OR OSSEOUS SYSTEM. 31 



for supporting and moving the six pairs of long articulated 

 members on each side are seen at b, c, and the branchiae at- 

 tached to the anterior or smallest pair of these haunches are 

 seen at h, h. The membranous conical long tube continued 

 from the anus is seen extending from the shell at e. These 

 articulated organs of the cirrhopods attached to the sides 

 of the abdominal portion of the trunk are in constant move- 

 ment during life, extending and retracting, like the branchial 

 organs of the post-abdomen in the branchiopodous crus- 

 tacea. 



IX. Annelida. The red-blooded worms lead us a stage 

 higher in the development of the articulated skeleton, and 

 especially in the organs of locomotion. Some, as the pleione 

 and the halithea, have the exterior covering of the body 

 still so soft and membranous, as scarcely to present an arti- 

 culated appearance. Some, as the leech, have only the trunk 

 of the body developed, and from the want of lateral setae 

 and cirrhi for progressive motion, have the segments sur- 

 rounding the body very short and numerous, and thereby 

 possess great flexibility of the trunk. In the earth-worm, the 

 segments of the trunk are larger and firmer, and each ring is 

 provided with eight very small curved, conical, hollow, sharp 

 pointed spines or setae, which are disposed on the sides of 

 the segments in two upper and two lower pairs. These 

 setse are surrounded each with a muscular sheath for their 

 advancement and retraction, and they serve as organs of 

 locomotion. In some annelides the setee are hooked at their 

 points, in others they are compressed, or spatulate, and in 

 others subulate. In the simplest forms of annelides we 

 sometimes find, as in the nais, but one long filament or seta 

 developed from each side of each segment, which however 

 still materially assists them in moving over a solid surface, 

 or through narrow passages, or in their serpentine motions 

 through the water. The softness of the skin in most of the 

 annelides compensates for the want of articulated feet, by 

 allowing greater flexibility to the trunk, and the delicate 

 sensibility of the whole skin compensates for the imperfect 

 development of their organs of sense. This delicate and 

 unprotected condition of their outer surface requires many 

 of them to shield their whole body in external solid adven- 

 titious tubes, often most artfully constructed, and some, as 



