868 ACALEPHA. 



of the animals which they inhabit. No trace of circulating vessels has been 

 detected ; and the vestiges of nerves are so obscure, that many naturalists 

 have doubted their existence. When these characters are found in an ani- 

 mal similar in form to tliose of this class, it is arranged along with this 

 division, though it does not inhabit the interior of another species. 



Linnaeus arranged this group of animals in a division of his great class 

 Vermes, including the genera Lumbricus, Sipttncvhis, Fasciola, Gordius, 

 Ascaris, Hirudo, and Myxine. Subsequent writers, such as Pallas, MuUer, 

 Blumenbach, Bloch, and Goeze, established new genera, or added new spe- 

 cies; and more lately, Cuvier, Lamarck, Rudolphi, and Bremser, from more 

 detailed examination of the animals, and a more intimate knowledge of their 

 structure, have proposed arrangements better suited to the present state of 

 the science. 



M. LaiTQarck divides the class into three orders, viz. Hispidae, Rigidulffi, 

 and MoUassse, the last of which is subdivided into three sections. In the 

 method of Cuvier, the class forms two orders, Les Cavitaires, andLesParen- 

 chymateaux, according to the structure of their body. And Ruliolphi, in 

 his work, entitled Entozorum, sive Ver7nium Intestinorian Historia Naturalis, 

 arranges them into five orders, viz. 1. Nematoides ; body elongated, cylin- 

 drical, elastic. 2. Acanthocephalus ; body cylindrical, slightly elastic, with 

 anterior simple or compound, prolongation covered with a series of bent and 

 retractile spines. 3. Trematodes ; body flattened, or slightly cylindrical, 

 soft, and provided with pores for suction. 4. Cestoidea; body elongated, 

 flattened, soft, of one or many pieces. 5. Cisticorus ; body terminated by 

 or adhering to a vesicle. This arrangement includes besides, three isolated 

 genera, which would not admit of being placed under the previous heads. 

 Latreille, in his Families du Regne Animal, disposes the intestinal worms 

 chiefly after the methods of Rudolphi and Cuvier ; combining in his sketch of 

 the class, the general views of these excellent naturalists. As the method 

 of Latreille is here followed with one exception, it is not necessary to repeat 

 the characters of the subdivisions. That branch of natural science which 

 treats of intestinal worms, is generally termed Helminthology. 



CLASS XII. — AC ALE P HA. 



Body gelatinous, circular, and radiated, ivith the skin soft and transparent, 

 susceptible of contraction and dilatation. 



The class Acalepha of Cuvier embraces the Radiaires, Medusaires and. Ano- 

 males of Lamarck, and besides includes the genus Actinia, which the latter 



