96 ZOOLOGICAL PHILOSOPHY 



of a bilateral symmetry of the parts, so that each part is opposite to 

 another exactly like it. 



In the worms we no longer find this bilateral symmetry, nor do 

 we yet witness the radiating arrangement of the organs both internal 

 and external which characterises the radiarians. 



After I established the annelids, some naturalists called them by 

 the name of worms ; and as they did not then know what to do with 

 the animals now under discussion, they united them with the polyps. 

 I leave the reader to imagine what may be the aflS.nities and classic 

 characters that justify the union in one class of Taenia or Ascaris with 

 a hydra or any other polyp. 



Several worms still appear to breathe hke insects by tracheae of 

 which the external openings are kinds of stigmata ; but there is reason 

 to believe that these Umited or imperfect tracheae are water-carrying 

 and not air-carrying like those of insects ; because these animals 

 never Uve in the open air, but are continuously in the water or bathed 

 by fluids which contain water. 



As no organ for fertilisation is distinguished in them, I suppose 

 that sexual reproduction does not occur in these animals. It may be 

 however that, just as there exists a primitive circulation in arachnids 

 so there may exist a sexual reproduction in the worms, as is suggested 

 by the various shapes of the tail of Strongylus ; but observation has 

 not yet fully estabhshed such reproduction in these animals. 



Objects which are found in some of them and supposed to be ovaries 

 (as in Taenia) appear to be merely clusters of reproductive corpuscles 

 which do not require fertihsation. These oviform corpuscles are 

 internal, hke those of sea-urchins, and not external like those of 

 Coryne, etc. Polyps exhibit similar differences in the situation of their 

 gemmules ; it is therefore probable that the worms are internally 

 gemmiparous. 



Animals like the worms, which have no head, eyes, legs or perhaps 

 sexual reproduction, provide further evidence in their turn of the con- 

 tinuous degradation of organisation that we are seeking throughout 

 the animal scale. 



RADIAEIANS. 



Animals with regenerating bodies, destitute of a head, eyes or jointed 

 legs ; with the mouth on the inferior surface and a radiating arrange- 

 ment of the parts both internal and external. 



In the usual order the radiarians occupy the twelfth rank in the 

 lengthy series of known animals, and constitute one of the three last 

 classes of invertebrates. 



