302 Dr. O. voni Rath on the 



second pair of antenna and their squama3, the whole of the 

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

 numerous 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 either to their arrangement and shape or 

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

 the sensory hairs of the antenna?, 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 tlie whole of the jointed appen- 

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

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

 pages I shall employ the convenient expressions — protopodite 

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

 branch). 



a. Sense-Organs of the Antennce. 



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

 important sensory hairs, since upon it are found both the 

 so-called olfactory tubes (" Riechschlauche ") 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. "^J'actile hairs, which run to a sharp point and are 

 not feathered, arc found distributed with a certain amount of 

 regularity in the immediate neighbourhood of the olfactory 

 tubes, and act to a certain extent as protecting sctw. 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. fj. 

 in Idothea ; frequently they are collected in bundles on 

 several joints, e. (j. in Astacusj but it is not unusual to find a 

 single structure of the kind only on several jouits, e. <j. in 

 Cajyrelld. 



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

 number i>f these organs is much more considerable than in the 

 f(,inale, and it was shown by Weismann * for Lcptodora and 



* AN'oisuiaun, " I'lbor Biui- imd LrLou.<er;mboiuuupon von L<jilmioia 

 hi/aliiia,''Zt'[ischr. fur wis*. Zool. 24 Bd., It74. 



