OXFORD UNIVERSITY SPITSBERGEN EXPKmTION. 1113 



doubt, it is the commoner species of the neighbouring lands that 

 are ah'eady known to us: and on the whole, no doubt, it is the 

 commoner species that, from their frequency, have most chance 

 of being transported. If it had been throughout a case of simple 

 transportation, therefoi-e, we might have expected to find that 

 a larger number of the species of woims were known from else- 

 where also. 



It seems hazardous to suppose that any of the present Oligo- 

 chsete fauna could be descended from ancestors existing in 

 Spitsbergen before the glacial period. It is true that we now know 

 species which can live their lives through, and reproduce them- 

 selves, on ice and snow. The food of such worms, according to 

 Welch, writing of the specimens from Mt. Rainier, seemed in one 

 case {Mesenchytrcens yelichcs) to be microscopic algfe having the 

 appearance of Pleurococcus. and the snow over which the worms 

 were crawling had a red colour due to a minute unicellular plant ; 

 in the case of the other Avorm [M. soUfugus var. rainier ensis) the 

 food could not be determined. 



But the food-supply of glacier and snow worms must, one would 

 say, be precarious : and it seems dangei-ous to assume its unfailing 

 presence throughout the long duration of the glacial period. 



On the other hand, there are appai-ently no facts which seem 

 to contradict " the most reasonable hypothesis of the origin of 

 the present fauna — namely, an 'accidental' peopling since the 

 period of maximum glaciation, during which time Spitsbergen 

 has been as widely separated from adjacent lands as it is now." 



On THE " PePIONEPHRIDIA " OF THE ENCHYTRiEID.E. 



Certain structures connected with the first portion of the 

 alimentary canal in a number of Enchytrteids are known as 

 *' peptonephridia," or sometimes as " salivary glands." 



(1) The best known of these structures are a pair of tubules 

 which arise from just behind the pharynx, and extend backwards 

 for some little distance free in the body-cavity. They are well 

 developed, for example, in EnchytrcKits alhidus, a worm which is 

 abundantly represented in the present collection. Here the 

 tubules open into a dorsal diverticidum of the alimentary canal 

 just behind the pharyngeal mass : they sometimes extend back- 

 wards into segm. v., while sometimes they do not transgress the 

 hinder limit of segm. iv. ; their general coui-se is sinuous or 

 contorted, and in addition the outline of the walls is marked by 

 numerous smaller sinuosities or crenations. The diameter of the 

 tubules varies — it may be about 50 /u, or may be less ; that of the 

 lumen may be 42 fi or less ; the diameter is least near the origin 

 of the tube from the alimentary canal, where the whole structure 

 is only 16 fx thick, and the lumen is very small. The walls are 

 mostly thin, about 4 jx thick, but in places are much thicker — 

 as much as 1.5 ju ; numerous nuclei are seen in the walls — in a 

 transverse section of one of the tubes there may be about six, 

 i. e, the lumen is intercellular. 



