THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 125 



echinus) and E. esculcntus or E. acidus, there is much less distinction 

 between the two latter, which we suspect to be merely varieties of 

 one and the same species, although according to Bell (2) the number 

 and disposition of the plates differ considerably. We were quite 

 unable, from a superficial examination, to tell to which species a large 

 number of specimens, brought into the laboratory during the course of 

 our experiments, belonged. In shape and character of their spines 

 they were as much E. esculentus as E. acutus, and without a close 

 examination of their plates it was quite impossible to identify them. 



In the "Ingolf" Echinoidea I (14), Mortensen mentions some 

 specimens, which " combine to a curious degree the character of both 

 E. esculentus and acutus, var. Fleminrjii, so that it is quite impossible 

 to decide with certainty to which of these species they belong, and 

 the supposition of their being hybrids between the two species seems 

 very obvious." In his recent paper (15) he gives a photograph of 

 one of these supposed hybrids, with a description. We ourselves 

 have long suspected that such hybridization must take place between 

 E. esculentus and E. acidus at Plymouth, where they live side by side 

 on the same beds, their breeding periods overlapping very consider- 

 ably. If crossing between these two forms is such an easy matter in 

 the laboratory, why should it not take place in a state of Nature ? 

 This is a question that has also been raised by Doncaster (5). We 

 believe that considerable hybridization does take place, and the forms 

 to which we have drawn attention above, and which agree in many 

 respects with the one figured by Mortensen (15), seem to bear out this 

 conclusion. In our full paper we shall give photographs and go into 

 details of the plates of these intermediate forms. 



Mortensen (15) also gives a figure of a form which he considers to 

 be a cross between E. esculentus and E. miliaris. We have already 

 mentioned in a footnote, page 124, that there are several varieties of 

 E. miliaris at Plymouth, and one of these bears a considerable resem- 

 blance to Mortensen's supposed hybrid. This variety differs, however, 

 in the character of its spines from the young hybrids of this cross 

 which we have reared. 



We have been unable to detect differences between the larvse of 

 E. esculentus and E. acutus, except that the skeletal apical rods of the 

 latter are more robust and less arched than those of the former. For 

 this reason, in the present paper, we have not considered in any detail 

 the development of E. acutus, beyond pointing out that, when crossed 

 with E. miliaris, it behaves similarly to E. esculentus. In our forth- 

 coming paper we will go into these points in full. 



We have tried to eliminate from our cultures, as far as possible, all 



