188 Misce JIa neons. 



ficial epithelium of the body, with the exception of the portion 

 lying above the mouth — the subsequent '■^ ceiihalic vesicle" — becomes 

 transformed, owing to active multiplication of cells, into a tissue 

 composed of little cylindrical elements. 



The cerebral ganglia arise from the epitlielium of the sensory plates 

 in the form of solid p>i'oliferations, which separate from their matrix 

 and soon become connected by a strong commissure lying above the 

 fore-gut, Soon after this has taken place the sensory plates bud 

 outwards on each side into three blunt papillae, and so form the 

 earliest rudiments of u'hat are suhsequently the two tentacles and the 

 oral lobes. The epitheJiimi of the rudiments of the tentacles gives 

 origin by proliferation to the tentacular ganglia., while soon after the 

 separation of the cerebral ganglia from the epithelium of the sensory 

 plates the latter give rise to yet other structures, which subsequentlj' 

 come to have relations with the cerebral ganglia, and possess the 

 highest interest for ns. These structures are the " cerebral tubes : " 

 I shall deal further with them below. 



Simultaneously with the cerebral ganglia the two pedal ganglia 

 arise in a precisely similar way from the epithelium of the foot-plate, 

 and soon become connected together by commissures as well as by 

 connectives with the corresponding cerebral ganglion of each side. 



It is not until later that a third pair — the visceral ganglia — appear. 

 They arise by p>roliferation of epithelium in the neighbourhood of the 

 orifice of the two p>rimitive kidneys, and after separating from the 

 epithelium lie beneath the hind-gut ; they become connected by 

 commissures with each other and with the corresponding cerebral 

 ganglion of each side. At this stage of development therefore the 

 nervous system of the Pulraonata exhibits a surprising agreement 

 with the typical disposition seen in many Lamellibranchs, e. g. Unio 

 or Cyclus, while the central nervous system of the perfect snail 

 {Clausilia or Lima.v) exhibits much more complicated and appa- 

 rently quite different arrangements. How these are produced in 

 the course of further development, how the separate parts become 

 displaced from their relative positions, will be explained in my 

 detailed work ; in the present paper a short outline only is given. 

 Tn the comparison of the embryonic with the fully developed nervous 

 system of the Pulmonata, that which at once strikes us and at the 

 same time renders more difficult the comparison of the latter with 

 the nervous system of the Lamellibranchs, is the circumstance that 

 the separate constituents of the central nervous system, the various 

 ganglia, are much more complicated in structure in the adult snail, 

 have apparently quite a different position with reference to one 

 another, are partly fused together, or at any rate touch one another, 

 and thus svirround the foremost section of the intestinal tract as a 

 single mass. They are therefore situated quite at the anterior end 

 of the body, while in the embrj'o the several pairs of ganglia — 

 corresjjonding to the primitive arrangement in the mussels — are 

 quite distinct and imbedded in widely separated regious of the 

 body. The following is ])robably the explanation of this apparent 

 difficulty: — After the several pairs of ganglia have separated in 

 the embryo from the epitlielium of the surface of the body and have 



