Chap, xii.] TACTILE ORGANS. 433 



in position in the Cceleiiterata, the absence of special 

 tactile organs in most of the members of the group is 

 not to be wondered at, for the tentacles, as a whole, 

 may be looked upon as having a general tactile sense. 



Among the Turtoellaria, trigger-hairs in con- 

 nection with nematocysts have been observed ; in 

 many cases tufts of delicate hairs have been found 

 scattered over the whole body, but more especially 

 well developed at its sides. In some there are 

 definite tactile organs in the shape of tentacles, which 

 are best developed in the anterior regions of the body, 

 and on which the sensory hairs are particularly nume- 

 rous. Thysanozoon, which is remarkable for having 

 the dorsal surface covered with villiform projections 

 of the body wall, has a bundle of such hairs at the tip 

 of each villus. In the earthworm, the whole body of 

 which is very sensitive to tactile impressions, the 

 anterior end is most remarkably so ; in the polychsetous 

 Annelids specially modified sense-cells are largely 

 developed on the protruding antennae and tentacles 

 which are developed on the praBstomium, and are 

 supplied by nerves which arise directly from the 

 cerebral ganglia ; these, as well as those on other parts 

 of the body, are, like the antennas of the Arthropoda 

 and of some Mollusca, very important aids to the 

 organism, for they are capable of movement laterally, 

 or of protrusion forwards, or of both ; they are, in 

 other words, able to feel about, and not, as is the case 

 with the earthworm, compelled to wait for the arrival 

 of food or foe. 



In the Hirudinea the widely distributed organs 

 of general tactile sense are purely of epidermic origin, 

 and are known to be supplied with nerve fibres ; at 

 the anterior end of the body these cells are aggregated 

 to form the so-called goblet-shaped organs. According 

 to Whitman, special papilliform aggregations are to 

 be found on every segment of the body. 

 cc 16 



