19 o DESCRIPTIONS OF PREPARATIONS. 



Freud, SB. Akad. Wien, Ixxxv. Abth. 3, 1882. In Decapoda, Yung, A. Z. Expt. vii. 

 1878. 



Stomatogastric system with figures, and posterior intestinal nerves. Lemoine, 

 A. Sc. N. (5), ix. 1868; Mocquard, A. Sc. N. (6), xvi. 1883. 



37. COMMON STARFISH (Asterias or Asteracanthion rubens), 



Dried. 



THE animal consists of a central disc, which is prolonged into five 

 lobes, the so-called arms, rays, or radii. The interval or part between each 

 radius is known as an interradius. Two surfaces may be distinguished : 

 one flat or somewhat concave, the ventral, oral, actinal, or ambulacral 

 surface ; the other convex, and termed the dorsal, aboral, abactinal, or 

 anti-ambulacral surface. In the centre of the ventral aspect of the disc 

 is the membranous peristome with the mouth. Five sets of spines, the 

 mouth-papillae, project over this area interradially, giving it a pentagonal 

 appearance : and there radiate from it the five avenues or ambulacral 

 grooves, one to each ray, which lodge the locomotor feet and hence give 

 this aspect of the animal the name of ambulacral. The feet in question 

 have been removed from two of the grooves for a short space, but are 

 left in situ and in a dried condition elsewhere. Examining the exposed 

 part of each groove attentively, it is seen to be formed by two series of narrow 

 parallel ossicles the so-called ambulacral or vertebral ossicles, the long 

 axes of which are at right angles to the axis of the ray. The ossicles of 

 one side of the groove are inclined at an obtuse angle, open ventrally, to 

 the corresponding ossicles of the other side ; and their dorsal ends are 

 articulated moveably together. The summit of the angle is median. The 

 groove lodges the radial water- vascular vessel, the inferior transverse 

 vertebral muscles, the radial perihaemal spaces and bloodvessels, and the 

 radial nerve-cord, with the feet. Between the ossicles are a series of 

 pores, one pore between each pair, formed by the apposition of two 

 grooves in adjoining ossicles. The ampullae of the feet which lie on the 

 dorsal side of the ossicles communicate through these pores with the 

 ventrally placed feet. The two first pores lie in the same straight line, 

 while the succeeding, to very near the tips of the arms, are arranged in 

 a zig-zag fashion, being alternately near to, and remote from, the axis 

 of the ray. Hence there appear to be four rows of pores and four rows 

 of feet to correspond. In the majority of Asteroidea, however, the pores 

 retain a straight linear arrangement for the whole extent of the grooves. 



The edges of a groove are bordered immediately by a series of fine 

 moveable spines, borne by the adambulacral ossicles (infra). In this 



