• ZOOLOaY AND BOTANY^ MICROSCOPY, ETC. 39 



excretion, cannot be well looked for in the course of currents whicli 

 pass into the organism, but may be well sought for along a line of 

 centrifugal currents. 



Interstitial Connective Substance of MoUusca.* — J. Brock finds 

 that the interstitial connective substance of molluscs is very ordinarily 

 found in the region of the central nervous system, and of the great 

 nerves and vessels lining the inner surface of the ccelom, and on and 

 between the viscera ; the amount present varies greatly in different 

 species, being, for example, richly developed around the central 

 nervous system of Opisthobranchs, though very sparsely so in Pul- 

 monates ; while the conditions are reversed when we come to examine 

 the viscera. The author deals in detail with Aplysia punctata, A. 

 fasdata, A. depilans, Pleurobranchus sp., Pleurohranchcea meckeli, Helix 

 pomatia, H. nemoralis, Limax agrestis, and Avion empiricorum. 



The observations of those who have studied the embryology of 

 the Mollusca appear to make it certain that, in the later stages of 

 their development, a large quantity of spindle-shaped or branched 

 mesodermal cells are to be found in the coelom ; it is from them that, 

 in all probability, the connective substance is derived. To connect 

 the one with the other it is only necessary for a homogeneous inter- 

 cellular substance to be secreted ; by means of their processes the 

 cells come into connection with one another, and so give rise to the 

 network. Other cells increase in length and break up into fibrils, 

 and thus the whole body becomes traversed by a connected network 

 of nucleated bundles of fibrils, which are surrounded by a plexus of 

 unaltered mesodermal cells. Yet other cells become altered in com- 

 position, and become filled with carbonate of lime or concretions of an 

 indefinite character. If this be the mode of genesis of the interstitial 

 substance the lowest conditions are to be found in the Opisthobranchs ; 

 the plasma-cells are exquisitely delicate bauds in Pleurobranclicea, and 

 large compact cells with sharp processes in Aplysia punctata. 



With regard to the vexed question of the cellular lining of the 

 ccBlom Broch comes to the following conclusion: the Enterocoelia 

 always have a peritoneal epithelium which is derived from the endo- 

 derm, and which, therefore, represents a true epithelium ; the Pseudo- 

 coelia have either no (?) coelomic epithelium, or a true endothelium 

 (Mollusca) which is derived from the mesoblast and has the morpho- 

 logical value of cells of connective substance. This character may 

 be distinctly retained, as in Opisthobranchs and Pulmonata, or may 

 attain to a higher degree of difterentiation, and taking on the form of 

 a true epithelium obscure its original character, as in Prosobrauchs 

 (? all) and in Cephalopods. 



Visual Organs in Solen.t— B. Sharp has been led to believe that 

 Solen ensis and S. vagina, the common razor-shells, are possessed of 

 visual organs, by observing that a number of these animals which 

 were exposed in a large basin for sale in Naples retracted their 

 siphons when his hand cast a shadow over them. Eepeating the 



* Zeitschr. f. Wiss. Zool., xxxix. (1883) pp. 1-63 (4 pis.). 

 t Proc. Acad. Nat. Sci. Philad., 1883, pp. 248-9. 



