296 MR. GEORGE M. THOMSON ON A FRESHWATER 



NercoMS System and Sense-Organs. 



The nerve cord (PI. XXVI. fig. 4) was easily traced along the whole ventral surface 

 of the hody from the extremity of the ahdomen to the mouth, but the organization of its 

 anterior end was not made out satisfactorily either by dissection or by means of 

 sections. 



In the abdomen there are six quite distinct paired ganglia, the cords being almost 

 united at the very base of the telson to form the largest of these. From each pair 

 of ganglia at its anterior and outer side there proceeds a dense jjlexus of nerves to 

 the surrounding masses of muscular tissue, while from its posterior outer side there 

 passes one rather stout nerve obliquely backwards and downwards to enter the 

 appendage. Each pair of ganglia is separated by two very distinct and somewhat 

 widely separated commissures. 



In the thorax the ganglia lie much closer together, the commissures, though quite 

 distinct, being parallel and touching throughout their length. The ganglionic portion 

 of the cord is not sharply defined as in the abdomen, the position of the different 

 centres bemg made out most readily by the nerves which pass into the appendages. 

 There appear to be seven (or eight) fairly well-defined ganglia in the thorax, but imme- 

 diately behind the gullet is a mass of ganglionic tissue which appears to supply all the 

 mouth-orsans, but which I was not able to resolve into its constituent elements; nor 

 could I trace the passage of the commissures round the mouth to the anterior ganglionic 

 mass from which proceed the powerful nerves which supply the eyes and the antennoe. 



The etjes are well-developed as in the higher forms of Crustacea. I have not 

 examined their minute structure. 



The organ ofhearing (PI. XXVI. fig. 10) is situated in the basal joint of the first pair 

 of antenna?. It consists of an oblong cavity, occupying about two-thirds of the length 

 of the joint, and communicating with the outside by a duct which opens at the end of 

 the joint under a tuft of terminal sette. The roof of the cavity is formed by the flattened 

 upper surface of the joint. This roof is lined by a colourless tissue formed of two (or 

 three ■?) layers of somewhat elongated hyaline cells, standing obliquely or nearly vertically 

 to the surface. The internal layer which forms the lining of the upper part of the 

 cavity appears to form a series of trumpet- or cup-shaped sockets, into which the auditory 

 hairs are jointed. Each of these hairs consists of a club-shaped stalk or pedicel, bearing 

 an elongated bell-shaped cell or capsule (fig. 11). I cannot make out Avithout fresh 

 material whether these cells are open or not ; I am inclined to think that they are closed 

 sacs. I have not been able to trace the se2iarate nerve-fibres passing into these auditory 

 tubes, except near their base, but a very considerable nerve enters the antenna, and has 

 been followed by me nearly up to the auditory cavity. 



I could not detect any trace of sand-particles such as are common in most of the 

 higher Crustacea, in which the auditory sac communicates with the outside water by 

 a canal, nor of an otolith such as occurs in those forms Avhich have a closed sac. Each 

 auditory tube or hair appears to possess a somewhat complicated structure in its capsular 

 portion. These sense-organs are quite unlike any others hitherto recorded among the 



