ANNELIDA. 



213 



of bundles which are remarkable during the life of the creature 

 for their beautiful red colour, derived from Fig. 87. 



the crimson blood which circulates copious- 

 ly through them. 



But the organs of respiration in the Dor- 

 sibranchiate Annelidans are not always arbo- 

 rescent ; on the contrary, they are not un- 

 frequently spread out into thin membra- 

 nous lamellee, or resemble fleshy crests or 

 vascular tubercles ; still, whatever their 

 form, their office is the same, and the vessels 

 spread over them, presenting an extensive 

 surface with which the water is brought in 

 contact, the blood is oxygenated as it passes 

 through them. 



(257.) The second class of organs to be 

 enumerated as entering into the composi- 

 tion of the lateral appendages, are soft, fleshy, 

 and sub-articulated processes called cirri 

 (Jig- 77, 2, c, d) ; these are generally two in 

 number, and belong one to the ventral and 

 the other to the dorsal oar : their precise 

 office is not well understood ; but as in 

 some of the segments, especially in the 

 neighbourhood of the head, they assume a 

 tentacular form, they have with some pro- 

 bability been regarded as instruments of 

 touch. 



(258.) The seta (fig. 77, 2, d) are per- 

 haps the most efficient agents in progression. 

 These are long and stiff hairs disposed in 

 bundles and implanted into strong muscular 

 sheaths. Each packet of setse can be retracted 

 within the body to a certain extent, and 

 again protruded by the action of the tubular 

 supports from which they arise, and, being 

 capable of independent action, these organs 

 must be looked upon as so many power- 

 ful fins, well calculated to propel the 

 creature through the element which it in- 

 habits. 



