STUDIES ON THE HYBRIDIZATION OF ECHINOIDS. 



By David H. Tennent. 



PART I. EMBRYOLOGY AND HYBRIDIZATION OF 



CIDARIS. 



In 1912, at Montego Bay, Jamaica, I obtained material and began 

 the study of straight-fertilized eggs of Cidaris tribuloides Lamarck, 

 of Cidaris eggs fertilized with the sperms of Lytechinus (Toxopneustes) 

 variegatus, of Cidaris eggs fertilized with the sperms of Tripneustes 

 {Hipponoe) esculenta, and of Cidaris eggs caused to develop par- 

 thenogenetically. A brief account of some of the facts determined 

 appeared in Publication No. 182 of the Carnegie Institution of Wash- 

 ington. The present paper includes my completed observations. 



Nature and Systematic Position of the Material, 



The nature and systematic position of the forms used demand more 

 than passing notice. Cidaris represents the lower extreme of a series 

 extending from Httle speciahzed to highly specialized Echinoids; 

 Lytechinus and Tripneustes represent the upper extreme. Jackson 

 (1912) has shown that the Cidaroida are primitive, extending from 

 the Lower Carboniferous to Recent times. He says: 



"The most primitive type of Echini, I believe emphatically, is Bothriocidaris 

 [p. 208J. . . . The order Cidaroida is placed as derived directly from the Both- 

 riocidaroida without known intermediate forms. The Cidaridae, as regards the struc- 

 ture of the young and adult, are the least removed from Bothriocidaris of any known 

 echinoid, living or fossil" [p. 21ll. 



Lytechinus and Tripneustes are members of the order Centrech- 

 inoida (Triassic to Recent), of the suborder Camarodonta, and of 

 the family Echinidse (Cretaceous to Recent). Again quoting from 

 Jackson (p. 210): 



"The sub-order Camarodonta may be considered the most specialized of modern 

 regular Echini on the basis of the lantern, and also in various genera by the sculptured 

 test, the degree of specialization of the ambulacrum, peristome, perignathic girdle, 

 or the elliptical form through a sidewise axis." 



In discussing the lantern, Jackson says (p. 187) that "Tripneustes 

 represents the most complex structure known in the Centrechinoida." 



H. L. Clark (1912, p. 365), in his consideration of the Echino- 

 metridae, says: 



"There can be little question that this family includes the most highly specialized 

 of the regular recent Echini, for the elongation of one axis, when combined with highly 

 developed ambulacra, indicates an unusual complexity of structure. And yet in the 



