394 L. DONCASTER. 



irregularly arranged, and not at all accurately transverse 

 (PI. 21, fig. 40, sep. sp.). In tlie places where the alimentary 

 canal is not in contact witli tlie body-wall tlie septa never 

 cross the space between them; they only touch the body- 

 wall Avhere the gut is in contact with it. In very young 

 minima they are not present, and after studying sections of 

 various ages I have come to the conclusion that they are not 

 septa, properly speaking, at all. The alimentary canal in 

 this species differs from that of others in being very wide, 

 almost entirely obliterating the coelom ; but this is less 

 conspicuously the case in the young. If a series of sections 

 of various stages is examined it is seen that the widening of 

 the alimentary canal is due to an enlargement of its walls, 

 while the lumen i-emains narrow as in other species. The 

 cells composing the walls become exceedingly large, and 

 ultimately lose most of their protoplasm; this change takes 

 place soonest in the lateral region, while dorsally and ven- 

 trally the cells remain less modified. At length, when the 

 animal is sexually mature, and its life is probably nearly at 

 an end, the alimentary canal consists of the inner wall 

 surrounding the lumen, and an outer wall, which is largely in 

 contact with the wall of the body ; and between these two is 

 a cavity crossed at intervals by sheets of tissue, which are 

 the remains of the cell walls of the endodermic cells. These 

 sheets are irregular in their distribution, and seem to corre- 

 spond in every way with the " septa " seen in the living 

 animal (figs. 41, 42). At the posterior end of the ali- 

 mentary canal, between the ovaries, this vacuolation and 

 degeneration does not take place, so by following forward a 

 series of sections of an adult S. minima the whole course of 

 changes can be observed. To test this further, some living 

 S. minima were put in a watch-glass with sea water in which 

 carmine particles were suspended, and as the animals swallow 

 water at intervals the carmine was taken into the gut and 

 gradually travelled backwards to the rectum, which it 

 reached in periods varying from five minutes upwards, and 

 was finally expelled at the anus. During the whole of its 



