Class ASTERIOIDEA. 



MORPHOLOGICAL FEATURES. 



The Asterioidea' have a polygonal or star-shaped body, in which 

 the rays are direct prolongations of the body itself, and contain exten- 

 sions of the body cavity and more or less of the viscera, especially one 

 or more pairs of gonads and a pair of digestive glands ; generally, 

 also, a lobe of the saccular stomach. 



The skeleton is made up of large numbers of ossicles or plates, 

 mostly articulated so as to be more or less movable, giving flexibility 

 both to the rays and to the disk, though in some species (certain 

 Goniasteridae) the flexibility is slight, except at the tips of the rays. 



The actinal side of the disk and rays has deep radial ambulacral 

 grooves, extending to the tip of the rays. The roof of the groove 

 is supported by the two rows of ambulacral ossicles, arranged like 

 rafters, or in close, inverted V-shaped pairs of compressed plates or 

 bars, between which there are rows of pores for the passage of the 

 ambulacral feet.' 



In the middle line of each ray and external to the ambulacral 

 plates are situated the radial nerve and blood-vessel. The radial 

 water tube supplies water to the locomotive tubes through the 

 medium of muscular ampullae, usually double, situated internally 

 above the ambulacral plates. In Brisingidae ampullae are lacking; 

 in Echinasteridae they are single. 



There is no median row of calcareous plates covering the ambu- 

 lacral areas and radial nerve and blood-vessel, such as exists in 

 Ophiuroidea. 



The tip of each ray ends in a special terminal ocular, or apical, 

 plate, supporting a pigmented ocellus, to which the radial nerve 

 extends. According to the studies of Fewkes, 1888, these are the 

 first plates to appear in the young. 



The grooves are bordered on each side by a row of plates called 

 adambulacral, which always bear spines. 



' This spelling of the name is preferred because it is derived from Asterias, 

 not from Aster. 



' In many paleozoic fossil starfishes the ambulacral plates are not opposite, 

 in pairs, but alternate. This rarely occurs in existing species, though I have 

 noticed it in Pycnopodia as an abnormal variation in some of the rays, and 

 also that it may occur from lateral bending. 

 (20) 



