cxlii Introduction. 



possessing- any pseudhaemal respiratory vessels, and in this as in 

 so many other points approximating" to the Annulata. 



The nerve-system consists, in the Trematodes and many Turhel- 

 laria, of a single pair of ganglia, placed in the anterior part of the 

 body, and connected with each other by a transverse commissure. 

 (See, however, p. i ^^ infra.) In the Nemertinea it consists of a double 

 pair of ganglia,connected by two commissures, one passing above and 

 the other below the proboscis. From these ganglia two main nerve- 

 stems pass off down either side of the body. Certain Trematodes, in 

 the free locomotor stages of their larval life, may possess eyes, but 

 with this exception, it has only been amongst Turbellaria that 

 either these organs or auditory capsules have been observed to be 

 present. In the Nemertine Turbellaria, two ciliated depressions 

 on the anterior part of the body, underlaid by certain solid organs, 

 are supposed to be sensory in function. It appears to be doubtful 

 whether the Cestodes really possess any nerve- system at all ; a 

 structure similar and similarly placed to the single ganglion of the 

 Nematelminthous Acanthocephali has been described as such. 



With the exception of the Nemertine Turbellaria and the proc- 

 tuchous Rhabdocoela, which are sometimes placed in the same 

 sub-order with them, all the Platyelminthes are hermaphrodite, 

 and provided with accessory intromittent male organs, and accessory 

 vitelligenous, uterine, and receptacular organs. The walls of the 

 efferent ducts are always continuous with the envelopes of the 

 sexual glands themselves, except in the Nemertinea, where the 

 generative glands are sessile upon the body-walls, and no ducts 

 exist as distinct from temporary, or permanent orifices in those 

 walls. All Platyelminthes are oviparous, except a few Nemertinea 

 and a few of the proctuchous Bhabdocoela amongst the Turbel- 

 laria. In a few fresh-water Trematodes, and Turbellaria with large 

 ova, the embryos undergo no metamorphoses after leaving the 

 egg ; but in all other cases the embryos of the Platyelminthes go 

 through more or less numerous stages of metamorphosis, which 

 furnish typical instances of what is known as ' digenesis with 

 heterogony/ and ^ alternations of generations.'' The histories of 

 these metamorphoses are complicated in the parasitic orders by the 

 fact that the different ' generations,' or, in other words, the sexual 

 and the asexual stages in the metamorphosis, require different 

 animals as ' hosts ' for their sustentation and lodgment ; and that 



