414 SUMMARY OF CURRENT RESEARCHES RELATING TO 



margin and in the middle. After maceration six secondary groups 

 — two superior, two inferior, and two lateral — can be made out. We 

 have to do here with a remarkable approximation of the cerebral, 

 pedal, and asymmetrical ganglia, for nerves converge to this central 

 mass from all the organs of the body. The author describes in 

 detail the careful dissections which were necessary to elucidate the 

 structure of this complex mass, and, comparing it with those which he 

 has previously described, says that, whereas in all of them all the 

 pedal and asymmetrical ganglia were found on the anterior surface of 

 the oesophagus, they are, in Tetliys, with one exception, all united into a 

 dorsal mass ; that exception is the extremely small genital asymmetrical 

 ganglion. 



Shell-formatioii in Lamellibranclis.* — Dr. F. MiiUer describes 

 the mode of shell-formation in Lamellibranchiata. His investigations 

 relate chiefly to Anodonta, Unio, and Cyclas, of vv^hich chipped-off edges 

 and sections were studied. The decalcification was effected by means 

 of dilute chromic acid, picrocarmin was used for staining, and cel- 

 loidin was found to be the only satisfactory imbedding material. 



The general result of Dr. Mliller's research is to corroborate 

 Nathusius in his account of the shell-growth by intussusception and 

 not by secretion. He does not, however, exclude the possibility that 

 apposition of organic elements may occur on the inner surface of the 

 shell, at those places where the shell is permanently united with the 

 body, i. e. from the muscles. The outer margin of the shell, that is 

 the thickened periostracum, and the inner surface next the mantle 

 are always soft. The calcification both of the prismatic and mother- 

 of-pearl layers, is due to small, roundish, irregularly distributed 

 bodies, which gradually increase in all dimensions, and become 

 prismatic by mutual pressure. 



During the metamorphoses of the young mussel, the shell has a 

 fibrillar structure ; the lamellation is secondary, probably beginning 

 along with the calcification. The original fibrillar structure is 

 associated with the development and differentiation of the shell- 

 muscles. The organic substance of the shell has a cellular origin. 



In their development the fibrils follow the directions of the mantle- 

 muscles. They assume a radial course at the ligament, but elsewhere 

 run parallel to the surface of the mantle, following the direction of the 

 muscle-fibres which run transversely round the animal, just under 

 the epithelium, and which uniting with the tooth-pad, the pallial line, 

 and the periostracum, thus exert influence on the fibrils. 



These transverse muscle-fibres aid in the opening of the shell. 

 Those radially disposed on the back of the animal flatten the liga- 

 ment in contracting, and thus also aid in opening, as those also do 

 which ascend on each side from the foot and are attached to the tooth 

 or tooth-pad. The muscle-fibres uniting the dorsal -muscle insertions 

 on either side act as adductors. The bundles of cross muscles on the 

 margin of the mantles, which are by one end attached to the shell 

 on the pallial line, and by the other to the free portion of the perio- 



* Zool. 13eitr. (Schneider), i. (1885) pp. 206-16 (3 pis.). 



