TRTSTOMTD^-. 41 



of the internal reproductive organs, wliicli it is here unnecessary to 

 particularize. 



TristomidcB. — The various members of this family display a 

 leech-like aspect, in consequence of which they have been placed 

 either along with the Malacobdellidas, or in some other allied family 

 of the suctorial annelids. It is admitted, on all hands, that the 

 tristomes are not entozoa in the literal acceptation of the term, yet 

 it will be seen that their internal organization conforms more 

 strictly to the trematode type than to that of the Hirudinidce. 

 Thus, they support two small suckers anteriorly and one large 

 sucker posteriorly, the body being externally smooth, devoid of 

 annulations, and more or less compressed. The mouth is placed 

 in front between the cephalic suckers, communicating with a beau- 

 tifully ramified and dendriform intestine, whilst the main divisions 

 of the latter, from either side, combine at the lower part of the 

 body, so as to form a closed system of tubes. The tristomes 

 have therefore no anus. In some species the large caudal sucker 

 is sessile ; in others it is stalked or pedunculated, being in either 

 case bordered by a membranous fold (Dujardin). All the species are 

 hermaphroditic, and display an arrangement of the reproductive 

 organs very similar to that which we find to be present in the 

 flukes, properly so called. These animals, as before implied, are 

 ectozoa, some of them living on the gills of fishes, others attaching 

 themselves to the general surface of the bodies of their piscine 

 hosts, selecting especially the neighbourhood of the fins, whilst a 

 third kind are parasitic on crustaceous parasites, the latter being 

 also attached to marine fishes. 



Development. — Our knowledge of the genetic history, in so far 

 as it relates to the typical members of this family, must be consi- 

 dered very imperfect; but in the case of the aberrant genus 

 Udonella, the mode of development is known to be remarkably 

 simple and direct. In this genus, according to the researches of 

 Van Beneden, the embryos are comparatively large, and they 

 acquire the form and characteristics of their parents whilst they 



