288 THE STARFISH. PHYLUM ECHI NODERM ATA 



is prolonged. From the junctions of the ossicles arise blunt 

 spines, each of which is surrounded by a cushion of skin. Crowded 

 upon these cushions, and scattered between them, are remarkable 

 little organs known as pedicellarise, each of which is a minute 

 pair of pincers, supported by httle ossicles, one at its base and 

 one in each jaw. The pedicellariae are defensive organs. They are 

 of two kinds, a smaller kind, found upon the cushions of the spines 

 of the back, in which the supporting ossicles cross at the base 

 like the blades of a pair of scissors, and a larger kind, scattered 

 between the spines, whose ossicles do not cross. In an interradius, 

 on the aboral surface, is a conspicuous button-like ossicle, 

 covered with fine grooves, and known, from its coral-like appear- 

 ance, as the madreporite. Its grooves are pierced with fine pores 

 through which, by the action of ciHa, water is drawn in. The anus 

 is a small opening, almost in the middle of the aboral surface, 

 but slightly displaced towards the interradius next (clockwise) 

 to that in which the madreporite lies. Each ambulacral groove 

 is crowded with tube-feet, delicate, cyhndrical tentacles, ending 

 each in a sucker, set in four rows. It is by these that the animal 

 crawls, and they are also used to hold prey. At the sides of the 

 ambulacral grooves stand a number of adambulacral spines, 

 which bear pedicellariae of the uncrossed kind and can be brought 

 together over the groove so as to protect the tube-feet. At the 

 bottom of the groove a longitudinal nerve ridge marks the position 

 of a radial nerve cord. At the end of the groove is a single sense- 

 tentacle, like a tube-foot but smaller and without sucker, which 

 subserves the olfactory sense and has at its base a red eye-spot. 



BODY-WALL, NERVOUS SYSTEM, AND CCELOM 



The body-wall is covered by a ciliated, columnar epithelium, 

 which contains gland cells and sense cells. Among the bases of 

 the cells lies a dense network of fine nerve fibrils, some of which 

 start as processes of the bases of sense cells, while others belong 

 to nerve cells which form part of the net. Above the nerve ridge 

 around the mouth and down each arm, this plexus is thickened 

 to form a special conductive system in the form of an oral nerve 

 ring with radial nerve cords, which send branches to the tube-feet 

 and end in the sense tentacles. Many of the fibrils in this system 

 are arranged to run in the directions of its strands. Under the 

 peritoneal epithelium is a similar but slighter system which 



