ANNELIDES. 89 



It is very common to find the first segment provided only with ten- 

 tacular cirri, destitute alike of setigerous tubercles and proper bran- 

 chiae ; and towards both extremities of the body its appendages are 

 either obsolete or less developed than they are near the middle. The 

 anal segment has its appendages commonly reduced to a pair of ter- 

 minal styles, which originate from each side of its aperture (fig. 6). 



Such are the most remarkable features in the external anatomy of 

 these annelidans ; but the bristles form so marked a character in 

 them, and influence their habits so much, that it seems necessary to 

 enter at length into a description of their peculiarities, — the more 

 especially as they constitute the principal means of defence against 

 the enemies that surround them. Some annelidans, when alarmed, 

 such as the Aphrodites, instinctively contract the body into a sort of 

 ball, bringing the head and tail into near contact ; and in this man- 

 ner seem anxious to elude the danger. Others, that have a long 

 vermiform body, wriggle and twist themselves with violence, as if 

 they would frighten away the foe by their contortions ; but these are 

 weak resources in comparison with the protection they derive from 

 their panoply of spines and bristles. And it is curious to remark 

 that as these diminish in size and piquancy, and become better 

 adapted to locomotion than offence, the worms, as if conscious of 

 their defenceless state, forsake the nomade life, and seek additional 

 security by digging themselves furrows for retreat in the sand and mud. 



Savigny was, perhaps, the first to notice that the bristles* are gene- 

 rally capable of being retracted within the body of the worm, and of 

 being again protruded at pleasure by means of appropriate muscles ; 

 but he does not appear to have studied these organs in relation to 

 their ofiice of defence, nor has he given any figures sufficiently mag- 

 nified to show how aptly they are fitted for this purpose. It was 

 left to Audouin and Milne-Edwards to fill up this blank in the history 

 of the Annelides, and we proceed to give their analysis of these 

 organs. No XII 



The Bristles are distinguished into spines (aciculi), 

 and bristles properly so called (festucae). 



The Spine is of one kind only (No. XII. fig. 11). It is 

 distinguished from the bristles by its central position and 

 its larger size. It is straight, conical, pointed, of a brown 

 or blackish colour, and always a shade darker than that 

 of the bristles with which it is associated. It is very 

 rare to find more than one in each branch of the foot, and 

 there are never more than two. Sometimes none can be 

 discovered. The worm, say our authors, employs this 

 weapon to give the " coup de grace " to the enemy against 

 which it is directed, — the comparative shortness and 

 stoutness permitting it to be used with a force and pre- 

 cision which it is impossible to impart to the weak and 0<5 

 flexible bristles; and the comparatively blunt point M- 

 making a wide wound. Each spine is composed of the shaft (a), 



* See Oersted's definition of the Bristles in his * Annul. Dan. Consp.' p. 5, and 

 copy. 



u 



