432 CHARACTERS OF INVERTEBRATE ANIMALS 



among other places, in the Mediterranean, and also recorded 

 from the British coasts. It is more complex in structure than 

 Dinophilus. 



A word of explanation may be here necessary of the statement 

 that the simplicity of these creatures is possibly not primitive, but 

 the result of degeneration. Take, for example, the absence of 

 setse as a character to be criticised from the two points of view. 

 This negative character may mean that the ancestors of the group 

 did not possess setae, and, if so, the character is a primitive one. 

 To maintain, on the other hand, that the absence of setae is due 

 to degeneration is to suppose that the ancestral forms did possess 

 setae, which have been lost in the course of evolution. That such 

 a thing is possible becomes clear if we compare Nereis with some 

 of the sedentary Polychaetes and with earth-worms. In the first 

 case well-developed foot-stumps provided with setae are present; 

 in the second, these structures are reduced; while ordinary earth- 

 worms possess setae but no parapodia. A further reduction would 

 give the condition found in Polygordius (and some Oligochaetes). 



Class II.— LEECHES (Discophora) 



Leeches are typically fresh-water or terrestrial forms, though 

 some of them are marine. Most of them are blood-sucking para- 

 sites. The average characters of the group are conveniently 

 exemplified by the common Medicinal Leech {Hirudo medicinalis). 

 The elongated body, capable of considerable variation in form, 

 is flattened from above downwards, and is provided with a 

 sucker at each end, by alternate attachment of which looping 

 movements can be performed, much as in a looper caterpillar 

 (see p. 364). Both the suckers face downwards, and the mouth is 

 in the middle of the front one, while the intestine opens outside 

 and just above the hinder one. Encircling grooves divide the 

 body into a large number of narrow rings, several of which go to 

 a segment. Feelers, foot-stumps, and setae are entirely absent. 

 Ten simple eyes may be seen as black specks on the front end of 

 the body, placed close to the margin of the upper surface. The 

 well-known three-rayed bite of the leech is made by the finely- 

 toothed margins of three horny jaws with which the muscular 

 pharynx is provided. 



Although the Medicinal Leech does occur in this country, a 



