Miscellaneous. 189 



become connected together by means of commissures they form an 

 integral system of organs, the further development of which pro- 

 ceeds quite independently of the increase in size and, ia dif- 

 ferent regions of the body in consequence of unequal growth, of 

 the different expansion of the external contours of the body. In 

 the embryo the visceral and pedal ganglia are relatively widely 

 separated from the cerebral ganglia, with which they are united 

 by relatively long commissures. All ganglia rapidly increase in 

 size, while tlie commissures uniting them together do not increase 

 in length in the corresponding ratio. It follows that a gradual 

 approximation of the ganglia must result from this, until, as the 

 ganglia continue to increase in volume, they finally come into con- 

 tact with one another, while at the same time the various ganglia 

 continually recede further from their place of origin, the surface of 

 the body, which is rapidly extending in all directions. 



We have already briefly alluded to the " cerebral tubes," struc- 

 tures which arise from the sensory plates after the formation 

 of the rudiments of the cerebral ganglia, and which subsequently 

 come into important relation with the central nervous system. 

 They are developed in this way: — Soon after the separation of the 

 cerebral ganglia a sac-shaped invagination of the epithelium of the 

 sensory plates tahes place on each side below the ocular tentacle, and, 

 growing continually deeper , finally comes into contact with the cerebral 

 ganglion of the corresponding side, though it still has for some time 

 a communication with the exterior by means of a long canal. The 

 duct of this " cerebral tube " subsequently closes, and loses its con- 

 nexion with the external epithelium ; the structure then lies, as a 

 thick-walled vesicle, upon the cerebral ganglion. While the lumen 

 of the vesicle gradually becomes narrower, and finall}^ disappcai's 

 entirely, an active multiplication of cells takes place in its walls, 

 and at subsequent stages of development we find the primitive 

 " cerebral tube " transformed into a roundish mass, which is completeh/ 

 fused ivith the corresponding cerebral ganglion ; nevertheless the 

 limits of the structure can still be determined with certainty, since 

 its small constituent elements take a much deeper stain than those 

 of the cerebral ganglia. 



These structures, the " cerebral tubes," were discovered and their 

 true importance recognized by Messrs. P. and F. Sarasin (cf. their 

 " Entwicldungsgeschichte der Helix Waltoni, ileeve,"in the'Ergeb- 

 uisse naturw. Forsch. auf Ceylon,' i. Bd. Heft 2, 1888). 



lu the tropical Hdix examined by the above-mentioned authors 

 two " cerebral tubes " — the structures were thus designated, and I 

 have accepted the term — were found on each side, and in this respect 

 therefore Helix Waltoni differs from Clausilia and Limax. As 

 regards the phyloyenetic importance of these peculiar organs, the view 

 taken by Messrs. Sarasin appears perfectly justifiable : — The cerebral 

 tubes of the Gastropods correspjond to the various organs described as 

 cephalic pits, nuchal organs (" Nackenorgane "), cf'c. in many Anne- 

 lids, and to the cephalic pits of the Nemertines. — Sitzungsberichte der 

 Naturforscher-Oesellschaft bei der Universitiit Dorpat, Bd. ix. Heft 2, 

 1891, pp. 277-282. 



