112 A. p. THOMAS. 



are situated one on either side of the middle of the body, in 

 each is a large cilium carried by a nucleated cell, and usually 

 directed forwards. The cilium is connected with the cell by a 

 disc at its base, it is tongue- or flame-shaped, and is con- 

 stantly in motion, waves passing along the cilium from the 

 base to the tip, and hence towards the apex of the rather 

 narrow infundibulum. Just behind the head-papilla is a 

 granular mass, which reacts with staining fluids differently from 

 the adjacent tissues. This, from comparison with other tre- 

 matode embryos, would seem to be a rudimentary digestive 

 tract. Behind, the rest of the body cavity is occupied by 

 delicate round nucleated cells — the germinal cells. 



The embryo is exceedingly active, and with head-papilla 

 retracted swims swiftly and restlessly through the water, not 

 unlike some of the larger infusoria, though more rapidly. 

 Sometimes it goes directly forwards, and then rotates on its 

 longitudinal axis, just turning a little from side to side, as if 

 searching for something. At other times, by curving its body, 

 it sweeps round in circles, or, curving itself still more strongly, 

 spins round and round without moving from the spot. When 

 the embryo, in moving through the water, comes in contact 

 with any object, it pauses for a moment, and feels about as if 

 trying to test its nature, and, if not satisfied, darts off" hastily 

 again. But if the object be a Limnseus truncatulus it at 

 once begins to bore. Prof. Leuckart has said of the head- 

 papilla of the embryo, that " it seems to have the function of 

 a tactile organ." But I have no doubt that it has the function 

 assigned to it in my former papers, viz. that it is a boring- 

 organ.^ The papilla is ordinarily short (about '006 m. in 

 length), and the end is quite blunt, or may have a slight 

 depression in the middle. A differentiation in the tissue of 

 the head-papilla is visible in the form of a delicate rod-shaped 

 structure, occuping the axis, certainly not distinct enough to 

 be called a spine, though the papilla seems to possess consider- 

 able rigidity. It is particularly evident in preparations of 

 embryos killed with osmic acid and stained with picro-carmine. 

 ' 'Roy. Agric. Soc. Journ.,' 1881, p. 7. 



