478 THE ANATOMY OF INVERTEBRATED ANIMALS. 



closely related to one another tliat they may be best consid- 

 ered together ; and as there is least difficulty in making out 

 their arrangement in the ambulacra, the study of them may 

 be commenced in this region. 



When the suckers of an ambulacrum are carefully cut away, 

 a longitudinal ridge is seen to lie at the bottom of the groove 

 between their bases. This ridge is the amhulacral nerve. 

 Followed to the apex of the ray, it ends upon the eye and its 

 tentacle ; in the opposite direction, it reaches the oral disk, 

 at the periphery of which it divides, and, skirting the margins 

 of the disk, joins the branches formed by the bifurcation of 

 the adjacent ambulacral nerves, thus giving rise to a subpen- 

 tagonal ring round the mouth. 



The eye ^ is a thick cushion-like expansion of the ectoderm 

 continuous with the ambulacral nerve. In it are imbedded 

 many clear oval bodies surrounded by pigment, which appear 

 to represent the crystalline cones of a compound eye. 



The tentacle which lies on the aboral side of the eye re- 

 sembles one of the pedicels in structure, but has no terminal 

 sucker ; its function appears to be tactile. 



In a good transverse section of one of the arms or rays of 

 the Star-fish, the nerve is seen to be a band-like thickening of 

 the ectoderm, the cells of which have become peculiarly mod- 

 ified, but which is continuous latterly with the ordinary ecto- 

 dermal covering of the pedicels. This band-like nerve consti- 

 tutes the superficial wall of a canal, which extends through 

 the whole length of the ambulacrum, and may be termed the 

 amhulacral Pleural canal. It is divided by a longitudinal 

 septum. At its oral end, as has been seen, each ambulacral 

 nerve, when it reaches the oral membrane, divides into two 

 divergent branches, which unite with the corresponding 

 branches of the other ambulacral nerves to form the oral ring. 

 Answering to the latter is a w ide circular neural canal, into 

 which the ambulacral neural canals open. 



In the transverse section of the arm, a second and much 

 larger canal is seen to lie between the conjoined ends of the 

 ambulacral ossicles and a strong septum, containing trans- 

 verse fibres, which separates it from the neural canal. This 

 is the radial canal of the ambulacral system of vessels. At 

 its oral end it opens into the circunioral ambulacral vessel, 

 which lies close to the ossicles to which the margins of the 

 oral membrane are attached. From opposite sides of the 



> Conf. Ilacckel, Zeitschriftfiir wiss. Zoologie^ 1860. 



