TAPE-WORMS OP THE SUB-FAJEILY AVrrELLIX[N.E. 375 



surrounded by smaller cells, derived from the walls of the 

 uterus. These cells, which have already been observed by 

 Fuhrmann (1909) in Stilesia sjostedti, Fuhrmann, can 

 be termed uterine nutritive cells; their function is without 

 doubt nutritive, as already suggested by Fuhrmann. The 

 uterine nutritive cells (fig. 49) soon fill all the space between 

 the eggs. They are rich in plasma at first ; later on their 

 plasma decreases, and finally they atrophy and disappear 

 entirely at or before the stage when the egg envelopes have 

 developed. 



Fuhrmann (1900) supposed the uterus of Stilesia to have 

 no cavity originally. This is not correct for Avitellina; 

 the uterus is a simple hollow tube at the time of the arrival 

 of the eggs, the uterine nutritive cells appearing shortly after- 

 wards. 



The absence of a vitelline gland, as can be seen, has had 

 such an influence on the cestode as to have caused cells of 

 two separate nnd distinct origins to arise in two different 

 organs to replace to some extent the nutritive function of the 

 missing organ, that is, if the vitelline gland has been lost in 

 the history of the genera. If, on the other hand, the lack of 

 a vitelline gland is a primitive character, the acquisition of 

 uterine nutritive cells must still be a recent adaptation, as one 

 would otherwise expect to meet such cells in other cestodes. 



The uterine wall cells, after having given rise to the uterine 

 nutritive cells, next supply the origin of the egg-pouches or 

 paruterine organ.' In the first stages of the development of 

 this organ we find parallel layers of plasma containing nuclei 

 splitting off the terminal wall of the uterus (fig. 51). The 



^ It is a question which I cannot attempt to decide, whether the par- 

 iiterine organ of Stilesia and Avitellina is homologous to the par- 

 uterine organ of other cestodes, as where it has been observed previously 

 it has genei'ally been held to arise outside the uterus. I am retaining 

 the name as being convenient and as referring to a more or less well- 

 known structure, but without prejudice as to its origin in other species. 

 The i^aruterine organs of various cestodes may quite possibly Ije of diffe- 

 rent origin, and may only be convergent structures, as Fulirmann bas 

 shown that they can arise independently in various unrelated genera. 

 VOL. 56, PART 2. NEW SERIES. 26 



