ANNULOSA CH^ETOGNATHA. 77 



the two anterior pairs. The front pair are termed the 

 cerebral ganglia. 



Simple eyes are possessed by most. They appear as 

 small dark spots, generally on the front of the head, but 

 sometimes on the gills. In other cases they are dis- 

 tributed over the various segments of the body. 



There are usually two or more antennae which are 

 without joints. 



104. Development. The sexes are sometimes distinct, 

 but the leeches and earth-worms are hermaphrodite (Gr. 

 Hermes i Mercury ; Aphrodite, Venus), that is, the sexes 

 are united in the same individual. The embryos are 

 generally furnished with vibratile cilia. 



105. Divisions. This class is generally divided into 

 two groups : 



1. The Branchiate section, which includes the sand- 

 worms and the serpulae. 



2. The Abranchiate group, comprising the earth-worms 

 and leeches. 



CLASS 2. Ch&tognatha. 



106. The class Chaetognatha (Gr. chaite, hair; gnathus, 

 a jaw) consists of only a single genus, Sagitta (Lat. sagitta, 

 an arrow). These animals were formerly placed among 

 the Annelida, but, according to Professor Huxley, " they 

 are so unlike them and every other group as to require a 

 class for themselves." They are small marine animals, 

 about an inch in length, and with a transparent skin. 

 The following is an abridgment of the account of these 

 singular creatures furnished by Huxley, (see Classifica- 

 tion of Animals, p. 53): 



" The head is furnished with six pairs of bristles. Two 

 of these are long and claw-like, and lie at the sides of the 

 mouth. The other four are short, and placed in front of 

 the head. The hinder part of the body is fringed by a 

 fin-like membrane (hence the name Sagitta), which 

 seems to be an expansion of the cuticle. The intestine 



