THE TREMATODA. 171 



appears probable that they form an annectent group between 

 the Rotifera and the Tarbellaria, which last approach the Ro- 

 tifera by such forms as Dinophilus, 



The free Rotifers present marked resemblances to the 

 telotrochous larvae of Annelids. The young Lacinidaria^ for 

 example, has a circular prge-oral disk provided with two eye- 

 spots and a second circle of cilia behind the mouth, and is 

 wonderfully like an Annelid larva (Fig. 39, I.). The append- 

 ages of Trlarthra and Polyarthra may be compared to the 

 lateral bundles of long setse of the larvie of i:ipio and Nerine^ 

 and the pharyngeal armature is essentially Annelidan. On 

 the other hand, in the sessile tubicolous Rotifera^ the trcchal 

 disk assumes the characters of the lophophore in the Polyzoa, 

 and of the tentacular circlet of the Gephyrean Phoronls. 

 Many years ago I drew attention to the points of resem- 

 blance between the Rotifera and the larvae of Echinoderms 

 ('• On L'lcinularia socialist'' I, c). Of any such close and 

 direct relations with the Crustacea., I see no evidence ; but 

 Pedalioa^ with its jointed setose appendages and curious 

 likeness to some JSTaupllus conditions of the lower Crustacea., 

 suggests that connecting links in this direction may be found.'^ 

 In fact, the Rotifera., as low Metazoa with nascent segmenta- 

 tion, naturally present resemblances to all those groups which, 

 in their simpler forms, converge toward the lower Metazoa. 



The Trematoda. — These are all parasitic, either upon the 

 exterior (ectoparasites) or in the internal organs (endopara- 

 sites) of other animals. Many are microscopic, and none 

 attain a length of more than an inch or tAvo. Most have a 

 broad and flattened form, one face being ventral and the 

 other dorsal, and the body is never segmented. 



In the adult, the ectoderm is not ciliated, but its outer- 

 most layer is a chitinous cuticula. In most Trematoda, one 

 or more suckers are developed upon the ventral surface of the 

 body, behind the mouth. These are sometimes armed with 

 chitinous spines or hooks ; and setae of the same character 



J Hudson, " On a New Rotifer." (Monthly Mkroaropiml Journal, 1871.) 

 2 The singular marine ^enus Echinoderes (Diijurdin) is perhaps such a link. 

 These are minute worm like animals, with a rouridod liead, followed by a num- 

 ber (ten or eleven) of distinct seorments, the last of which is bifurcated. There 

 are no limbs, but the head is provided with recurved hooks, and the bodv seg- 

 ments with paired sette. The nervous system appears to be represented by a 

 single gantflion, which lies in the head and presents eye-spots. Tlie develop- 

 ment of ^t7ij/iO(/6res is unknown. (See Greef, " Archiv fiir Naturereschichte," 

 1869.) ^ ' 



