PHARYNX OF MICEOSTOMA CAUDATUM 313 



enough. This paralysis, however, is confined only to the re- 

 gions of the body taken into the pharynx, and is not complete, 

 for within an hour the animal had fully recovered. 



The observations last cited indicated a striking difference of 

 reaction on the part of the Hydra, depending upon the region of 

 the pol}T3 which was attacked by Microstoma. We have fre- 

 quently seen Microstoma snip away a tentacle without disturb- 

 ing other regions of the body than the tentacle involved. How- 

 ever, when the attack is made upon the aboral end of a Hydra 

 (lying free — not fixed), the response of the polyp is general — the 

 entire polyp (body and tentacles) contracting. This suggests an 

 interesting line of speculation. Parker ('17 a) found that the 

 tentacles of actinians were but 'slightly sensitive,' while the ab- 

 oral or pedal regions were highly sensitive to mechanical or con- 

 tact stimuli. On the other hand, the tentacles of these polyps 

 were very sensitive to chemical stimuli (juice of mussels) while 

 the aboral or pedal regions were but slightly sensitive to such 

 stimulation. In the light of Parker's observations, it is suggested 

 that there may be in Hydra, as in actinians, paths of 'specialized 

 transmission.' If it be that the tentacles of Hydra are more 

 sensitive to chemical stimuli and the aboral end is more sensitive 

 to mechanical stimuli, then our observations indicate that the 

 preliminary phase of Microstoma's attack upon Hydra is mechan- 

 ical, the chemical phase ensuing only after the prey lies within 

 the pharynx and enteron. 



The histology of the pharynx indicates that it is prepared to 

 function in a chemical manner, for it is highly glandular. The 

 pharynx, in its resting condition, is spindle shaped, with a lumen 

 in the form of an oblate spheroid. The lumen is lined with a 

 strongly ciliated, sparsely nucleated epithelium of low cells (fig. 

 9, Ej)). This epithelium is frequently interrupted by the ducts 

 of the many unicellular glands which radiate into the mesenchyme 

 from all sides of the pharyngeal wall. There are many indefinite 

 layers of these gland-cells. Those nearest the lining epithelium 

 are the smallest, while those lying at the periphery of the phar- 

 ynx are the oldest and largest (fig. ^, A, B, C). The smallest 

 gland cells show no sign of secretion formation, having neither 



