156 Proceedings of the Royal Physical Society. 



ceive, may be divided into tactile, auditory, and olfactory 

 setae. 



Those whose lumen is closed, and to which no nerve can 

 be traced, may be called "fringing setm,'' from the fact that 

 they mainly form the fringes which are found on so many of 

 the appendages and segments of the body. I do not propose 

 to discuss in detail the structure of the olfactory or auditory 

 setse, and shall therefore pass at once to the consideration of 

 the tactile setse. 



Tactile Setce. — These setse, whose distribution will be dis- 

 cussed in detail, vary slightly in length and thickness in 

 different parts of the body, and are perhaps seen to most 

 advantage on the terminal segments of the abdominal swim- 

 merets. Here they are many times longer than their breadth, 

 cylindrical, simple {i.e., without secondary bristles), hollow, 

 with the lumen about as thick, in optical section, as one of 

 the walls, and sometimes with a few granules in the interior ; 

 forming a cylinder of the same thickness from the base to a 

 point whose distance varies from a quarter to a half of the 

 way up, where there is an internal thickening of the wall all 

 round, and an appearance of jointing. According to Braun 

 {loc. cit., p. 149) — and I see no reason to doubt the correct- 

 ness of his observations — this corresponds to the point at 

 which the seta is doubled on itself before the moult (see his 

 figures). From this point onwards the seta tapers very 

 gradually to a fine point (as in PL VIII., Fig. 1), though this 

 is rather more usual in the stouter setae of the chela, or else, 

 after becoming very fine, swells out again into a slight knob, 

 on whose point there is a slight depression, the whole looking 

 rather like the extremity of an elephant's proboscis on a 

 small scale. The terminal part of the seta is rarely straight, 

 but almost always more or less bent, sometimes so as to form 

 nearly a right angle, and generally the seta, as a whole, is 

 slightly curved. The lumen of the seta extends very nearly 

 to the point, but there is no opening. It is filled with some 

 fluid, probably blood which has transuded from the vessels, 

 and if setae be examined soon after the moult granules and 

 nuclei in process of breaking down may be seen. These are 

 the remains of the " papilla " of hypodermis, which assists in 



