Invertebrate Gallery of the Indian Museum. 43 



PHYLUM III OF THE CCELOMATA. 



ANNELIDA. 



[flSlcstcm (ilaU-rascs 43-44]. 



The next phylum is formed by the Annelida, including 

 the Sea-worms, the Earth-worms, the Leeches, and the 

 Wheel-animalcules, or Rotifera, The members of this 

 phylum usually possess blood and blood-vessels. In most 

 of the Annelida we distinguish very plainly the following 

 external characters :—(i) a body made up of a succession 

 of equal rings or segments, (ii) an anterior or head segment 

 behind which is the mouth, (iii) a posterior or anal segment 

 in which is the anus, (iv) ventral and dorsal surfaces, and 

 (v) bilateral symmetry. 



The segments behind the head are not only alike exter- 

 nally, but internally their contents correspond or nearly 

 correspond, — each containing an equivalent portion at 

 least of digestive tube, of blood-vessel, of excretory organ, 

 and of nerve-cord. 



In many Annelida each segment carries a pair of loco- 

 motor appendages, one on each side, which are known as 

 "parapodia" (or foot-like organs), and each parapodium 

 commonly consists of two elements — a dorsal element or 

 " notopodium " and a ventral element or " neuropodium," 

 both carrying a tuft of bristles and a finger-like process or 

 "cirrus." This is especially well seen in the sections of 

 the large Sea-worm — Eunice. 



The alimentary canal usually passes straight along the 

 body-cavity from one end to the other, and is held in place 

 by two longitudinal folds of the membrane that lines the 

 body-cavity — one above (the dorsal mesentery) the other 

 below (the ventral mesentery), and by transverse partitions 

 or "septa" which correspond to the segments. By these 

 septa the cavities of the segments are shut off from one 

 another. 



