ANNULOSA : TREMATODA. 237 



CHAPTER XXIV. 



TREMATODA AND TURBELLARIA. 



ORDER TREMATODA. Leaf -like internal (sometimes external} 

 parasites, provided with one or more ventral suckers ; a mouth 

 and alimentary canal, but no anus. No body-cavity. Integument 

 of the adult not ciliated. Sexes generally united. This order 

 includes a group of animals, which, like the preceding, are 

 parasitic, and are commonly known as " suctorial worms " or 

 " Flukes." They inhabit various situations in different ani- 

 mals mostly in birds and fishes and they are usually flat- 

 tened or roundish in shape. The body is provided with one 

 or more suctorial pores for adhesion. An intestinal canal, 

 with one exception, is always present, but this is simply hol- 

 lowed out of the substance of the body, and does not lie in a 

 free space, or " perivisceral cavity." The intestinal canal is 

 often much branched, and possesses but a single external 

 opening, which serves alike as an oral and an anal aperture, 

 and is usually placed at the bottom of an anterior suctorial disc. 

 A " water- vascular" system is always present, and consists of 

 two lateral vessels which generally open on the surface by a 

 common excretory pore. The nervous system consists of two 

 pharyngeal ganglia. With few exceptions, the sexes are united 

 in the same individual; and the young may be developed 

 directly into the adult, or may pass through a complicated 

 metamorphosis, which varies in different cases, and does not 

 admit of description here. In many cases, the larvae are 

 " cercariiform," or " tailed ; " and one of the early stages of 

 their existence is often spent in the interior of fresh-water 

 molluscs, from which they are finally transferred to their de- 

 finitive host. 



The "Flukes" inhabit, in their adult condition, the most 

 varied situations. Most are internal parasites, living in the 

 intestines or hepatic ducts of mammals, birds, or batrachians, 

 the vitreous humour or lens of the eye, the blood-vessels, &c. 

 A few are external parasites, living on the skin and gills of 

 fishes, crustaceans, and other animals. 



From the absence of a perivisceral cavity, the Trematoda were formed by 

 Cuvier into a separate division of Entozoa, under the name of Vers Intes- 

 tinaux Parenchymateux, along with the Tceniada and Acanthocephala, in 

 which no alimentary canal is present. By Owen, for the same reason, they 

 are included in a distinct class, under the name of Sterelmintha. 



