78 DEVELOPMENT OF 



small interval between it and the cyst, but it continues 

 its movement not the less for many weeks, though 

 perhaps with diminished activity. 



I conclude that a long time elapses between their be- 

 coming pupae and their quitting that condition, partly 

 because I have had pupae under observation for several 

 weeks, and partly because I have examined in a state of 

 nature, hundreds of pupae which clearly showed that the 

 animals had lain for a long time in that condition. I 

 have several times endeavoured to preserve a considerable 

 number of pupae in the living state between slips of glass, 

 but have only once succeeded in keeping them alive until 

 some of the animals crept out, which occurred in about 

 ten days from the commencement of the experiment. All 

 the rest remained in their previous condition, untu at 

 last, by imprudently wetting them with cold water, I 

 killed them outright. 



Several of the external and internal organs are visible 

 through the transparent pupa case, which without great 

 difficulty may be referred to those of the Cercaria, since 

 during the act of the creatures assuming the pupa state, 

 we have the opportunity of tracing the metamorphosis of 

 at least some organs, and the position of these may then 

 throw some light upon the others. The most prominent 

 of these is of a horse-shoe shaped figm-e in the hinder part 

 of the pupa ; this marks the position of the semicircular 

 lateral projections which surround the root of the tail, 

 and w^hich have apparently become thicker and larger in 

 consequence of the contraction of the body. Between the 

 two arms of the horse-shoe {t) is situated the opening of 

 the proper excretory organ (vide 4 f/, 4 e, 4/, 4y). A 

 more or less circular spot or ring in the centre of the 

 animal is the acetabulum {u) ; a three-lobed organ, which 

 when regularly contracted is shaped like a trefoil leaf, is 

 without doubt a digestive organ or stomach, with its 

 club-shaped lateral prolongations, for immediately ante- 

 rior to the central lobe of this organ, under favorable 

 circimistances, an opening or mouth may be perceived on 



