76 A FURTHER KNOWLEDGE OP THE CYSTIC CESTODES, 



a cyst from the liver of Cyhium regale, described by Linton* under 

 the name of an embryo Tetrarhynchobothriitm, and is probably 

 identical with an immature Synhothrium from the spiral valve of 

 Trygon centrura, which the same author describes! as a new 

 species under the name of Syndesmohothrium filicolle. In view 

 of Linton's descriptions and figures I have not thought it neces- 

 sary to describe the structure of the fully-formed scolex, but 

 proceed at once to the desciiption of the developmental stages I 

 have been able to examine. 



Fig. 14 represents a median longitudinal section of the youngest 

 blastocyst observed. It had an elongated form of uniform thick- 

 ness, except for a slight constriction near its middle, and measured 

 7'5mm. in length by 1mm. in breadth. The blastocyst in its 

 natural position is surrounded by an outer fibrous layer derived 

 from the peritoneum, and representing the cyst of the Cysticerci 

 properly so-called. The part enclosed by the cyst, and here, after 

 Linton termed the blastocyst, really represents the caudal vesicle 

 of these forms, and is, without doubt, the direct derivative of the 

 embryo. The blastocyst is in the fresh condition of an opaque 

 whitish colour and contains scattered calcareous corpuscles. 



The structure of the blastocyst is comparatively simple. It is 

 invested by a thin layer of cuticle ; below this is a thin granular 

 layer with scattered nuclei. Internal to this is a vacuolated 

 ti.ssue, with here and there scattered nuclei, calcareous corpuscles, 

 and a series of longitudinally running fibres, by means of which 

 the contractile movements of the living blastocyst are probably 

 performed. The central mid-portion of the blastocyst is occupied 

 by a somewhat irregular cavity. Two well-marked excretory 

 vessels with thin cuticular walls run along, one on either side of 

 the blastocyst, to its posterior end, where they unite to open to 

 the exterior by a short terminal canal. 



* " Notes on two Forms of Cestode Embryos," American Naturalist, 1887, 

 p. 199. 



t "Notes on Entozoa of Marine Fishes," United States Commission of 

 sh and Fisheries, 1891, p. 861, pi. xv. figs. 2-4. 



