188 [August, 



experiments, probably occurs in nature at the moment of pairinoc of the worms. 

 The young larva finds its way into the seminal vesicle, and then falls into a 

 state of hibernation, in which it passes the winter. The long duration of this 

 state is due to the low temperature, and it can be ended, and the cycle hurried 

 forward, by artificially raising the temperature at any time. The presence of 

 these ' primary ' larvae in the worms can he detected by examining the latter 

 under strong transmitted light. 



About the beginning of May the larva awakes, and comes out of the seminal 

 vesicle — if it has not already passed into the general body-cavity, whicli may 

 have occurred during the winter. It then migrates forwards in the body-cavity 

 from the genital segments to the front end of the worm, working its waj'^ with 

 the hooks on its body and with its mouth-armature. This migration occupies 

 1-4 daj^s. The last part of the journey is effected hind end foremost ; arrived 

 at the prostomiiim of the worm, the larva tears this with its hind end, and thus 

 puts its i^osterior spiracles in commiinication with the outer air by way of the 

 worm's mouth. 



Henceforth the worm ceases to feed or move about. The larva, on the 

 other hand, commences the more active and the feeding part of its life. Six 

 days after perforating the prostomium it undergoes a moult, passing into the 

 second stadium, in which it grows considerably and destroys the partition 

 separating it from the worm's pharynx, into which it then passes, and which it 

 distends. Several photographs show the hind end of the larvae projecting from 

 the front end of the worm. After nine days in the second stadium it moults 

 again, passing into the third stadium, in which it begins to devour the tissues 

 of the host. The body-walls of the worm, distended by the larva, become 

 inflamed and disintegrate from before backwards, leaving the larva more and 

 more exposed. The larva ends by being attached by its front end to a few of 

 the hindmost segments of the worm, Avhich still have some life in them. It 

 leaves these remains, biu'ies itself more deeply in the earth, and pupates. There 

 is but one generation in the year. 



As many as four young larvae can penetrate into a worm, but only one 

 completes its development. The others appear to be wounded by the random 

 strokes of the mouth-armature of their comrades, and are then i>erhaps over- 

 come by the phagocytic reaction of the host. The survivor probably owes its 

 success largely to arriving first at the front end of the worm, in which position, 

 with its own head-end directed backwards, it is likely to wound any other larvae 

 that come so far forward. 



The phagocytic reaction of the host has been mentioned. ' Primary larvae ' 

 are often found in winter completely enclosed in phagocytic cysts. If uninjured, 

 they generally manage to emerge from these cysts in spring and continue their 

 development. Wounded or weakly larvae may, however, succumb to the phago- 

 cytes. The worm can also react in another way, by expelling the larva 

 mechanically from its body under certain conditions, as when the larva makes 

 any error in its movements : e.g., if it perforates the prostomium head first 

 (instead of tail first) it can be expelled by the contraction of the worm. Or if 

 it tears the pharyngeal wall too soon, or goes too deep into the pharynx, it can 

 be swallowed and expelled thi'ough the anus. 



