208 



HISTOLOGY 



static cells also tells the animal when its body has moved or turned in 

 certain directions. 



With a few exceptions (the Crustacea), and on account of the great 

 delicacy of the stimulus, the static cells, unlike the coarser tactile cells, 

 receive the impression directly upon their perceptory end-organ. This 

 would mean that they possess no intermediate tissues unless we may 

 consider the crystals of lime, chitin, and foreign matter, that are operated 

 on them by gravity or inertia, as intermediate tissues. Since these 

 carbonate of lime concretions, chitinous plates, and even foreign bodies, 

 as grains of sand or streams of fluid are necessary, and often organic, 

 parts of the apparatus, we shall designate them as the intermediate sub- 

 stances. 



On account of the delicate nature of the tissues and their operations, 

 the surface that contains them is usually invaginated into some sac or 

 follicle inside the body. This leaves a mouth or duct leading to the 

 exterior and remaining open in primitive forms of the organ, but closed 

 and cut off entirely in the more specialized forms. 



The perceptory end-organs of the static nerve cell, which are rod-like 

 or hair-like processes, are placed directly upon the cell body, which is a 

 part of the surface of the static epithelium. The only exception to this 

 is the same case mentioned above as an exception, and it will well serve 



as our first subject of study, the static 

 organs of the Crustacea. 



These structures will show us bet- 

 ter than any others the relation of the 

 static organs to the tactile organs. In 

 the prawn, Palcemonetes, the static 

 organ consists of an invaginated hol- 

 low or chamber in the side of a joint 

 of the claw. This chamber remains 

 in communication with the exterior by 

 means of its opening at the point of 

 invagination. 



Inside of this pocket we find some 

 particularly delicate " touch hairs." 

 That is to say, they are formed like 

 touch hairs in general, except that 

 they have finer and more numerous 

 ends and that they are bent so as to 

 have their sides most accessible to any- 

 thing that may touch it (Fig. 188). 



These hairs are situated in groups on various parts of the interior surface 

 of the sac and are played upon by grains of sand, etc., which the shrimp 



fi 



FIG. 188. A static sensory hair from the 

 statocyst of the shrimp, Palamonetes. 

 Formed similarly to the tactile hair (Fig. 

 1 86) but with a thinner cuticle on the 

 hair; cu., cuticle; hy.c., hypodermal 

 cells; h.c., hair cells; nv.fi., nerve fiber. 

 (After PKENTISS.) 



