EARTH-WORM, NAIS. ORDER SUCTORIA. 311 



ova, when they have been fertilised, quit the ovarium, not by 

 oviducts or passages leading outwards, but by finding their way 

 into the loose tissue of the body, beneath the muscular layer ; 

 through this they are propelled by a series of strong undula- 

 tions, until they reach a sort of receptacle, in which they undergo 

 the first part of their development, and within which, in some 

 species, the eggs are hatched, so that the young come forth alive. 

 The eggs are provided with a curious valve-like structure at the 

 end through which the young worm emerges ; and it is remark- 

 able that, in a great number of instances, two embryos may be 

 observed in a single egg. 



846. The genus Nais is nearly allied to the Lumbricus ; but 

 the worms which it includes are aquatic in their habits, living 

 in holes which they perforate in mud at the bottom of water. 

 Some of them have small black points upon their heads, which 

 have been regarded as eyes. 



ORDER IV. SUCTORIA. 



847. This Order contains the common Leech and its allies, 

 which are all animals of aquatic habits, but not all agreeing in 

 its blood-seeking propensities. The group derives its name, as 

 already stated, from the suckers which terminate the two ex- 

 tremities of the body, and which constitute its chief means of 

 locomotion ; for having fixed its anterior extremity, the animal 

 draws the posterior one close up to it, by bending its body, and 

 then, fixing the latter, re-advances the first, by straightening 

 and extending the body. In this manner the Leech can advance 

 much more rapidly over solid surfaces, than the Earth- Worm 

 can by its crawling movement ; and it can also swim with 

 facility. 



848. The Leech, in its general structure, bears a considerable 

 resemblance to the Earth- Worm ; but differs as to the conforma- 

 tion of its mouth and digestive apparatus, which are in accord- 

 ance with its suctorial habits. Its mouth is situated in the 

 middle of the cavity of the anterior sucker ; and three little 



