220 HIGH SCHOOL ZOOLOGY. 



mologues of these are to be recognized in tlie green gland of the 

 crayfish, and the coxal glands of various other pi-imitive Arthro- 

 l)ods, such as the scorpions and mites, which have a similar 

 jjosition, but are not repeated in every segment as in the 

 Annelids. 



The excretory organs of Annelids find a closer parallel in the kidneys 

 of the more primitive Vertebrates, which are also disposed segmentally ; 

 the internal apertures may be retained, but the separate external aper- 

 tures are replaced by a single collecting duct leading to the outside. 



4. Instead of the elaborate muscles which are pi-esent in the 

 highest Arthropods, we find that the mviscles and the skin 

 are closely united into a tube surrounding the ccelom. Im- 

 mediately underneath the skin is a layer of circular fibres, 

 within that, one of longitudinal fibres, and both are penetrated 

 by radial fibres which extend from the skin inwards. Although 

 locomotion is always effected by the alternate contractions 

 and relaxations of this cutaneo-muscular tube, yet the precise 

 way in which it is carried out differs in the two subclasses of 

 Annelids — the Chsetopoda and Discophora — to which the earth- 

 v/^orm and the leech respectively belong. In the former, loco- 

 motion is assisted by bunches of strong bristles (setse) attached 

 to the sides of the segments, and worked by special muscular 

 slips, while in the latter, one or more regions of the tube ai'e 

 modified into suckei-s, which fix the body, while the muscu- 

 lar tube alternately contracts and elongates. 



5, To the Chsetopods belong the bulk of the Class, marine 

 forms with numerous bristles (Order Polychaeta) fixed on short 

 pi'ojecting stumps (parapodia), which may also carry feelers, 

 gills, or protecting scales. The marine Annelids are either car- 

 nivorous in their habits, living a free life, and swimming or 

 creeping about the seashore, or sedentary forms, which buri'ow 

 in the sand (Fig. 149), or live in tubes of chitin or sand or lime, 

 which are constructed with the aid of secretions from the skin. 



