THE VICTORIAN NATURALIST. 27 



groups of two to twelve, surrounded by a very thin transparent 

 gelatinous substance, enclosed in a definite cell wall (fig. 3). 

 When the cysts were ruptured, both anterior and posterior 

 suckers were clearly defined in the embryo, as well as a circlet of 

 small spines near the anterior sucker. 



Another curiously modified cyst also occurs in immense 

 numbers in Isidora texturata, and occasionally in Isidora tenui- 

 striatus. This cyst is about ^ s in. in length, being somewhat 

 oval in shape, with the anterior end just a little wider than the 

 posterior one. The whole of the posterior region is obscured by 

 a large number of dark pigmented cells, with the exception of a 

 small circular patch which occurs near the posterior border, in 

 the region probably of the excretory pore (fig. 4). If these cysts 

 are kept for any length of time within the snail, the pigmented 

 portion becomes less in size and lighter in colour, and it is quite 

 probable that the pigmented cells are food yolk cells, upon which 

 the young embryo is supported during its temporary imprisonment. 

 During the last three months these cysts have been frequently 

 met with in the same gathering of snails, and it is probable that 

 they will exist in a living condition for some considerable time 

 when protected by the tissues of the snail. The embryo which 

 issues from this cyst does not appear to be an *' armed " variety, 

 no large spines being visible. The anterior portion, however, is, 

 as in the other cases, covered with very minute spines. 



Another pigmented variety of cyst, t }q in. in size, occurs in 

 Limnea brazieri. In the case of this form encystation was seen 

 to take place directly it was taken from the snail. A granulated 

 material was passed over the cercaria and the tail eventually 

 cast off. A very large amount of pigment was present in the 

 cercaria, and when the cyst was complete the centre of it was so 

 dark as to be quite opaque to transmitted light. The outer 

 border of the cyst presented a serrated appearance in all the 

 examples observed on this occasion (fig. 5). No spines were 

 noticed on any of the embryos. 



The embryo fluke (cercaria stage) with one spine springing 

 from the anterior sucker has already been described as occurring 

 in Isidora texturata, I. gibbosa, Limnea lessoni, and L. brazieri. 

 Encystation has been observed in one or two instances, the 

 resulting cyst being about r \^ in. in diameter. The central proto- 

 plasm is very granular, but a single stylet is clearly seen shining 

 through (fig. 6). If the embryo is released, both anterior and 

 posterior suckers come into view, as well as the typical arrange- 

 ment of the alimentary track. A depression with an opening 

 on either side of the anterior sucker is visible at a slightly higher 

 level than the openings referred to in the first embryo described. 



The last cyst to be described is one found in company with 

 Segment ina victories — a few only of the cysts being noted, and 



