302 Dr. 0. vom Hatli on the 



second pair of antennae and their squama3, the wliole of the 

 mouth-parts, and all the pairs of limbs are the bearers of 

 numerons sensory hairs ; in a similar way I always found 

 sensory hairs at the end of the tail, on the margin of the last 

 abdominal segment ; in rarer instances free sensory hairs are 

 also found on the segments, e. g. in Branchipus. The sensory 

 hairs of the mouth-parts and legs have hardly been noticed at 

 all by authors, and I know of no precise statements in litera- 

 ture with reference eitlier to their arrangement and shape or 

 to the finer histological structure of the nerve-end apparatus ; 

 the sensory hairs of the antennse, on the other hand, have been 

 described by a number of writers. 



Before passing on to speak of the various sensory hairs, I 

 would remind the reader that the whole of the jointed appen- 

 dages of the Crustacea, with the exception of the first antennaj, 

 are reducible to the typical biramose limb, and in the following- 

 pages I shall employ tlie convenient expressions — proto})odite 

 (shaft), exopodite (outer branch), and endopodite (inner 

 branch). 



a. Sense- Organs of the Antennce. 



The antennule, or first antenna, is the bearer of the most 

 important sensory hairs, since upon it are found both the 

 so-called olfactory tubes (" Eiechschlauche") and also, at 

 least in the Decapods, the auditory organs ; besides these we 

 find on the most widely different regions of this first antenna 

 sensory hairs of various shapes, which are regarded as tactile 

 organs. Tactile hairs, which run to a sharp point and are 

 not feathered, are found distributed with a certain amount of 

 regularity in the immediate neighbourhood of the olfactory 

 tubes, and act to a certain extent as protecting setse. The 

 number and arrangement, as well as the outward form and 

 size, of the olfactory tubes are extremely varied and charac- 

 teristic in the orders and families, and to a large extent even 

 in the different species. In certain cases a number of them 

 are found on the terminal joint only of the first antenna, e. g. 

 in Idothea ; frequently they are collected in bundles on 

 several joints, e. g. in Astacus] but it is not unusual to find a 

 single structure of the kind only on several joints, e. g. in 

 CapreUa. 



It is worthy of note that in the male sex the size and 

 number of tiiese organs is much more considerable than in the 

 female, and it was shown by Weismann * for Leptodora and 



* Weismann, " Ueber Ban- und LeLeusevsclieinungen von Lijitodura 

 fnjidina,'' Zeitschr. I'iir wiss. Zool. 24 IJd., 1874. 



