V] TREMATODA 103 



of the body. The nervous system is remarkable for the fact that 

 several trunks of equal size are given off from each side of the brain. 

 The reproductive organs resemble those of a Rhabdocoele like 

 Mesostoma; thus the germarium is developed only on one ovary, 

 of which it is a basal branch, and the testes each consist of a 

 lobed organ directly continuous with the vas deferens. The main 

 peculiarities are as follows : there is no spermatheca ; the sperma- 

 tozoa from another individual enter either by a dorsal pore or two 

 lateral pores, leading into a canal or canals which join the oviducts 

 where they unite with one another. These ducts, totally unrepre- 

 sented in Turbellaria, are called the " canals of Laurer." Further, 

 the genital atrium is situated on the anterior part of the body in 

 front of the ventral sucker. There is no uterus comparable with 

 that of Turbellaria; the so-called uterus being a long coiled tube 

 composed of the conjoined oviducts (vitellarian ducts). Finally 

 the testes are so large that there is not room for them side by 

 side, but in order to stow them away one is situated behind the 

 other. 



Trematoda are divided into two orders called respectively the 

 MONOGENEA and the DIGENEA. In the first named the egg gives 

 rise to a larva which develops continuously into the adult; the 

 main organ of adhesion is a disc situated at the posterior end of the 

 body and armed with suckers or hooks, usually both, and there are 

 two lateral vaginae from which the canals of Laurer lead inwards. 

 The excretory system opens by two dorsal pores. One of the 

 commonest of the Monogenea is Polystomum integerrimum, found 

 in the bladder of the Frog. This animal has an adhesive disc 

 bearing six suckers. The fertilised egg of Polystomum is discharged 

 through the cloaca of the Frog into the water. After some time a 

 larva hatches out which has a forked alimentary canal, but which 

 is without genital organs and has no suckers, although the posterior 

 adhesive disc is clearly differentiated. It is provided with a 

 number of transverse bands of cilia by means of which it swims 

 about until it finds a tadpole, to the skin of which it attaches 

 itself. It creeps into the branchial chamber of its host and loses 

 its cilia, and commences to develop genital organs and suckers. 

 About the time of the tadpole's metamorphosis the Trematode 

 wanders down the alimentary canal into the bladder. Sphyranura 

 osleri is an allied form parasitic on the skin of the Urodele 

 Necturus. Its posterior disc carries two large suckers and two 

 hooks. 



