Anatomical structure of Aspidogaster conchicola. 505 



as will be remembered discharge their conteats round the borders of 

 the foot but above the ventral disk. The first of the three above 

 mentioned possible functions appears to be the only rational one. 

 From this point of view it is clear why there is such an aggregation 

 of these organs towards the anterior end either in the mouth sucker 

 or in the neck. The mouths of the circum-pharyngeal glands are 

 advantageously placed for the immediate application of an irritating 

 fluid to a tear or bruise brought about by the friction of the mouth 

 sucker or a pinch produced by the protrusible anterior end of the 

 pharynx. That these are salivary glands is not impossible but their 

 similarity of structure and contents to the others leave it doubtful, 

 and besides that conception would also implicate the notion of a solid 

 food which is also probable but not proved. On account of the 

 structure of the intestinal epithelium it is next to impossible to re- 

 cognize food particles in the lumen of the intestine even if they are 

 present. The long, finger-like epithelial cells dipping into and bathed 

 by the fluid contents of the intestine supply an immense surface for 

 osmotic intercourse with the inner parenchyma of the animal. This 

 great surface would seem to point to the necessity for a great quantity 

 of fluid dift'usion, and this to the conclusion that the fluid is more or 

 less poor in dissolved food ingredients, and this poverty again might 

 exclude the use of solid food, upon which hangs the necessity for 

 salivary glands. 



Again, if the animal rubs off surface cells and produces raw in- 

 flamed patches on its host, this would, especially where many para- 

 sites are present, be most likely to disturb the health of the host. 

 I have very frequently, and always, so far as I can recollect, 

 where a number of Aspidogasters was present, observed a languid, 

 unhealthy behaviour on the part of Anodons associated with a yellowish, 

 litharge-like coloration of the foot, mantle, etc. and a shrunken, starved 

 appearance of the whole mussel. That this was due to the parasites 

 I can not affirm, but at any rate it is worth keeping in mind for 

 future observation. Also whether the sickness were brought about by 

 inflamed surfaces, or by loss of nourishment from the blood, or by a 

 storing of the blood with excretion products from the parasites, which 

 act to some extent as poisons, are questions that at present must be 

 left unanswered. 



According to the same trend of reasoning the foot glands would 

 also represent irritating organs, and then arise the questions why 

 opening round the foot and why a definite arrangement? As regards 



