ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 195 



Gasteropoda from which the Heteropods sprang was of great impor- 

 tance. The free-swimming habits which these active molluscs have, 

 caused its reduction to a rudimentary organ. 



Growth of the Molluscan Shell.* — Mr. H. L, Osborn points out 

 that the structure of the molluscan shell has been studied by means 

 of sections of adult shells by Carpenter and others, who have found 

 that it presents an outer, membranous, horny epidermis, and an 

 internal stony portion. Such a method could not give any idea of 

 the actual process of shell-formation, and a knowledge of this could 

 be gained only by study of the first steps. To this end, edges of the 

 shell were snipped away and a thin glass circle thrust between the 

 animal and its shell, care being taken to prevent injury to the mantle. 

 After the lapse of twenty-four hours the shell was opened and the 

 glass circle carefully examined, others were allowed to remain two 

 days, or three days, or for periods of weeks. 



In twenty-four hours it was found that a film had been left upon 

 the circle; in forty-eight hours, this film was plainly stony. The 

 earliest traces of this film when treated with colouring reagents, 

 stain, but, when treated with acids, show no traces of lime, nor any 

 evidences of structure ; it is simply a structureless membrane. Later 

 films, when treated with acetic acid, present the appearance of a 

 tesselated pavement, and when examined with the polariseope and not 

 treated with acetic acid, show beautifully the presence of lime. 



It would thus appear that the epithelium of the mantle pours out 

 a. secretion of horny matter which forms the epidermis ; that this 

 secretion holds lime in solution ; and that from this the stony internal 

 portion of the shell is formed. Experiments were successfully made 

 upon the shells of the oyster and Pinna and several other Lamelli- 

 branchs, and some Gasteropods were tried, but thus far in vain. 



MoUuscoida. 



Individual Variation in Ascidians.f — Prof. Herdman points out 

 that the specific determination of Ascidians has always presented con- 

 siderable difficulties, owing to the apparent absence of reliable 

 characteristics. His own investigations have shown him that the 

 " olfactory " or dorsal tubercle is very variable, and the same is 

 largely true of the branchial sac. Further observations on the 

 common Styela grossularia have led the author to recognize a " con- 

 tinuous series connecting a well-developed branchial fold with the 

 most rudimentary indication of where the fold ought to be." We 

 may have, that is, the projection of the sac disappearing, while the 

 bars are still close ; the bars may then become more open ; then there 

 may be only three rows of narrow meshes ; then two, then one row ; 

 this single row may extend only part of the way, and at last, the 

 right-hand fold may be entirely absent. All the branchial sacs 

 described were found in individuals with fully developed genital 

 organs. In addition to some illustrative figures, an ingenious diagram 

 illustrating this variability is given. 



* Johns Hopkins University Circulars, 1882, Nov., p. 7. 

 t Proc. Lit. and Phil. Soc. Liverpool, 1882, pp. 12 (2 pis.). 



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