ECHINODERMATA. 



193 



Section of ray of Asterias rubcns, showing arrangement of 

 calcareous pieces. 



gelatinous Radiaria by its mode of swimming : the movements of 

 its pinnate arms exactly resemble the alternating stroke given by the 

 Medusa to the liquid element, and with the same effect of raising the 

 animal from the bottom, and propelling it back foremost. 



The rays of the ordinary star-fishes are not cirrigerous or bifur- 

 cated : their soft ex- 

 ternal integument is 

 supported by a tough 

 coriaceous membrane, 

 strengthened by calca- 

 reous matter {Jig' 94. )> 

 disposed in a coarsely 

 reticulate form upon the 

 dorsal and lateral as- 

 pects of the radiated 

 body, and arranged in 

 series of more compact and regularly-formed transverse pieces, a, by 

 which bound each side of a longitudinal furrow, extending along the 

 under surface of each ray from its attached to its free extremity. 

 The sides of this groove are perforated by alternating rows of minute 

 fissures, and external to these are situated the largest and most 

 numerous spines. 



The tube-feet are protruded, in two {Ast. aurantiacd) or four {Ast. 

 rubens,Jig. 95 c, c) rows, through the marginal pores of the furrows, 

 which are termed " ambulacra." These tube-feet have muscular pa- 

 rietes, and they communicate with internal vesicles, d, c?, full of fluid, 

 which form, in fact, the bases of the feet. By the contraction of the 

 parietes of the vesicle the fluid is injected into the tube-foot, c, c, and 

 protrudes and extends it : when the muscular parietes of the tube-foot 

 contract, the fluid is returned into the sac, and the tube-foot is short- 

 ened and retracted. The basal vesicles are in communication with, 

 and are supplied by, a system of vessels, i, small brown sacs, m, and 

 larger pendent pyriform sacculi, which are lodged in the central disc 

 or body of the star-fish, and surround the oral aperture, a. 



There are other kinds of soft contractile appendages to the integu- 

 ment, some tufted, others of simple form ; but the tube-feet just de- 

 scribed are the most important organs for prehension and locomotion. 

 The tegumentary processes called " pedicellariae," which resemble 

 miniature pincers, will be more particularly described in connection 

 with the skeleton of the Echinus. In the star- fishes they are of the 

 kind called Pedicellarice forcipatcB and Pedicellarice valvulatcp. 



Various are the forms of the calcareous parts which strengthen and 

 defend the skin of the star- fishes ; but in all the echinoderms in w^hich 



o 



