BY W. A. HASWELL AND J. P. HILL. 375 



EXPLANATION OF PLATES. 



Plate xix. 



Fig. I. — Anterior portion of a specimen of Didymogaster sylvatica, Fletcher, 

 laid open along the middle dorsal line in order to show the 

 numerous rounded cysts containing Polycerci. Slightly magni- 

 fied. 



Fig. 2. — Five of the cysts as seen under a low magnifying power (10 

 diameters) ; the polygonal areas represent the contained Cysti- 

 cercoids. 



Fig. 3. — Early stage in the development of Polycercus from the hooked 

 embryo ; first traces of proliferation. 



Fig. 4. — A somewhat later stage. 



Fig. 5. — Still later stage than fig. 4, the cellular mass divided externally 

 into numerous broad lobes, a, wall of cyst. 



Fig. 6. — Stage in which some of the buds have assumed the general form of 

 the future Cysticercoid and are connected to the central mass 

 only by narrow stalks ; other buds at earlier stages in their 

 formation. 



Fig. 7. — Portion of a section of a young bud, with the spindle-shaped cells 

 from which the hooks will be developed situated in the centre 

 of the cellular mass. Magnified about 250 diameters. 



Fig. 8. — Developing Cysticercoid of Polycercus, in which the rostellum, 

 with its hooks, and the suckers have become formed ; the 

 rostellum retracted within the head and not yet connected with 

 the external aperture, r, anterior, and r', posterior parts of 

 rostellum ; s, suckers ; e, excretory aperture ; c, internal cavity. 



Plate xx. 



Fig. 1. — A stage in the development of Polycercus similar to that repre- 

 sented in fig. 9 of Plate xix., but showing the external aperture 

 and the two layers of the body-wall, a, aperture in wall of 

 cyst ; other letters as in preceding figure. 



Fig. 2. — Sheath of the rostellum of the same more highly magnified to 

 show the spinules along the border. 



Fig. 3. — Similar stage, with the rostellum partly evaginated and extending 

 towards the aperture. Letters as in fig. 1. 



