SECT. v. TARD1GRADA. 161 



Errantia and Tubicola have no zoological regions cha- 

 racterized by one or more special types like the other 

 classes of animals; they have representatives in all seas. 

 But it is exactly the contrary with regard to species. 

 The number of species common to any two seas, or the 

 shores of two continents, is very small; there is not a 

 single species common to the Atlantic coasts of France 

 and the Mediterranean. The sea- worms are not affected 

 by climate, but they are said to be more abundant on 

 granitic and schistose coasts than on the calcareous. 7 



With regard to fossil remains, worm-tracks are seen 

 in the Forest marble, long calcareous tubes occur in 

 the Upper Silurian and Carboniferous strata, and in all 

 the later formations tubercular Annelids abound, espe- 

 cially of the genera Serpula, Spirorbis, and Yermilia. 8 



Tardigrada. 



The Tardigrades are slow creeping animalcules, which 

 seem to form a link between the Worms and the Eotifers, 

 though they are more nearly allied to the former in 

 having a vermiform body divided transversely into five 

 segments, the first of which is the head, and each of the 

 others has a pair of little fleshy protuberances furnished 

 with four curled hooks. They resemble the Eotifers in 

 their jaws, in their general grade of organization, and 

 in the extreme length of time they can remain dried up 

 without loss of life. When in the dried state they can 

 be heated to a temperature of 250 Fahr. without the V 

 destruction of life, although when in full activity they 

 cannot endure a temperature of more than from 112 to 

 115 Fahr. When alive the transparency of their skin 

 is such as to show a complicated muscular system, the 

 fibre of which is smooth; and as no respiratory organs 



7 * Comptes rendus,' July 1864. 



8 ' Palaeontology,' by Professor Owen. 

 VOL. II. M 



