40 ASTERIAS VULGARIS. 



d) In the spaces between the calcareous plates are dermal pores 



through which project very small, soft, tubular processes, the 

 dermal branchiae, which function as organs of respiration and, 

 perhaps also, of excretion and sensation. They are capable of 

 being retracted, and will probably be seen with difficulty if at all. 

 In the dried specimen the pores will be closed and the branchiae 

 shriveled up. 



e) The pedicillariae, very small pincher-like organs situated at the 



bases of some of the spines and in the intervals between them. 

 Their function is supposed to be that of keeping the surface of the 

 body free from particles of dirt or minute organisms that might 

 prove harmful. 



Exercise i. Draw the aboral surface on a scale of two-thirds. The 

 drawing may be natural size if the specimen is small enough to be 

 conveniently figured. 



Exercise 2. Draw the madreporite, enlarged three or four times, using 

 the lens to determine the structure. 



ORAL SURFACE. Study the oral surface for: 



a) The mouth, situated in the center of the disc. It is surrounded 



by a tightly stretched membrane, the peristome, capable of 

 considerable dilation and contraction. The mouth is protected 

 by a number of specialized spines. 



b) The arms on this surface are channeled by the ambulacral grooves 



into which project, from the interior of the body, the tube feet. 

 In the dried specimen these are much shrunken. How many 

 rows of tube feet in each ambulacral groove ? What is the 

 arrangement of the rows and of the tube feet in the row ? 



c) In the center of each ambulacral groove is a faint line marking the 



position of the radial nerve. These five nerves proceed from 

 a nerve ring in the central disc, and each ends in a pigment 

 fleck, the eye-spot, at the tip of the arm. The nerve ring can not 

 be demonstrated in this exercise. If the eye-spot has faded out, 

 look for it later in a specimen preserved in liquid. 



