paratively few North European Echinoderms a great percentage still 

 remain unknown as regards their development, our knowledge having for 

 a great part not reached beyond Joh. Miiller. To a no smaller extent 

 this holds good also for the North American types. 



It is a strange fact that in the numerous works on hybridization of 

 Echinoderms it is generally only the first larval stage which is taken into 

 consideration. Beyond this stage the investigators rarely go. Thus e. g. 

 Sphserechinus granulans, which is used over and over again for hybridi- 

 zation experiments, has not yet been reared to its full larval shape; in 

 fact, it is only by inductive evidence that we may now with a fair degree 

 of certainty refer one of the larvae described by Joh. Mull er to that spec- 

 ies. Similarly the larvse of Strongylocentrotus franciscanus and purpuratm 

 used so very much for hybridization studies by American investigators 

 are as yet known only in their first stage. AsMacBridei) justly says: 

 "to judge from much of what has been written on this subject, no one 

 would ever suspect that the larva of an Echinoid had more than four arms." 

 A highly praiseworthy exception from the rule forms the work by Shearer, 

 Morgan and Fuchs "On the experimental hybridization of Echinoids" 2), 

 these investigators having studied not only both the young and the final 

 stage of the normal as well as the hybrid larvae, but also reared the hybrids 

 beyond metamorphosis and even nearly to the full size of the sea-urchin. 

 Hybrids of the same forms (viz. Echinus esculentus, acutus and Psamnv- 

 echinus miliaris) were likewise reared through metamorphosis by De- 

 baisieux*). Also Tennent*) has reared some of the larvae used for his 

 experiments to their full shape. But, as stated, the general rule is that 

 only the first larval stage is used in the hybridization and heredity studies, 

 the experimentators having apparently no idea of what the normal larvae 

 look like in their full shape. 



I do not mean to deny, of course, that the young larvae do very often 

 afford striking characters already in the first stage, so that something 

 may be concluded from the mixing up of these characters in the young 

 hybrid larvae as to the inheritance and dominance of the maternal or 

 paternal characters. But it seems selfevident that much more valuable 

 results would be gained from these hybridization studies if carried through 

 at least to the final larval form. The ideal must be, evidently, to 

 rear not only the hybrid larvae to their full size, but to get 



1) studies in HerediLy. I. Tlie effects of crossing the Sea-urchins Echinus esculentus and 

 Echinocardium cordatum. Proc. R. Soc. B. Vol. 84. 1911. p. 398. 

 ") Philos. Transact. Ser. B. Vol. 204. 1914. 

 ^) Quart. Journ. Micr. Science. N. S. Vol. 58. 1913. 



*) David H. Tennent. Echinoderm Hybridization. Publ. No. 132. Carnegie Inst. Wash- 

 ington. 1910. 



