INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 417 



one another by distinct commissures. At first the ganglia are all 

 formed of compact groups of small cells; later on the nerve-cells 

 pass to the periphery, and the central region comes to be occupied by 

 their processes only. 



The eye first commences as an invagination of part of the ectoderm 

 of the rudimentary tentacle ; the lens is a metamorphosed cell of the 

 wall of the optic vesicle ; as it becomes spherical, its protoplasm gets 

 to be homogeneous and highly refractive, while the nucleus almost 

 completely disappears. The otocysts arise from a somewhat indistinct 

 invagination of the side wall of the foot ; later on, they approach one 

 another, and become connected with the pedal ganglia. 



As the cells of the mesoderm increase in number they undergo 

 various modifications ; those near the ectoderm form a cuticular layer ; 

 others form connective tissue ; the serous layer of the enteron forms 

 a thick pigmented investment to the stomach. The muscle-fibres, best 

 seen in the columella muscles and in the retractor of the radula, 

 are merely elongated mesodermal cells, in each of which the nucleus 

 remains distinct. 



Formation of the Shell in the Snails.* — This subject is dealt 

 with by MM. Lougc and E. Mer. The shell of Helix is composed 

 of two principal layers of organic and mineral nature covered by a 

 cuticle solely organic. The first layer is composed, commencing 

 exteriorly, of a structure showing confused striation, having nearly 

 the same thickness as the cuticle, and of another, thiclvor, formed of 

 vertical prisms. It is to this that the general coloration of the sliell 

 is due as well as that of the spots and bands. The second layer 

 (nacre) is colourless and consists of several strata of prisms arranged 

 horizontally and whose axes in two successive layers arc nearly 

 perpendicular to each other. 



The cuticle and the calcareous layers are produced by difibrent 

 regions of the mantle, approaching nearer the collar in proportion as 

 the layers are more superficial. The cuticle is formed by an 

 apparatus which tlie authors think has not hitherto been described, 

 and which they call the " appareil cutogene." This consists of two 

 special organs iunuediately behind the collar. The one is composed 

 of a groove named by the authors the pallial grot^ve, parallel to tho 

 border of the mantle, at the bottijm of wliich glandular cieca open ; 

 the otlier, behind the fir.st, appears on an antero-posterior section liko 

 an epithelial wedge forced into the substance of the mantle. It is 

 formed of long vertical cells, like buttles, wliose orifices open at tho 

 base of the organ provisionally termed the epithelial organ. These 

 cells enclose granules separable in i)otaKh and a nucleus in their 

 deeper-seated i)art. They must bo regarded as ditlcrentiated epithelial 

 cells. 



The cutogenic ajiparatus exists in the embryo when still enclosed 

 in tho ovular envelopes, at which period tho shell is already provided 

 witli a cuticle. It subsists during the whole growth ot tlio young 

 Ilelix ; the epithelial organ showing itself under tho aspect of a 



* 'Comptoa Rciulus," xc (1880) p. 882. 



