416 ZOOLOGY SECT. 



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 (Actinometra), 

 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 

 ambulacral grooves, one of which extends along the oral surface 

 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 of entero- 

 ccelic origin 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 respiration and 

 excretion through the medium of this fluid are the dermal branchiae 

 or papulce, 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 compartment of the ccelome enclosing 

 Aristotle's lantern (lantern-ccelome) is taken by Stewart's organs, 

 simple or arborescent bodies which project inwards from the peri- 

 stome. 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 body, to the ambulacral system of vessels. A 

 ring-like circum-oral vessel (ring-vessel) in nearly all cases sends off 

 a series of radial branches, one passing along each of the rays or 



