COMMON STARFISH. 193 



of Luidia, where they possess three as in Echinoidea. But the stalk of the Asteroid, 

 unlike that of the Echinoid pedicellaria, is formed entirely of soft structures. There 

 are in the Asteroidea, which possess four rows of ambulacral feet, two kinds of pedi- 

 cellariae. Both agree in having three calcareous pieces, a basilar piece bearing 

 two blades opened and shut by muscles. But in one kind which occurs chiefly 

 isolated the blades are articulated opposite to one another, in. the other the blades 

 cross one another at a certain point like the blades of a pair of scissors. The latter 

 are termed crossed pedicellariae. They occur in rings or semi-rings upon the spines. 

 In other Asteroidea there is a form of pedicellaria termed ' valvulate,' in which the 

 blades are broader than they are long. The pedicellariae are used to take hold of 

 objects such as algae until the feet can be applied. 



The apical system of plates is not traceable in this nor in the majority of 

 adult Starfish. It is disguised by the formation of new plates and ossicles during 

 growth. See the general account of the Echinodermata and Asteroidea. The 

 ossicles are developed from a calcareous network as is usual in Echinodermata, and 

 the calcareous matter is chiefly Calcium carbonate. 



The series of ambulacral ossicles end at the tip of each arm with a terminal 

 plate which supports the tentacle on its ventral surface, and is formed in the young 

 Starfish at an early period. All new ossicles and plates are added between it and 

 already existing ossicles and plates. A series of plates on the dorsal surface con- 

 nects it to a radial plate of the apical system in the young Starfish and some adults. 

 And when the radial becomes indistinguishable, this median row of plates may 

 remain conspicuous. In Asterias it is often well marked, but the ossicles are 

 similar to the other ossicles of the general perisoma which are arranged in linear series. 



According to Viguier, ten muscles correspond to each pair of ambulacral 

 ossicles : four vertical muscles (two on each side) uniting the ambulacral and adam- 

 bulacral ossicles : four longitudinal (two on each side, one superior and one inferior), 

 and two transverse muscles. One of the transverse muscles is ventral and deepens 

 the groove when it contracts : the other dorsal and antagonistic to the ventral. 



The dermal branchiae of the dorsal surface are delicate contractile tubular 

 processes of the perisoma. 



The madreporite contains tubular ciliated canals radiating conformably with 

 its superficial furrows. These canals send up minute vertical non-ciliated tubes 

 which open in the furrows by ciliated funnels. Of these openings A. rubens has 

 about two hundred. The radiating canals join the stone-canal. This canal has at 

 its dorsal end an ampulla, which is seven-lobed. Its cavity is partially divided by a 

 septum, but in many Asteroidea it is completely broken up into many tubes. To 

 the circumoral water-vascular ring are attached in pairs nine racemose vesicles or 

 Tiedemann's bodies. A tenth is missing, and the stone canal opens where it should 

 be. The bodies consist of branched canals lined by cubical cells. The radial 

 vessels spring from the circumoral. They give off laterally, corresponding to the 

 interval between two ambulacral ossicles, the branches for the feet. These open 

 into the bases of the feet in such a manner that water cannot regurgitate. A branch 

 from the base of every foot passes through a corresponding ambulacral pore and 

 swells into an ampulla on the dorsal aspect of the series of ossicles. Longitudinal 

 muscles exist in the walls of the ampullae and feet, circular in other parts of the 

 water-vascular system. 



O 



