240 EMBRYOLOGY 



and then remain in the damp earth. Here they moult and 

 metamorphose into the sexually mature animals. These 

 copulate, deposit their eggs in the earth, and the young 

 worms developing from them then migrate into insect larvae 

 again, that of Mermis albicans, for example, into young 

 caterpillars. 



The mode of development just described for many 

 Nematodes, in which the worms pass through a Rhabditis 

 stage, may well be regarded as most nearly resembling that 

 form in which parasitism in the Nematoda originated ; that 

 is to say, a more or less f ally developed worm resorted to the 

 body of another animal, or at first only became attached 

 to it in order to gain nourishment from its juices. The 

 parasitism only gradually became permanent ; it is precisely 

 the Nematodes that offer all transitions from a partial to a 

 complete parasitic life, which eventually leads to a total 

 transformation of the form of the body. Such a metamor- 

 phosis of the most extreme kind is realized in Sphserularia 

 hombi, which was first investigated by Ant. Schneider and 

 more recently in detail by Leuckart (No. 7). This worm in 

 the adult condition consists of a thick warty sac, which 

 lies in the body cavity of female humble-bees. To it is 

 attached a diminutive worm, which can be recognized as a 

 Nematode only upon careful examination. The entire sac 

 owes its origin to the fact that the vagina of the worm 

 became everted, and, growing to an enormous size, included 

 the other sexual organs in it. The entire animal now con- 

 sists, with the exception of the small attached worm, simply 

 of a sac filled with sexual products. In it the eggs develop. 

 The young worms reach the body cavity of the humble-bee, 

 and from there the outside world, where they attain to 

 sexual maturity. They copulate in the free condition, and 

 probably the fertilized females migrate into the humble-bees 

 when these seek their winter quarters in the ground. Then 

 the remarkable transformation of the female begins, 

 r A transition to Sphserularia is represented by Atractonema 

 gibbosujn, discovered by Leuckart, in which an e version of 

 the vagina likewise takes place, although it attains no 

 greater size than about that of the worm itself. It is 



