STATIC TISSUES 



209 



Stl. 



FIG. 189. Tentaculocyst (statocyst) of a medusa, Rhopa- 

 lonema. stl., statolith inclosed in a pedicle which sways 

 with the animal's motion and records its movements by the 

 hairs that project from its surface. (From LANG after 

 HERTWIG.) 



manages to insert in the sac for that purpose. The animal is conscious, 

 when a hair is touched by the sand grain, of which part of the sac the 

 hair was in and is able 

 to guide its movements 

 accordingly. By " con- 

 scious " is not meant the 

 same thing that the 

 word would imply in a 

 mammal, but a correla- 

 tion of the particular 

 hairs touched with the 

 use of certain muscles 

 to maintain the animal 

 in an upright position 

 with regard to gravity. 

 Compare Figure 1 88 with 

 the drawing of the tac- 

 tile hair shown in Figure 186 and see how essentially alike the two are. 



Whether the action of the sand particles or statoliths upon the static 

 hairs, in this creature, will convey to the nerve centers an accurate 

 measure of spatial movement or not is not known. 



An example of an organ probably used to determine the direction of 

 movements of the body in space is to be seen in some of the static organs 

 of medusae. In these forms a body surface is invaginated into a more 

 or less complete sac, and from the lower wall of this sac a pedicle arises, 

 containing in its tissue a crystal or concretion of lime, or sometimes 

 several of them (Fig. 189). 



The cells lining the walls of both the cavity and the pedicle, which 

 is called a tentaculocyst, are provided with sensory hairs, and the least 

 motion of the body must convey a record of action by the touching of the 

 sensory hairs on one side or the other of the tentaculocyst and cavity. 

 While gravity probably makes some record of its pull, the waving of 

 the body edge to and fro in swimming must cause a much greater stimu- 

 lus, and the organ therefore records the motions of the body in space 

 by the inertia of its heavy statoliths. 



Auditory cells have been described as formed on the walls of these 

 cysts. While admitting the possibility of this, the writers do not think 

 that the cells described can be used to hear sound. 



A beautiful example of a statocyst, used to orient a part of the body, 

 is to be seen in the organ that occurs in the "foot " of some plecypod mol- 

 lusks. Figure 190 shows a picture drawn from a section of the statocyst 

 in a small plecypod, Cyclas, a fresh-water form. The cavity of this cyst, 

 which was invaginated and cut off from the ectoderm, is lined with an 



