56 SIR C. WrVILLE THOMSON ON THE MODE OF 



mediately from the segmented yelk without the formation of a 

 " pseudembryo," or at all events with no further indication of its 

 presence than certain obscure temporary processes attached to 

 the embryo, to which I have elsewhere (Phil. Trans, for 1865, 

 p. 517) given the name of " pseudembryonic appendages." 



I have not at present an opportunity of consulting autho- 

 rities ; but if I may trust my memory, this direct mode of de- 

 velopment has been described in Holothuria tremula by MM. 

 Koren and Danielssen, in Synaptula vivipara by Professor 

 Oersted, in a " viviparous sea-urchin " by Professor Grrube, in 

 EcUnaster and in Fteraster by Professor Sars, in Aster acantliion 

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

 OpUolepis sqnamata by Professor Max Schultze, and in " a vivi- 

 parous ophiurid" by Professor Krohn. Ko less than four of 

 these observations were made on the coast of Scandinavia. In tem- 

 perate regions, where the economy of the Echiuoderms 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 normal 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 and subarctic seas 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 

 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 connexion with her until they are 

 sufficiently grown to fend for themselves. The receptacle, in cases 

 where a special receptacle exists in which the yovmg are reared, 

 has been called a " marsupium " (Sars), a term appropriately 

 borrowed from the analogoiis arrangement in their neighbours the 

 aplacentai mammals of Australia. The young- do not appear to 

 have in any case an organic connexion with the parent ; the im- 

 pregnated e^g from the time of its reaching the "morula " stage 

 is entirely free ; the embryos are indebted to the mother for prb- 



