ANNULOSA. 561 



organization demonstrates, not meiely their annulose 

 nature, but their position among the Annuloida, and 

 which exhibit precisely the same indistinct segmentation, 

 the same general absence of appendages, and whose means 

 of locomotion are in like manner confined to one or two 

 ciliated circlets at the anterior part of the body. The 

 connexion between the Annelida and the Rotifera is 

 further illustrated by such remarkable forms as the 

 Polyophthalmus of De Quatrefages, a true Annelid, which, 

 nevertheless, possesses on each side of the head a ciliated 

 lobe, capable of being voluntarily protruded and retracted, 

 and presenting a close resemblance to the trochal disc of a 

 Rotifer. Hydatina senta has been so well and accurately 

 described by Dr. Cohn, 1 and others, that it may be taken 

 as the typical form of the Rotifera. The trochal disc in 

 the species, undergoes great changes of form. In Hydatina, 

 it is circular, and its margin is skirted by two distinct con- 

 tinuous bands of cilia, the one immediately in front of, the 

 other behind the mouth. In Brachionus the ciliated circlet 

 fringing the edges of the trochal disc is horseshoe-shaped, 

 but the circlet is produced into three lobes or processes, 

 which stand out pei-peudicularly to the surface of the 

 trochal disc. In Stephanoceros it fringes the edges of a 

 number of tentaculiform processes, into which the trochal 

 disc is produced, so as to give the whole animal somewhat 

 the appearance of a Polyzoon. 



The Turbellaria, a group of serpent-like worms, for 

 the most part the inhabitants of fresh and salt waters, a 

 few only being found in damp siuations on land, are 

 characterised by the ciliation of the entire surface of the 

 body. In their internal organisation they approximate in 

 many respects to the Trematoda, while in others they 

 exhibit a certain affinity with the other great group of 

 parasitic Annuloida, the Nematoidea. Polycelis losvigatus, 

 one of the Dendrocoda so well described by De Quatrefages 

 in his Monograph on the Marine Planarioz, may be most 

 advantageously selected as a type of the group. The 

 Nemertidce have engaged the attention of the same learned 

 authority, the ova of which undergo a remarkable kind of 



(1) Ueber die Foriflawzung der Raderthiere, Von Dr. F. Cohn, in Breslau. 

 1855. 



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