308 Dr. O. vom Rath on the 



lumen of the sensory hair, however, is by no means exclu- 

 sively occupied by the terminal cord ; I observed in many 

 cases, and with especial distinctness in the olfactory tubes, 

 that the hypodermis-cells send distinct processes into the 

 hair ; the cells which do this are those which form the matrix 

 of ihe hair. The number of sense-cells belonging to each 

 sensory hair varies very much : in the ease of the Decapods 

 1 was always able to count a large number of them, but in 

 the J'hylloj)ods and Cladocera only a few. The groups of 

 sense-cells are. sometimes rounder, sometimes more elongate 

 or linear in shape. The nuclei of these cells are usually 

 round and possess a corresponding network of chromatin- 

 fibres ; they are readily distinguishable from the more elon- 

 gate and always darker-coloured nuclei of the hypodermis. 

 It is only shortly after ecdysis (as is seen especially clearly in 

 Astacus) that the difference in external appearance between 

 the nuclei of the hypodermis-cells and those of the sense-cells 

 is small. The group of sense-cells often lies a very long way 

 from the hypodermis and the sensory hair, and the terminal 

 cord is then of considerable length*, as, for instance, in the 

 first antennce of the Caridinas and Brachyura. Each group 

 of sense-cells is surrounded by a sheath, which consists of 

 flat cells with flattened nuclei, and appears as a continuous 

 prolongation of the neurilemma of the nerves. It can usually 

 be distinctly seen that this sheath also surrounds the terminal 

 cord. I believe that the cells of the sheath do not essentially 

 differ from those of the hypodermis. When the groups of 

 sense-cells are collected in greater numbers near one another, 

 and lie at some little distance from the sensory hairs, we 

 always detect between the terminal cords elongate dark- 

 coloured nuclei, which belong to elongated hypodermis-cells. 

 From these cells it is not always easy to distinguish those 

 of the above-mentioned sheath of the terminal cord. If the 

 sensory hairs, as is often the case, are united into a bundle, or 

 stand close together in larger numbers upon a common sensory 

 field, the groups of sensory cells belonging to the separate 

 hairs may be compressed into a compact mass. Even then, 

 however, the separate elongated groups or bands of sense- 

 cells can, be distinguished with tolerable clearness within the 

 apparently single ganglion, and we observe between them 

 the flat nuclei belonging to their sheaths of connective tissue. 

 The terminal cords, too, are approximated to one another, 

 and between them lie flat nuclei, which belong partly to the 



* lu the Insects the group of sense-cells is usually found iu the neigh- 

 bourhood of the hair, and is even frequently situated within the hypo- 

 dermis. 



