GENERAL CHARACTERS OF VERMIFORM ARTICULATA. 297 



exist any proper tegumentary skeleton or hard envelope, as in 

 most of the higher classes of Articulata. In the lowest mem- 



FJG. 519 LEECH. 



bers of the class, indeed, we cannot even trace an integument 

 distinct from the contained tissues; all being alike soft, and some- 

 times almost jelly-like. Even here, however, the division into 

 segments is most distinctly marked, by the repetition, in each 

 segment, of nearly all the organs of the body ; so that the 

 animal is much more capable of sustaining severe injuries, than 

 is the case in the higher classes ; and it has also greater power 

 of repairing them, new segments being developed to replace 

 those which had been lost. The greater number of the Vermi- 

 form or Worm-like Articulata are aquatic in their habits, living 

 either in water or in moist situations ; and as a general rule, their 

 respiration is performed either by gills, or by the general surface 

 of the body. They have usually a distinct circulating appara- 

 tus ; which serves not only to convey the nutritious fluid through 

 the several tissues, to whose growth it is to contribute, but also 

 to carry it to the organs in which it is to undergo aeration. 

 This apparatus consists, as in Insects, of a dorsal vessel, running 

 along the greater part of the body, and contracting from behind 

 forwards, so as to expel the blood through the branches which 

 proceed from its anterior extremity ; the fluid then flows back- 

 wards through the body, affording nutriment to its several parts 

 in its course ; and it is then made to flow over the walls of the 

 alimentary canal, so as to take up from its cavity any new 

 materials, which may have been prepared for it. The pro- 

 visions for carrying it through the respiratory organs vary 

 greatly, in proportion to the variety in the arrangement of the 

 respiratory organs themselves, which we shall see to be great, 

 as it is in the Mollusca : but there is not unfrequently a distinct 

 contractile cavity, or respiratory heart (ANIM. PHYSIOL. 281), 

 for propelling the blood through every pair of gills, as in the 



