ECHINODERMA TA 225 



Class I. ASTEROIDEA (Starfishes). 



Characteristics. — Echinodermata whose body is flattened dorso- 



ventrcdly, and is produced into arms or rays, which are usually 



five or more in number. These arms are longitudinally 



grooved on the ventral surface, and the tube-feet lie in this 



groove. The madreporic plate is dorsal and interradial in 



position. The alimentary canal sends caecal diverticula into 



the arms. The generative organs are interradial in position 



at the base of the arms. Pedicellariae usually present. 



Asterias ruhens is oue of the commonest of starfishes, and is 



constantly left stranded on our shores by the retreating tide. 



Its body consists of a central disk, from which five arms or 



radii project. The surface on which it habitually rests or 



moves, and on which the mouth opens, may be termed the 



ventral, the upper and more convex, where the anus is situated, 



may be called the dorsal. 



From the mouth five grooves radiate along the arms, these 

 are the arnhulacral grooves, and they lodge the tube-feet j between 

 each two grooves, and consequently interradial in position, are 

 five sets of oral spines, which project over the mouth and perhaps 

 assist in feeding. If the tube-feet be removed from each ray, 

 it will be seen that the ambulacral groove is formed of two rows 

 of ambulacral plates, situated right and left of the middle line of 

 the radius (Fig. 131). Each right plate is so placed as to form 

 an angle, open ventrally, with the corresponding left plate, and 

 between the adjacent plates of each side certain pores exist 

 which give exit to the tube-feet. The groove is covered in by 

 the integument, and lodges two radial canals, of these the most 

 ventral is divided by a vertical septum, and is called, for 

 reasons mentioned below, the " peri-haemal " space. The dorsal 

 canal is the radial trunk of the water-vascular- system. At the 

 outer end of the ambulacral plate a series of adambulacral 

 ossicles are situated, and these support three rows of moveable 

 spines. Those spines which are nearest to the centre of the 

 disk form the oral spines mentioned above ; these are borne by 

 the first adambulacral ossicles, one set on each side of an inter- 

 radius. 



At the distal end of each arm the ambulacral plates end in 



15 



