THE TREMATODA. 171 



appears probable that they form an annectent group between 

 the Rotifera and the Turbellaria, 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 Lacimdaria, 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 Triarthra and Polyarthra may be compared to the 

 lateral bundles of long setae of the larvae of Spio 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 Phoronis. 

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

 blance between the Rotifera and the larvae of Echinoderms 

 (" On Lacinularia socialist I. c). Of any such close and 

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

 Pedalion, 1 with its jointed setose appendages and curious 

 likeness to some JSTauplius conditions of the lower Crustacea, 

 suggests that connecting links in this direction may be found. 2 

 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 two. 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 setaa of the same character 



1 Hudson, " On a New Rotifer." (Monthly Microscopical Journal, 1871.) 



2 The singular marine genus Echinoderes (Dujardin) is perhaps such a link. 

 These are minute worm-like animals, with a rounded head, followed by 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 body seg- 

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

 single ganglion, which lies in the head and presents eye-spots. The develop- 

 ment of Echinoderes is unknown. (See Greef, " Arclnv fiir Naturgeschichte," 

 1869.) 



