78 NOTES AND MEMORANDA. 



To sum up ; among the Crustacea, tlie Insecta, and the Annelida, the 

 structure of the nerves differs from that of the Vertebrata by the com- 

 plete absence of the substance endowed with great refractive power, 

 called myeline, which in these latter is interposed between the 

 cylinder-axis and the proper wall of the tube, the grey fibres of the 

 great Sympathetic excepted. 



In the Gasteropodous and Acephalous Molluscs the nerves are 

 further simplified ; the sheath proper, or sheath of Schwann, is wanting 

 in almost all the nerves. The nerve-tuhes only, represented by the 

 cylinder-axis, form bundles which it is difficult to dissociate. 



One more character remains yet to be added to those which we 

 have referred to. The nerve-cells of Crustacea were of an extreme 

 fragility. The contents of their tube are displaced very easily. In 

 the snail the cell takes a certain consistency. The cylinder-axis of 

 the nerves opposes also more resistance to pressure and to chemical 

 agents. 



The author adds in a further foot-note : — In the Bryozoa we 

 have observed a nerve layer situated under the ectoderm. This layer 

 was composed of cells very distant from one another, and united by 

 bundles of rectilinear filaments possessing small oval nuclei in their 

 thick part, resembling those which are formed in the nerve-fibres 

 during development in all animals. From this sort of plexus start 

 very fine threads which extend along the tentacles, others go to the 

 retractor muscle. The characters observed in the nerve-tubes of all 

 the animals which we have passed in review allow us to conclude that 

 the cells with the filaments which depend from them, and which we 

 have seen in the Bryozoa, are truly nerve elements. Here, the 

 nerves, closely allied in their structure to those of the Molluscs 

 properly so called, would be reduced to the cylinder-axis. 



Development of Cephalodia on Lichens. — M. Babikof has under- 

 taken some investigations with the view of settling the origin of the 

 peculiar excrescences found on the surface of some lichens, known as 

 Cephalodia. The result is contained in a paper* presented to the 

 Imperial Academy of Sciences of St. Petersburg. 



The author says that the structure of the cephalodia is known in 

 a small number of lichens only, and that there are but few exact 

 notions as to their development-history. Some hypotheses, very 

 probably correct, have been suggested, but they have not yet been 

 established by facts. A summary is given of the views of Messrs. 

 Nylander, Th. Fries, Schwendener, and Bornet, on the cephalodia of 

 Stereocaidon, and it is pointed out that what they have observed has 

 been simply different degrees of development of the alga by the hypha, 

 and not the complete progress of the development of the cephalodia. 



" It is therefore only in consequence of simple isolated facts that 

 the authors have supposed that the cephalodia are abnormal forma- 

 tions produced by a local growth of the lichen under the influence of 

 algaj accidentally fallen upon it. Their hypothesis has, however, been 

 completely confirmed by experiments which I have made under the 

 guidance of Professor A. S. Famintzin, on the development of the 

 * ' Bull, de rAoade'niie des Sciences de St. Petersburg,' vol. xxiv. p. 548. 



