196 PROCEEDINGS OF SOCIETIES. 



there is a median dorsal vessel which is not ciliated. Distoma 

 Okeni and Bilharzia hoematobium are the only Trematods of 

 distinct sexes. The latter is a dangerous parasite of Egypt, 

 causing the death of hundreds of the poorer class. The male 

 permanently embraces the female, so that they present through 

 life this appearance (fig. 5). No complete case of Trematod 

 development is yet known. Leuckhart has found that the 

 common Fluke gives rise to a ciliated larva (fig. 6), but he 

 has been unable to trace it further. But by comparing the 

 Monostonum of birds, the Redia and Cercaria of water-snails, 

 which subsequently become encysted, and give rise to a Dis- 

 toma, we are able to frame some notion of^the order of 

 development. It is evident that two hosts are necessary, of 

 which the second is nearly always higher in the animal series 

 than the first. "VVe get, then, the following order : — 1. Ciliated 

 embryo ; 2. Redia, which may produce other Redise by in- 

 ternal budding, but eventually produces, 3. Cercarise, whi<;h 

 become encysted, and emerge, as 4. Distomata, which lay 

 eggs. In some cases the Rediae are simple oval masses, and 

 are then called Sporocysts. In the fresh-water Mussel, a 

 form of Cercaria is found which has not yet been traced out ; 

 it has two long tails instead of one, and is known as Buce- 

 phalus. D'qjlozoon is a Trematod ; the individuals are hatched 

 separately, but come together and fuse or conjugate as in the 

 Infusoria, and then the sexual organs develop. 



Lecture YI. — The Turbellaria are very near to the Trema- 

 toidea, but none are parasitic, they never have prehensile 

 hooks as some Trematods do, nor any suckers. In the inte- 

 gument are bodies resembling thread-cells and aciculi. The 

 alimentary canal exhibits the simple and the branched form, 

 as in the types Nemertes, Opisthomum, and PoIyceUs. The 

 proboscis, which some Nemertians have in front of the mouth, 

 but usually packed in the body-cavity, is a very remai'kable 

 structure. The water-vascular system in some has more than 

 one pore. In Nemertians it is open when young, but in 

 adults it is closed definitely, forming a contractile system of 

 vessels like that of Annelida. The nervous system consists 

 of a couple of ganglia, giving off two long stems, but there is 

 no gangliated chain. The reproductive organs present two 

 extremes of complexity ; in Nemertians they are simple 

 masses which escape by dehiscence, the sexes being distinct ; 

 in Planarians they are as complicated with accessory parts, 

 &c., as in any group of animals. The development of Tur- 

 bellarians presents many points of interest, and is not yet 

 known in more than a few forms. A certain species of 

 Planaria presents a larva of the form in fig. 12, presenting 



