ANNELIDA. 189 



ternal surface ; they are moreover almost all hermaphrodite, and 

 generally require the congress of two individuals for mutual im- 

 pregnation. 



(233.) These animals are separated by Cuvier into three distinct 

 orders, distinguished by the nature and position of their organs of 

 respiration ; they are as follows : 



ABRANCHIA. In this order there is no respiratory apparatus 

 visible externally, but on each side of the body a series of minute 

 apertures may be detected, whereby the surrounding medium is 

 admitted into numerous internal delicate sacs, over which the 

 blood-vessels are seen to ramify ; these form apparently the re- 

 spiratory system : the sacculi themselves, and the ducts by means 

 of which they communicate with the external apertures, are de- 

 lineated in^g. 80, 2, ra. 



This order comprises two distinct tribes, that differ widely in 

 their habits and external appearance : the first comprehends the 

 LEECHES (Annelida suctoria), distinguished by the existence of 

 a prehensile sucker at each extremity of the body ; while, in 

 the second, instruments of attachment are totally wanting, the 

 only external appendages to the body being a number of minute 

 and almost imperceptible bristles, which project from the different 

 segments and assist in progression : such are the EARTH-WORMS, 

 &c. (Annelida terricola.) 



DORSIBRANCHIATA. In the second order the respiratory appa- 

 ratus consists of numerous vascular tufts, a pair of which is ap- 

 pended to the outer surface of every ring of the body, or, in some 

 cases, only to those near the middle of the animal. The organs of 

 locomotion, which are likewise attached to each segment, assume 

 various forms, but are generally composed of short moveable spines, 

 or packets of retractile bristles, probably destined to perform the 

 office of oars. In the annexed figure, (fig. 77, 1,) which repre- 

 sents the Leodice antennata, the general form of these animals is 

 well seen, as is the most usual arrangement of the branchial tufts 

 and locomotive setae, \nfig. 77, , showing an imaginary trans- 

 verse section of one of the segments, the relative positions of the oars 

 (c, rf, e), and of the branchial appendages (&), are likewise indicated. 



TUBICOLA. The two preceding orders of Annelidans are 

 erratic ; but in the third we find creatures inhabiting a fixed and 

 permanent residence, which encloses and defends them. This is 

 generally an elongated tube, varying in texture in different species. 

 Sometimes it is formed by agglutinating foreign substances, such 



