2'1:j 



nkjuo do IM. (c/j/icrr. l)c iiieiiu' Asterias Miillrii n'esl sans doule qu'une 

 vaiietc i)occiloi»()iik|iu' scpU'iilrionalo du vulgaire.l.s7<'r/r/.s qUiciaUs des cotes 

 do France." In Ihese Iwo cases lliere is, of course, no doubl as lo the cor- 

 ix'cLness of the observations. Hut the .said forms are far from being only 

 "poieilogenetic varieties" of one and the same species of Aslerina or Aslc- 

 lias; as anybody having some systematic knowledge of these great "genera" 

 of starfishes will agree, lliey are very well separated species which, on 

 modern classilicalion, sliould at least be referred to dilTerent subgenera. 



In spite of the ])a(l luck of the instances of poicilogony among Eciiino- 

 derms produced by (liard 1 do not pretend to maintain that poicilogony 

 does not exist within tliis animal group. In fact, we have an almost typical 

 instance in Asterias Mulleri — gioenlandica. These two forms are so closely 

 related that they are hardly distinguishable and apjx^ar to be merely var- 

 ieties of the same species. But while .1. Midlcri, as already observed by 

 M. Sars, hatches its eggs in a brooding cavity made by the raised arms, 

 A. tjroenlandica, according to the beautiful observations of I. Lieber- 

 kindi) hatches its eggs within the stomach. It is unknown as yet whether 

 there is any difl'erence in the embryonal development of these two forms; 

 but at any rate the breeding habits of these two forms are remarkably 

 difl'erent. 



The cases of nearly related species showing (juile a dilTerenl mode of 

 development, although not lo ])e termed directly poicilogony, are of very 

 considerable interest as related phenomena. Such cases were known hith- 

 erto only in the genus Asterias, but have been proved now to exist also 

 within the genus Aslerina (s. lat.) and among the sea-urchins in the genus 

 Ileliocidaris, where //. luherculala has a typical pelagic larva, II. erijlhro- 

 (jramma a barrel-shaped larva showing no likeness whatever lo a Pluteus. 

 (It must be emphasized that there can be no doubl Ihal the two said 

 sea-urchins are really nearly related species, decidedly belonging to the 

 same genus). In a good many cases of nearly related species one has pel- 

 agic larva>, while Ihe other is viviparous (Echinociiamus pusillus and ini- 

 Irix, several Ophiurids). l^pou Ihe whole more extended researches will 

 be sure to disclose many cases of dissimilar development in nearly related 

 forms. In this connecliou menliou nuisl also be nuule of llie interesting 

 observalioii by Nach I sliei in'-) Ihal in Echinaster seposiliis the eggs may 

 dilTer very conspicuously in size; the development, however, is the same 

 in both larger and smaller eggs, and the larva^ developing from them dilTer 

 only in size. 



') I. LicbiTkiiul. On a slarlisli (.\storias grocnlaiidica) which liaU-lics its youiifl in its 

 stomach. Vid. .Medd. D. Nat. Foren. 72. 1921. 



-) H. Nachl siici ni. l^bcr die Knt\vicl<hini; von Echinaster scposUus (Gray). Zoo). An?.. 

 .\I.I\-. I>)ll. J). 601. 



31* 



