THE TREMATODA. 171 



appears probable that they form an annectent group between 

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

 tifera by such forms as Dinophilus. 



The" free Rotifers present marked resemblances to the 

 telotrochous larvge of Annelids. The young Laclnularla, for 

 example, has a circular prae-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 Ti'larthra and FolyartJtra may be compared to the 

 lateral bundles of long set£e of the larvas of Splo and Nerine^ 

 and the pharyngeal armature is essentially Annelidan. On 

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

 disk assames the characters of the lophophore in the Polyzoa, 

 and of the tentacular circlet of the Gephyrean Rhoronis. 

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

 blance between the Rotifera and the larvae of Echinoderms 

 ('• On Lacinularla socialist'' I, <?.). Of any such close and 

 direct relations with the Crustacea^ I see no evidence ; but 

 Pedalion^ with its jointed setose appendages and curious 

 likeness to some JSfaupHus 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 bodj^ 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 



^ Hudson, " On a NewEotifer." (Monthhj Microacopieal Journal, 1871.) 

 2 The sino^ular marino irenus EfMnoderes (^Dujardin) is perhaps such a Unk. 

 These are mhiute worm like animals, with a rouiided head, followed bv a num- 

 ber (ten or eleven) of distinct segments, 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 set;e. The nei-vous system appears to be represented bva 

 sinarle gancclion, which lies in the head and presents eve-spots. The develop- 

 ment of Echiaoden^s is unknown. (See Grecf, '' Archiv fiir Naturi^eschichte," 

 1869.) "^ ' 



