paralively lew Xoitli I Miiopciiii Mchinodcrms a jJit-al percenlage still 

 remain unknown as regards Iheir devolopnu-nl. our knowledge having for 

 a great pari not reached beyond .loli. 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 liybridi/ation of 

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

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

 Sphierechiniis granularis, 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 larva> described by .Job. Miiller to that spec- 

 ies. Similarly the larvae of SlroirgiilocciUiolus franciscdiius and purpuidlus 

 used so very much for hybridization studies by American investigators 

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

 "to judge from imich ol wlial has been written on this subject, no one 

 would ever suspect that the larva of an b>hinoid had more than four arms." 

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

 Morgan and Fnchs "On the experimental hybridization of bkhinoids"^)^ 

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

 stage of the normal as well as the hybrid larva', 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 cscnlentus, aculiis and Psamm- 

 echinus milidiis) were likewise reared through metamorphosis by De- 

 baisieux^). Also Ten nen t^) has reared some of the larva' u.sed for his 

 experiments to their full shape. Hut, as staled, 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, thai the young larva? 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 larvse as to the inheritance and dominance of the maternal or 

 paternal characters. But it seems selfevidenl that imuli 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 larva' to their full size, but to get 



') Sludics in HcTi-dily. I. 'I'lie flTi-cls ol crossing tlio Sea-nrcliins Hcliinus csculenlus and 

 Echinocardiiini conlaUini. Proc-. H. Soc. B. Vol. SI. 1011. p. .H(»«. 



°) Philos. Transact. Ser. B. Vol. 204. 1014. 



') Quart, .lourn. .\licr. Science. N. S. Vol. .')S. I'.ti:!. 



*) David H. Tcnnent. Echinodcrm Hyhridization. Piibl. No. KVi. Carnegie Inst. Wash- 

 ington. 1910. 



