44 ^ Guide to the Zoological Collections in the 



The body-cavity contains blood, but there are also closed 

 blood-vessels, the two principal trunks of which traverse the 

 whole length of the body — one above (the dorsal blood-ves- 

 sel) and the other below (the ventral blood-vessel). When 

 "branchiae/' or gills, exist, as they do in most of the Sea- 

 worms, many small blood-vessels run into them, the 

 branchiae in short being organs specially developed for the 

 aeration of the blood. There are special excretory organs . 

 — a pair in each segment. 



The nervous system consists of a cerebral ganglion placed 

 above the gullet, and connected, by means of a nerve- 

 rino- which embraces the gullet, with a pair of usually very 

 closely approximated jierve-cords that run along the ventral 

 wall of the body. This can be seen in one of the dissec- 

 tions of the Earth-worm in Case 44. 



The internal morphological characters sketched above 

 can be seen by reference to the dissections of the large Sea- 

 worm Eunice from the Andamans, and by tne several dis- 

 sections of the common Earth-worm in Case 44. Repro- 

 duction is usually sexual, but many Annelida are reproduced 

 asexually by budding and fission from the parent. 



The Annelida consist of three groups or natural Classes — 

 the Chsetopoda or Worms, the Rotifera or Wheel-animal- 

 cules, and the Hirudijiea or Leeches. Along with them we 

 may briefly consider, as an appendix, the Gephyrea or unseg- 

 mented Sea-worms, the Byyozoa or Polyzoa^ and the 

 Brachiopoda, which along with the Annelida, the Platyhel- 

 minthes and the Nemathelminthes used to constitute the 

 phylum Vermes of older systematists. 



I. ANNELIDA CH/ETOPODA. 



[SUcst^rn gSall-ras^s 43-44]. 



The Chsetopoda possess, on each segment, either paired 

 parapodia bearing cirri and setae and surmounted by breath- 

 ing organs (branchiae), or at least paired tufts of setae. 



A head, often surrounded with tentacles, is generally 

 present. 



