Mr Gullancl on the Sense of Touch in Astacus. 1G7 



In the thoracic appendages of Thysanopoda, which all 

 function as swimming feet, the tactile setae, which are long 

 and slender {see PL IX., Figs. 14 and 15), are found only on 

 the tip of the last joint and the anterior margin of the two 

 last joints (of the endopodite). There are no tactile setae on 

 the exopodite or the abdominal appendages, which all func- 

 tion purely as swimming feet. 



Cephcdothoracic Carapace. — The only setae on the surface 

 of the carapace are the exceedingly minute ones mentioned 

 by Huxley (" Crayfish," p. 239), which are to be found on 

 the lateral surfaces projecting forwards from the anterior 

 part of the base of the large tubercles which stud the carapace 

 in that region. They occur singly, are very small, and, as 

 far as can be made out, have the characters of the tactile 

 setae; but since in all the spots where the setae are un- 

 doubtedly functional, and where the cuticle is as thick as it 

 is here, they occur in groups, and since these setae are so 

 very small and are practically protected from all outside 

 influences by the tubercles, I greatly doubt whether they 

 receive tactile impressions, and whether they are not to be 

 res^arded as a degenerate form of the tactile setae. It is not 

 difficult to understand why the cephalo thoracic carapace should 

 be without any definite tactile apparatus on its dorsal surface 

 when we remember that the carapace is an outgrowth from 

 the head-segments for the protection of the important organs 

 situated in the thorax, and has, morphologically, no connec- 

 tion with the parts which immediately underlie it, and when 

 w^e further consider that the whole of it is within the sweep 

 of the antennae, which, when intact, reach as far back as the 

 second or third abdominal somite. At the anterior and in- 

 ferior part of the carapace, however, the setae are larger and 

 more numerous, and probably functional. When the crayfish 

 has its back protected in any way, its antennae are directed 

 forward, but when its dorsal surface is unprotected the an- 

 tennae are turned back and lie along its sides. 



Eound the margin of the carapace, and arising slightly 

 beneath and behind its edge, is a single row of " fringing 

 setae ; " on the dorsal surface, where the carapace overlaps 

 the first abdominal somite, they are shorter, and the 



