604 W. S. NFCJKERSON, 



next the duct is probably sigiiificaiit of sonie relatioii to it. Sensory 

 cells have not been demonstrated iu tlie marginal organs of other 

 luembers of this family. 



A condition of the organ observed in only oiie case is shown in 

 Fig. 13. The terminal portion of the duct is everted and the interior 

 of the body of the organ is protruded tho' the opening to the ex- 

 terior. Voeltzkow ('88) observed that the marginal organs of 

 Aspidogaster were capable of Protrusion and retraction and upon this 

 fact based his conclusion that the organs are sensory in function. It 

 is possible that the Protrusion of the organ aids both in the exercise 

 of its sensory function and in the discharge of its secretion. The 

 study of these organs with the aid of intra vitani methylen blue 

 staining would doubtless show many details of the nervous structure 

 which it is quite impossible to make out by ordinary stains used 

 upon preserved material. They offer an interesting field for investig- 

 ation by this method wheu the necessary living material can be 

 obtained. 



In Fig. 12 I have also represented a peculiar condition observed 

 only in the case shown. Immediately beside the duct of the organ 

 figured lie two vesicular structures closely in contact and hyaline and 

 uustained. I can offer no explanation of their presence. 



It is a fact of some interest in this connection that in the two 

 closely related genera Cotylogaster and Cotylaspis the marginal organs 

 which must unquestionably be regarded as strictly homologous difler 

 markedly in structure. Through the courtesy of Prof. H. L. Osborn 

 I have been perniitted to examine his sections of Cotylaspis and he 

 has stated to me his conclusions ^) concerning the structure of these 

 organs. It would appear from Voeltzkow's description that the 

 organs of Aspidogaster resemble those of Cotylogaster more closely 

 than those of Cotylaspis though it is perhaps probable from general 

 anatomical features that Cotylaspis and Aspidogaster are more closely 

 related than are Cotylogaster and either of the other two forms. 

 Not enough is known concerning the marginal organs of Macraspis 

 (Jägerskiöld '99) to enable a comparison with them to be made. 

 Stichocotyle lacks these organs. 



It is perhaps probable that the tentacles described by Mac- 

 donald ('77) from the ventral shield of Aspidogaster macdonaldi will 



1) To be published in a forthcomiog number of the Journal of 

 Morphology. 



