243 



nique de VA. cephea. De meme Asterias Mulleri n'est sans doute qu'une 

 variete poecilogonique septentrionale du \u\gSiire Asterias glacialis des cotes 

 de France." In these two cases there is, of course, no doubt as to the cor- 

 rectness of the observations. But the said forms are far from being only 

 "poicilogenetic varieties" of one and the same species of Asterina or Aste- 

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

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

 modern classification, should at least be referred to different subgenera. 



In spite of the bad luck of the instances of poicilogony among Echino- 

 derms produced by Giard I do not pretend to maintain that poicilogony 

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

 instance in Asterias Miilleri — groenlandica. These two forms are so closely 

 related that they are hardly distinguishable and appear to be merely var- 

 ieties of the same species. But while A. Mulleri, as already observed by 

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

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

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

 there is any difference in the embryonal development of these two forms; 

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

 different. 



The cases of nearly related species showing quite a different mode of 

 development, although not to be 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 Asterina (s. lat.) and among the sea-urchins in the genus 

 Heliocidaris, where H. tuberculata has a typical pelagic larva, H. erythro- 

 gramma a barrel-shaped larva showing no likeness whatever to a Pluteus. 

 (It must be emphasized that there can be no doubt that 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 larvae, while the other is viviparous (Echinocyamus pusillus and nu- 

 Irix, several Ophiurids). Upon the whole more extended researches will 

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

 forms. In this connection mention must also be made of the interesting 

 observation by Nachtsheim^) that in Echinaster sepositus the eggs may 

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

 in both larger and smaller eggs, and the larvae developing from them differ 

 only in size. 



1) I. Lieberkind. On a starfish (Asterias groenlandica) which hatches its young in its 

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



2) H. Nachtsheim. Uber die Entwicklung von Echinaster sepositus (Gray). Zool. Anz. 

 XLIV. 1914. p. 601. 



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