J. Johnstone 371 
secretion superposed on the fibrous tissues of the larval cyst. (See 
Ward, 1907.) 
The cysts of Tetrarhynchus erinaceus and the mode of development 
of the Plerocercoid larva into the segmented worm have been described 
by P. J. van Beneden (1850, PI. XXXIII) but I give here some figures 
less diagrammatic than those quoted. Text-fig. 1 represents several 
cysts dissected out from the mesenteries of a Trigla, and some other 
hosts and Text-fig. 2 represents a section through one of these .structures 
taken from a Megrim (Arnoglossiis laterna) and cut parallel to the 
Fig. 1. Cysts containing Plerocercoid larvae of Tetrarhynchus erinaceus. A, B and C 
represent the entire cysts; D is a larva dissected out from the receptaculum. From 
various hosts. 
plane in which the larva lies. The whole structure represented is the 
larva. We see that the scolex with its proboscides invaginated lies 
within a cavity at one pole of an egg-shaped body, and is coiled up 
in a S-shaped form. The whole scolex is invested in the cuticle 
characteristic of the Cestodes, and at the base, just posterior to the 
proboscis bulbs, this cuticle is thickened and becomes continuous with 
