ISOMETRA VIVIPARA. 31 



III. ISOMETRA VIVIPARA Mortensen. 

 (Plates XIV to XXIII.) 



In 1904, Dr. K. A. Andersson published an account of a viviparous Crinoid 

 found in the Antarctic Sea by the Swedish South Polar Expedition. 20 This 

 was the first instance of care of the brood known among Crinoids. K. A. 

 Andersson referred the species to Antedon hirsuta P. H. Carpenter. A. H. 

 Clark, 21 however, pointed out that it was more closely related to Carpenter's 

 Antedon angustipinna, for which the genus Isometra was established, and in 

 my report on the Crinoidea of the Swedish Antarctic Expedition I described 

 it under the name Isometra vivipara. 22 



Dr. K. A. Andersson gives only a short account of the development of the 

 species and expected to write a more detailed report later, but, not finding 

 opportunity for so doing, he kindly left the interesting material to me for 

 study, for which I thank him very cordially 



The preservation of the material simply by means of alcohol is, of course, 

 not of the best, but it suffices except for the finer histological details. 



For a more detailed account of the viviparous habit and the way in which 

 the care of the brood is carried out reference must be made to the papers by 

 K. A. Andersson and the present author as quoted here. 



I. CLEAVAGE; FORMATION OF ENTODERM. 



The egg is about 0.3 mm. in diameter. The reticulated structure of part 

 of the egg mentioned by K. A. Andersson (op. cit., p. 4) I believe to be the 

 result of preservation instead of a normal structure. It is very rich in yolk 

 and quite opaque. As pointed out by Andersson, the eggs are found in various 

 stages in the ovary, not ripening all at the same time, as is the case in other 

 Crinoids, and they are not emptied all at the same time as normally in 

 Crinoids, but gradually as they become ripe. This accounts for the fact that 

 the embryos are in all different stages of development one may find fully 

 formed larvae together with embryos in the first cleavage stages in the same 

 marsupium a very fortunate circumstance for the study of the develop- 

 ment, as there is thus no difficulty in finding all stages of development in 

 this species, such as was the case in Compsometra serrata and Notocrinus 

 virilis. On the other hand, there is no possibility of ascertaining how long a 

 time is required for development, and the direct age of the different stages 

 can not be given, as it was for Tropiometra and by Seeliger for Antedon. 



"" K. A. Andersson. Brutpflege bei Antedon hirsuta Carpenter. Wiss. Ergebn. d. Schwed. Sildpolar- 

 Exped. 1901-3, vol. v, 1, 1904. 



21 A. H. Clark. Die Crinoiden der Antarktis. Deutsche Siidpolar Exped., xvi, Zoologie, Bd. VIH, 

 1915, pp. 106, 146. 



'" Th. Mortensen. The Crinoidea of the Swedish Antarctic Expedition, Wiss. Ergebn. d. Schwed. 

 Sadpolar-Exped. 1901-3, vol. vi, 8, 1917, p. 10-15, plate I, figures 6 to 10; plate n, figures 5 to 7. 



