44 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Structure and Formation of the Shell of Lamellibranchs.* — The 

 obscrvatious of E. Elireubanm are founded on the species of lamclli- 

 brauchs most common in the Bay of Kiel. After some introductory 

 remarks of an historical and critical character, Mijtilns cduUs is dis- 

 cussed ; the pei'iostracum is divided into (1) an extremely delicate 

 very clear marginal fringe, (2) a delicate cuticular lamella, (3) 

 layer tif cavities, (4) second cuticular lamella, and (5) a dark-coloured 

 finely homogeneous cuticular layer, which forms the boundary of the 

 blue shell-substance. The author was unable to detect the vertical 

 canals which have been seen and figured by earlier authors in the 

 nacreous substance of the shell ; those so regarded by Tullberg are 

 thought to be couchiolin-membriiues, and they are not so straight and 

 parallel to one another as they are represented in figures. Two 

 interesting modifications of the white substance arc described. 



Cyprina islandica, Astarfe borealis, and species of Cardium, Scrobi- 

 cularia, and Tellina arc discussed in order; these are followed by 

 Corhula tjihha, Solen pelliicidus, and Mya arcnaria. 



The proportion of inorganic to tlie organic constituents of the 

 shell varies considerably, and to this we must ascribe the great 

 difierences in the specific gravity and hardness of lamellibranch 

 shells. 



Nothing of importance has been added to our knowledge of the 

 physiological processes of the secretion of the hard part of tbe shell 

 since the studies of C. Schmidt in 1845, althougli indeed Hartiug 

 has made some important experiments on the process of crystalliza- 

 tion ; the problem of shell-formation might be reduced to a mechanical 

 question by further observations along the lines of that distinguished 

 observer. Our knowledge of the formation of the epicuticle is much 

 better, owing especially to the studies of Tullberg. The great result 

 of the author appears to be to confirm the old doctrine that all the 

 parts of the lamellibranch shell are to be regarded as true cuticular 

 structures cr, in other words, as cell-secretions. 



MoUuscoida. 



a. Tunicata. 



Follicular and Granular Cells of Tunicates.f — A Sabatier deals 

 with the criticisms made by Fol on his work on the ovum of Ascidians. 



With Fol and Eoule he thinks that the follicular cells arise in the 

 interior of the ovum, near the nucleus, and that they then make their 

 way to the surface of the yolk ; the difference between them is as to 

 the mode of formation of these elements. Fol and Eoule think that 

 they start from one element which is derived from the germinal 

 vesicle, Sabatier believes that they arise from the elements of the 

 yolk which are differentiated in the layer which surrounds the 

 germinal vesicle ; and that the latter takes no essential share in 

 their formation. 



Sabatier now points out that if we observe young ova of Ciona at 



» Zeitbchr. f. Wiss. Zool.. xli. (1SS4) pp. 1-47 (2 pis.)- 

 t Rev. Sci. Nat., xii. (1SS4; pp. 106-40 ,2 pis.). 



