132 PHYLUM ECHINODERMATA. 



fact that they open directly to the exterior and apparently have 

 no relation to the coelom. The researches of MacBride have 

 however shown us that the cells composing them are, in Asteroids, 

 Echinoids and Ophiuroids, coelomic in origin, being derived 

 from lining cells of the body-cavity, and this will probably 

 be found to hold throughout the phylum. Another peculi- 

 arity of them consists in the fact that in all classes, with the 

 possible exceptions of Holothurians (see below), they are 

 connected with a cellular cord, disposed in different ways in 

 the different classes and called the generative rachis. Indeed 

 they may be regarded as swollen, fertile portions of this rachis, 

 the rest of which is sterile and does not produce generative 

 cells. As stated above the axial organ contains a prolongation 

 of the generative rachis. 



There are no accessory glands of any kind. The generative 

 products when ripe are in nearly all cases discharged directly 

 into the sea, where the ova are fertilized and undergo their 

 development. In rare cases the eggs are retained in special 

 brood pouches or on the skin of the mother. Secondary sexual 

 characters are as a rule not developed. 



The generative organs usually consist of tufts of branched 

 tubes which open directly on the surface. In Crinoids they 

 are contained in the pinnules as hollow structures without any 

 opening to the exterior, and the manner of escape of the genera- 

 tive cells is not certainly known, but it probably takes place 

 by dehiscence. In Asteroids, Ophiuroids, and Echinoids 

 they are pentamerously arranged in the interradii and parti- 

 cipate in the radial symmetry of the body ; they open inter- 

 radially and sometimes dorsally. In Holothurians, which are 

 sometimes said to be without the generative rachis,* they are 

 present as a single tuft of tubes which open in the adult-dorsal 

 middle line. 



The Echinoderms are exclusively marine animals, f They are 

 found on the floor of the ocean from the littoral to the abyss. 

 Very few pelagic forms are known. They are almost all slowly 



* Holothurians possess a cord containing germ cells and passing from 

 the point of union of the genital tubes along the generative duct towards 

 the body wall (Theel, Bih. Svenska Akad. Handl, 27, Afd. 4, No. 6, 1902). 

 This is probably the generative rachis. 



t Synapta similis inhabits brackish water of the mangrove swamps of 

 Bohol (Ludwig). 



