ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 737 



■wHcli isolates tlie filaments of the plexus and the fibrils. The author is of 

 opinion that the medullary substance in the central nervous system of the 

 Acephala is the homologue of the white substance in the same organ of 

 Vertebrates ; cells surround the medullary substance in cortical fashion. 

 Between the cells of the cortex and in the medullary substance there is no 

 structure either homologous with or analogous to the neuroglia of Ver- 

 tebrates. The processes of the inner envelope which sink in between the 

 cells of the cerebral and visceral ganglia of Pecten are homologous with 

 the pericellular tissue which is found in the intervertebral ganglia of 

 Vertebrates. The peripheral nerves of the Acephala consist of simple 

 axial fibrils, and exhibit no tendency to group themselves into broader 

 fibres. There is a considerable exchange of fibres between ganglia of 

 dififerent names (e. g. cerebral and pedal), and an incomplete crossing 

 between the fibres of the similarly named organs (e. g. visceral ganglia). 



This last character is very interesting physiologically ; if one touches 

 an open lamellibranch on any part of its body, it immediately closes up, 

 and, if it be a siphoniate form, it draws in its siphons ; in other words, the 

 least peripheral stimulus leads at once to a combined action of all the 

 muscles of the edge of the mantle, of the foot, and of the shell, and must 

 therefore be perceived in all parts of the body equally and simultaneously. 

 The connectives and commissures may be regarded as containing a system 

 of association-fibres of the most highly developed form. The importance 

 of this extreme sensitiveness of the tactile organs to these (nearly always) 

 blind animals must be very great. 



The morphological conclusions to which the author has been led do not 

 diflfer in any important particular from those reached by previous investi- 

 gators. He difiers, however, from his predecessors as to the systematic 

 position of the Ostraeidae, for while they have regarded that group as a 

 very low one, he regards it as having a high position. This view is based 

 on the high value which the author attaches to the development of the 

 visceral ganglion ; he points out that this stands in relation to the develop- 

 ment of the edge of the mantle as the chief concentration-point of the most 

 important sensory organs ; as the Ostraeidfe have the most highly deve- 

 loped visceral ganglion, they are the highest of the MoUusca Acephala. 

 Dr. Eawitz combats the view that the development of the visceral ganglion 

 is correlated with that of the gills. 



Jouannetia CTuningii Sow.* — Herr E. Egger has made a morphological 

 study of Jouannetia cumingii Sow., which, with its much contracted body, 

 represents one extreme derivative of the Pholas type, while the long Teredo 

 represents the other. The material for the investigation of this rare mollusc 

 was obtained from Prof. Semper's Philippine collection. Instead of fol- 

 lowing the morphological details, it will be more profitable to compress the 

 author's summary of the main rtsults. 



Most of the peculiarities of Jouannetia cumingii, as compared with other 

 members of the Pholas family, are associated with the considerable reduc- 

 tion of the longitudinal axis. These peculiarities are to be regarded as 

 modifications in more or less direct response to the external conditions 

 of life. Those organs would be first influenced which have most to do 

 with the external world, viz. the shell, the musculature, and the boring 

 apparatus. The shortening of the shell is almost a mechanical result of 

 the peculiar boring. The consequence is the approximation of the adductor 

 muscles ; the posterior one especially appears to be shunted forward towards 

 the centre of revolution of the shell. By this shortening of what corre- 



* Arbeit. Zool.-zoot. Inst. Wurzburg, viii. (I8S7) pp. 129-99 (4 pis.). 



