THE TUKMATODA. 171 



appears probable that they form an annectent group betw. n 

 the Rotifera and the Turbellaria, which last approach the/fo- 

 tifera by such forms as IHnophihto. 



Th<- IV: liMt.ifer.s present marked resemblances to the 

 telotrochous larvae of Annelids. The young Xr/'/////.//-/.- 

 example, has a circular prae-oral disk pn.vMrd with tu< 

 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 8pio and Nerine, 

 :m<l thr phiryngeal armature is essentially Annelidan. On 

 the other hand, in the sessile tubicolous Rotifera. tin- n 

 disk assumes the characters of the lophophore in tin- /'<>/ 

 an 1 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 Echinodrrms 

 (" On Lddnularia socialis," I. c.). 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 Nctupliua conditions of the lower Crustacea, 

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

 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 Tremntoda, 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 



1 Hudson, " On a New Rotifer." (Monthly M/<<ro*capical Journal, 1 ^7 1 ^ 



2 Tiie singular marine genus Echinoderes ^Dujardin) isjn rhaj-s such a link. 

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

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

 are no limbs, but the head is provided with recurved hooks, and the bod 

 meats with paired setee. The nervous system appears to be represented by a 

 single trunnion, which lies in the head and presents eye-spots. The dt \ 

 meat of Echiiwderes is unknown. (See Greef. " Archiv far Naturgeschichte " 

 1869.) 



