D.— ZOOLOGY. 79 



the bottom by means of its pedal cilia. Calcification of the plates then 

 sets in, again in order from before backwards. Simultaneously the 

 creeping sole becomes enlarged by the development of muscles, and these 

 effect attachment above to the developing plates. A cephalic plate in 

 front of the prototroch, and an anal plate behind, are added, thereby 

 completing the typical eight. The prototroch is then absorbed, and the 

 adult life begins. The larval history is thus very similar to that of a simple 

 Polychsete, although segments, in the strict sense of the term, are absent. 

 As in Polychsetes, also, the adult characters are not completed until the 

 creature has descended to the bottom. 



I In the case of Dentalium ' the trochosphere starts with a much more 

 powerful prototroch of three rows of ciliated cells, and goes much further 

 than that of Chiton in its development of adult characters during its free- 

 swimming career, for it not only establishes the complete form of its 

 tubular shell — which is much more elaborate than that of Chiton — but 

 also develops its characteristic digging foot. There is plainly an adaptive 

 connection between these two features : development of the additional 

 adult characters has been conditioned by the greater ability of the larva 



Fio. 2. — Larvae of Dentalium. 



to carry them. The ciliated prototroch is actually extended over'part 

 of the surface of the larval body by means of an internal duplicature of 

 the skin behind it, the locomotive girdle projecting freely over the front 

 of the body, like a collar over a coat. 



This adaptive modification is carried to an even greater extent in the 

 Protobranch Bivalves Nucula and Yoldia? The collar becomes a great 

 ciliated cloak or overall, and the duplicature is so deep and precocious 

 that the whole post-trochal region is developed under cover of its five 

 rows of ciliated cells, the middle three of which bear powerful flagella. 

 At the end of the larval period a diminutive adult, fully formed, is de- 

 posited on the bottom by disruption of the prototrochal envelope or ' test.' 

 Quick-change artistes are obviously not limited to the human species. 



Embryologists are familiar with many other illustrations of this kind 

 of development, e.g. the North Sea Pohjgordius, Sifuncidus. It shows by 

 easy steps how the more dramatic metamorphosis of Pilidium into a 



' Dentalium, Kowalevsky, Ann. Mvs. HlH. Nat., Marseille, T, 1883. According 

 to the earlier account by Lacaze-Duthiers, the young trochosjihere has no less than 

 seven ciliated girdles, four of which give rise to the prototroch by a process of concen- 

 tration, but their relation to the rows of cells was not described {Ann. Sci. Nat. 

 (4) VII. 18.57). 



» Nttcula and Yoldia, Drew, Q.J.Micr. Set., XLIV, 1901. 



