FURTHER RESEARCHES 19 



that Ascidians are not invertebrates but members of 

 the great sub-kingdom of back-boned animals. He 

 also confirms previous statements that the "test" or 

 firm investment which characterizes members of the 

 group contains cellulose, a substance once thought to be 

 peculiar to plants, and of which cotton may be taken as a 

 good example. 



3. " On the Morphology of the Cephalous Mollusca, 

 as illustrated by the Anatomy of certain Heteropoda and 

 Pteropoda collected during the voyage of H.M.S. Rattle- 

 snake in 1846-50" (Phil. Trans. Roy. Soc, cxliii, 1853, 

 pp. 29-66. Read at the Brit. Assoc. Meeting of 1852. 

 Sci. Mem., i, xvn, p. 152). — The "cephalous" Mollusca 

 are those which possess a distinct head, e.g., cuttle-fishes 

 and snails, and the groups named include pelagic animals 

 belonging to the latter category. After remarking on the 

 unsatisfactory state of knowledge as regards Mollusca, 

 the author says that this appears, — 



" to result from two causes ; — first, from the want of a clear and 

 definite conception of the fundamental varieties of molluscous 

 structure, and of the nature of the changes in the relations of 

 parts which constitute those varieties ; and secondly, from the 

 want of a due regard to the facts presented by the development 

 of the various families, which must stand in the relation of cause 

 to the varieties of form. 



" Now in order to the former end (the obtaining of a definite 

 conception of the varieties of molluscous form), I propose to set 

 forth the structure of certain Heteropoda and Pteropoda ; pelagic 

 animals so transparent, that a perfect knowledge of the arrange- 

 ment of their parts may be arrived at by simple inspection, with- 

 out so much as interrupting a beat of their heart. 



"Afterwards, I shall inquire how far the known laws of 

 development account for these forms, and thence of what arche- 

 typal form they may be supposed to be modifications." 



Many additions to our knowledge were made in the 

 descriptive part of this memoir, though the mistaken 



