WYVILLE THOMSON, ON SYNAPTA INHiEKENS. 145 



these young Holothurise were under observationj the structure 

 and appearance of the calcareous framework changed remark- 

 ably, a fact which invalidates to a great extent the reference 

 of an embryo to a certain species, on account of the form of 

 the calcareous bodies first produced. 



Professor Oersted (' Videnskabelige Meddelelser fra den 

 Naturhistoriske Forening i Kjobenhavn' for the year 1849)* 

 states that Synaptula vivipara (Oersted) produces its young 

 li"\4ng, and somewhat similar in form to the parent. 



It appears, then, that in the Holothuridse, the germ- mass, 

 the product of the complete segmentation of the yelk, changes 

 either entirely into sarcode, or into a thick wall of sarcode 

 still containing some of the granules of the germ-mass within. 



The body thus formed is oval, and becomes covered v/ith 

 cilia developed upon the structureless sarcode surface ; it is 

 of course locomotive, and absorbs nourishment through its 

 whole surface. In some cases a depression at one extremity 

 now indicates the mouth of the embryo, an alimentary canal, 

 which contains the unaltered granular matter of the germ- 

 mass, if there be any, passes through the long axis of the 

 oval ; the outer sarcode covering is studded Avith plates and 

 spiculse, losing its cilia; and the whole germ-mass takes the 

 form of a young Holothuria, which proceeds to its complete 

 development, by a mere differentiation of its organs from its 

 sarcode substance, and Avithout any further metamorphosis. In 

 other cases the ciliated sarcode-germ increases rapidly in size, 

 and sends out symmetrical lobes and borders, Avhich are clothed 

 with a ciliary fringe, disposed doubtless with reference to the 

 position of the centre of gravity of the body. A mouth and 

 anus, united by a loop-like alimentary canal, indicate a special 

 absorbent surface, and the peculiar intermediate being is 

 formed, Avhich I have called in this paper a pseudembryo, and 

 Avhich I have regarded as analogous to the provisional eipi- 

 bryonic appendages in the higher animals. This zooid enjoys 

 for a certain time a perfectly independent existence. Its 

 fringes and lobes then contract, it becomes simply cylindrical 

 in form, the cylinder hooped by bands of cilia. The pro- 

 visional mouth and oesophagus disappear, and the development 

 of the Holothuria from this sarcode-cylinder now proceeds, 

 exactly as it did in the former case from the sarcode-germ. 



I have little doubt that we shall find in the Holothuridse 

 gradations between these, by no means extreme, forms of 

 development. At present I know of only one case which 

 appears to be perfectly intermediate. In Comaiula a sarcode- 

 cylinder closely resembling the pupa of Miiller's Holothui'ia, 

 * Quoted by DanitUseu and Korcii, op. cil. 



