454 STANLEY C. BALL 



e. Sex organs and vitellaria 521 



f. Cilia 522 



g. Embryonic digestion 522 



h. Formation of the intestine 526 



i. Glands 527 



9. Post-embryonic development 528 



a. The definitive intestine 528 



b. Mesenchyme 528 



c. Ovaries and testes 529 



d. Vitellaria 531 



V. Discussion 532 



VI. Summary 535 



VII. Bibliography 538 



I. INTRODUCTION 



1. Determination of the species 



In the Brooks Memorial Volume of The Journal of Experi- 

 mental Zoology, vol. 9, 1910, Prof. Edwin Linton described 

 a new species of Rhabdocoele Turbellarian which he had dis- 

 covered living in the ribbed mussel, Modiolus demissus. He 

 referred it with some hesitation to the genus Graffilla (Jhering). 

 Von Jhering ('80) in estabhshing the genus Graffilla charac- 

 terized it in the following words: 



Pharynx nach Art des Mesostomeenschlundes aus Muskelfasern 

 und eingelagerten Bindegewebszellen bestehend, nicht in einer Tasche 

 gelegen, sondern direkt mit dem Munde zusammenhangend. Bursa 

 copulatrix und Receptaculum seminis durch ein Organ vertreten. 

 Ovarien langgestreckt, bandfomig, Dotterstocke sehr reich verastelt. 



Most important in the determination of the genus in which 

 Linton's Turbellarian is to be placed is the question of the 

 receptaculum seminis. Von Jhering in his figures of Graffilla 

 muricicola indicates that two reservoirs are connected with the 

 atrium (called by him the 'uterus'). The more anterior of 

 these is the vesticulum seminalis which bears a short conical 

 penis on its lower surface and projecting into the atrium. The 

 receptaculum seminis lies just posterior to the seminal ves- 

 icle, and likewise dorsal to the atrium. It s to be noted 

 that the latter is not merely a dorsal lobe of the atrium but 

 is a distinct spherical reservoir, fully as large as the seminal 



