PHARYNX OF MICROSTOMA CAUDATUM 311 



worm lay head-on within the enteron, ingestion of it was renewed 

 and continued until completed, although the Microstoma was so 

 gorged that the pharynx of the posterior zooid, not yet being in 

 communicMion with the enteron, was everted (figs. 4 and 5). 

 Thus it is seen that the mouth of the pharynx is distensible to a 

 surprising degree and that the pharynx operates as a prehensile 

 organ which can bring considerable force to play upon prey that 

 is being ingested. The mechanism by which this forcible in- 

 gestion is accomplished is shown in figures 6 and 7. Figure 6 

 shows a sagittal section of an animal with its mouth nearly closed. 

 The passage-way from pharynx to enteron is closed by a region 

 of slightly peculiar cells. The anterior sac of the enteron is 

 conspicuous and lies well up within the 'head' (fig. 6, A,S.). In 

 forming a sucking apparatus or piston, the lips of the enteron are 

 everted into the pharyngeal cavity. This eversion involves drag- 

 ging the anterior sack of the enteron ventrally and posteriorly. 

 The widened pharyngeal cavity now contains an everted knob 

 of enteric tissue (fig. 7, E.K.), which can be drawn back into its 

 resting position and thus cause a vacuum within the pharyngeal 

 cavity. It is by the repeated everting and drawing back of this 

 region of the alimentary canal, together with an alternate dis- 

 tending and contracting of the pharynx, that objects can be quite 

 forcibly crowded into the enteron of Microstoma. 



This prehensile function of the pharynx is supplemented by the 

 work of the glands. Martin ('08) kept some Hydras in a solu- 

 tion of neutralroth, and "the vacuoles of the ectoderm of the 

 hydras, which had been stained a pink color by the neutralroth, 

 took a yellowish-brown color under the action of the digestive 

 fluids, indicating that the secretion (of Microstoma) was of an 

 alkaline nature, and possibly allied to trypsine." Martin said 

 that under the conditions of ingestion the "tentacles of Hydra 

 do not grasp the Microstoma, but remain extended almost par- 

 allel with its body, and it would appear as though the pharyngeal 

 secretion has a paralyzing action on the Hydra." He further 

 says that "in many cases, after a time, the Microstoma leaves 

 its prey, and in such a case the Hydra does not seem much the 

 worse for the attack" (p. 268). 



