10. Viviparity as a Factor Producing Direct Development.

It is clear that viviparity must act as a factor producing direct development for the simple reason that it automatically effaces the pelagic stage from the ontogeny. It had been my hope that my study of the development of Amphipholis squamata would provide material for understanding the effects of viviparity. Unfortunately, however, Amphipholis, as already noted, has a yolky egg, and practically all the alterations in development noted are attributable to this factor. The only other viviparous ophiuroid of whose development we have much information is Ophiomyxa brevirima, of which I hope to publish a further paper at a later date. Here again the embryo proves to be extremely yolky and has a schizocoelous coelom (Fell, 1941). Indeed, from such facts as we possess at present, it seems that a yolky egg is a condition frequently associated with viviparity in ophiuroids.

One result of viviparity, however, is certain. This is, that the young sea-star is enabled to pursue its development to a much more advanced stage in the viviparous forms, before emerging into the outer world and seeking its food itself. Thus, in Amphipholis squamata, the newly born star has arms with ten to fourteen armsegments, whereas the newly metamorphosed star of oviparous forms is still in the unsegmented “Asterina” condition.

By means of experimental culture in vitro of explanted embryos of Amphipholis squamata it was shown that the embryo is unable to develop on its own yolk material alone (Fell, 1940 b). Certain substances must be added to the culture medium to enable normal development to proceed. This result taken in conjunction with anatomical evidence led to the conclusion that the developing embryos in Amphipholis are nourished by a secretion from the wall of the bursa. Here, then, is one undoubted effect of viviparity upon development in Ophiuroidea—that of prolonging embryonic life to a later stage by the secretion of nutritive substances. This effect is well illustrated by the following data:—Ophiocomina nigra, a non-yolky form with pelagic Ophiopluteus, has a functional alimentary canal by the third day, and continues to take in food for the remaining 35 days of larval life, before metamorphosis. A similar food-gathering larval period is found in Ophiothrix fragilis, which also has a functional