicles forming 

 the skeleton of the central disc are the basals and the radials : with 

 the latter articulate externally the brachials, a single row of which 

 gives support to each of the arms and its branches, while similar rows 

 of smaller ossicles support the pinnules — the lateral appendages 

 which fringe the arms in a double row. In the free forms 

 the stalk is absent in the adult condition, though present 

 in the larva, and from its terminal ossicle and other neighbouring 

 plates is formed by coalescence a plate — the centro-dorsal ossicle 

 of the disc. To the centro-dorsal ossicle are attached whorls 

 of many-jointed, slender, curved cirri. 



The mouth in all the Crinoidea, with one exception (Aclinomctm), 

 is situated in the centre of the oral (upper) surface, and the anus 

 in all, with the same exception, is eccentric and inter-radial. 

 Running outwards from the mouth are a series of very narrow 

 ambulacra I grooves, one of which extends along the oral sur- 

 face of each arm, giving off branches to the arm-branches and 

 to the pinnules. Bordering the ambulacral grooves and their 

 branches are a pair of rows of short' tubular tentacles, which 

 correspond morphologically with the tube-feet of the other classes, 

 but are devoid of the terminal suckers, and are not locomotor, 

 but probably sensory and respiratory in function. 



The ccelome in the Echinoderms is a wide cavity lined by 

 a ciliated ccelomic epithelium and containing a corpusculated 

 fluid. Prolongations of it pass out into the rays, and, in the 

 Ophiuroidea and Asteroidea, between the layers of the body-wall. 

 In the Crinoidea it contains numerous strands of connective tissue. 

 Special organs providing for the respiration of this fluid are the 

 dermal branchial or papula, the Stewart's organs, and the respiratory 

 trees. The first of these, which are confined to the Asteroidea 

 and Echinoidea, have been described in the accounts of the 

 Starfish and Sea-urchin. In most Asteroidea they occur only on 

 the dorsal surface, but in some forms they are present on the 

 ventral surface as well. In some of the Echinoids the place of 

 dermal branchiae in providing for the respiration of the compart- 

 ment of the ccelome enclosing Aristotle's lantern (lantern-ccelome) 

 is taken by Stewart's organs, arborescent bodies which project 

 inwards from the peristome. The respiratory trees are referred 

 to below in connection with the enteric canal. 



Some reference has already been made, in describing the general 

 form of the bodv, to the ambulacral system of vessels. A 



