79 



CHAPTEE VI. 



WORMS (vermes) — Gmiinued. 

 Annelida oe Segmented Worms. 



■Of the true segmented worms two groups are of interest to the 

 agriculturist — namely, the Earthworms and the Leeches : the 

 former are beneficial to the soil, the latter harmful to horses. 



Annelids are segmented worms with a brain, a circum-sBso- 

 phageal nerve-ring, a ventral nerve-cord, and a definite vascular 

 system. Generally Annelids are round, but some — such as the 

 Leeches — are flattened ventrally. In form most are cylindrical, 

 with a thick muscular skin. The body is made up of a num- 

 ber of successive segments, constricted off externally. Those 

 segments following the head are similar externally and internally. 

 The tail segments differ, and give origin during the growth of 

 the worm to new segments anteriorly. The internal divisions are 

 called dissepiments, and may correspond to the external segmen- 

 tation, or may equal three, four, or five of the external rings. 



Annelids are found in all manner of places. The Glwetapoda 

 are marine and terrestrial. The marine species live in the 

 sand, on rocks, shells, &c. The terrestrial live in damp earth, 

 and others live in fresh water. They all require damp media 

 to flourish in. Very few annelids are parasitic ; those that are 

 eo are mainly external parasites. 



Most of these worms are oviparous ; a few produce living 



