382 THE VOYAGE OF H.M.S. CHALLENGER. 



than certain obscure temporary processes attached to the embryo, to which I have else- 

 where given the name of ' pseudembryonic appendages.' 1 



" This direct mode of development has been described in Holothuria tremula by MM. 

 Koren and Danielssen, in Synaptuia vivipara by Professor Oersted, in a ' viviparous sea- 

 urchin ' by Professor Grube, in Echinaster and in Pteraster by Professor Sars, in Astera- 

 canthion by Professor Sars, Professor Agassiz, Dr. Busch, and by myself, in Ophiolepis 

 squamata by Professor Max Schultze, and in ' a viviparous Ophiurid ' by Professor Krohn. 

 No less than four of these observations were made on the coast of Scandinavia. In 

 temperate regions, where the economy of the Echinoderms has been under the eye of a 

 greater number of observers, the development of the free-swimming larva appeared to be 

 so entirely the rule that it is usually described as the noi'mal habit of the class ; while 

 on the other hand, direct development seemed to be most exceptional. I was therefore 

 greatly surprised to find that in the Southern Ocean and sub-antarctic regions a large 

 proportion of the Echinoderms of all orders, with the exception perhaps of the Crinoids 

 (with regard to which we have no observations), develop their young after a fashion which 

 precludes the possibility, while it nullifies the object, of a pseudembryonic perambulator, 

 and that in these high southern latitudes the formation of such a locomotive zooid is 

 apparently the exception. 



" This modification of the reproductive process consists in all these cases, as it does 

 likewise in those few instances in which direct development has already been described, 

 of a device by which the young are reared within or upon the body of the parent, and 

 are retained in a kind of commensal connection with her until they are sufficiently grown 

 to fend for themselves. The receptacle, in cases where a special receptacle exists in 

 which the young are reared, has been called a ' marsupium ' (Sars), a term appropriately 

 borrowed from the analogous arrangement in their neighbours the aplacental mammals of 

 Australia. The young do not appear to have in any case an organic connection with the 

 parent ; the impregnated egg from the time of its reaching the ' morula ' stage is entirely 

 free ; the embryos are indebted to the mother for protection, and for nutrition only 

 indirectly through the mucus exuded from the surface of her perisome, and through the 

 currents of freshly aerated water containing organic matter brought to them or driven 

 over them by the action of her cilia. 



" Animals hatching their eggs in this way ought certainly to give the best possible 

 opportunities for studying the early stages in the development of their young. Unfor- 

 tunately, however, this is a kind of investigation which requires time and stillness and 

 passable comfort ; and such are not the usual conditions of a voyage in the Antarctic 

 Ocean. Specimens have been carefully preserved with the young in all stages ; and I 

 hope that a careful examination of these may yield some further results. 



" Cladodactyla crocea is one of the forms in which there is no special marsupium 



1 Phil. Trans., p. 517, 1865, 



