92 INTRODUCTION TO ZOOLOGY chap. 



verbebrates, used formerly to be considered as allied to true 

 worms, but an investigation into its larval history has 

 disclosed distinct affinities with the Echinoderms,^ and suggests 

 an ancestral link between this group and the Vertebrates. 

 So the discussion of the vertebrate origin is further involved, 

 for these two groups, with both of which it is possible the 

 Vertebrates may have affinity, are in themselves strikingly 

 dissimilar. 



Classification of the Annelida mentioned in Chapters VI. and VII. 



Class I. Chaetopoda or Bristle-worms. 



Sub-class 1. Oligochaeta. — Those with no definite " bristle- 

 stumps" (parapodia), but only a few single bristles on each 

 segment. No distinct head. Chiefly land or fresli- water 

 forms. 



Tubifex rivulorwn, The Biver-worm. 

 Lv/mbricus terrestris, The Earth-worm. 

 Sub-class 2. Polychaeta. — Those with definite parapodia, each 

 usually bearing many long bristles. A definite head is pre- 

 sent, bearing eyes and tentacles. Special filamentous gills 

 are often developed. Chiefly marine forms. 

 Aphrodite, The Sea-mouse. 

 Phyllodoce, The Paddle-worm. 

 Pectinaria, The Comb-worm. 

 Terebella, The Mason-worm. 

 Arenicola, The Lob-worm. 



Sabella "i All tube-forming, and peculiar in having 

 Serpula I distinct thoracic and abdominal re- 

 Pomatoceros j gions in the body, marked by dif- 

 ^ Spirorbis J ferences in the chaetae. 

 Class II. Hirudinea or Leeches. 



Hirudo medicinalis, The Medicinal Leech. 

 Haemopsis gulo, The Horse Leech. 

 Nephelis vulgaris, The Small Pond Leech. 



Practical Notes on Sea-worms and Leeches 



Some of the marine bristle-worms are so beautiful 

 and so easy to keep for a time, that they should be 

 studied in the sea-water tank, as well as in their natural habitat. 

 Sabella, Terebella, and Pomatooeros are fairly easily found in 



' See page 66. 



