PHARYNX OF PRORHYNCHUS APPLAXATUS 477 



animal does not as a rule carry about with it in its pharyngeal 

 sheath a chitinous penis which could be used defensively as do 

 some Rhabdocoela. Prorhynchus applanatus, has of course, no 

 defensive and offensive proboscis. 



The structure of this pharynx and the conditions attending it 

 all suggest that it is a bulbous stomodaeuni which has assumed 

 the function of a proboscis as well as that of a pharynx. 



It is, therefore, interesting to see that Kepner and Taliaferro, 

 ('16), recorded the fact that this animal uses its pharynx as a 

 defensive organ. "Correlated with the absence of a chitinous 

 penis, which in the other forms of Prorhynchus is used as an 

 organ of defense, the pharynx of Prorhynchus applanatus serves 

 as a defensive structure." Mr. W. H. Tahaferro, while work- 

 ing in this laborato'ry, was the first to make the observation that 

 such use is made of the pharynx by Prorhynchus applanatus. 

 \^Tiile observing a specimen under a microscope, he saw it plunge 

 its pharynx quite through another animal which passed by, and 

 further, that its pharynx was very quickly withdrawn from the 

 victim to retreat into its pharyngeal sheath. 



This method of functioning imposes a peculiar stress upon 

 surrounding organs, both when the pharynx is discharged at an 

 object, and especially when its recoil is to be met. Were it not 

 to have some special method of meeting this recoil, the sur- 

 rounding parenchjona and other tissues would be greatly taxed 

 to withstand the impact of the rebounding pharj^nx. 



This organ is cushioned laterally by parenchyma. At its 

 base, however, it rests upon the rim of the enteron. Thus, a 

 more delicate situation encounters the recoil of this pharynx 

 than is that which receives the rebound of the highly developed 

 proboscis of the other Rhabdocoeles. The proboscis of the 

 higher Rhabdocoeles is cushioned by an extended lateral mass 

 of parench^Tna as well as by a basal pad of mesenchyme, a con- 

 nection between the enteron and the proboscis not being involved. 



In Prorhynchus applanatus the pharynx, of course, com- 

 municates with the enteron. Its Imnen opens into that of the 

 enteron, while its wall is confluent with, or merges into the wall 

 of the latter. There is thus an intimate connection betw^een 



