130 



INVERTEBKATE AKIXIALS. 



individual. The embryos are almost always ciliated, and many of 

 them pass through a metamorphosis. 



The so-called " Abranchiate " Annelides are the Leeches and 

 Earth-womis, in which respiration is carried on principally through 

 the soft moist integument. On the other hand the marine Tube- 

 worms and Sand-worms possess, as a rule, external processes adapted 

 for respiration in water, and these are known as the " Branchiate " 

 Annelides. 



Order I. Htrudinea. 



The Hirudinea or Discophora are well known under the name of 

 Leeches, some forms being marine, while others inhabit fresh water, 

 and a few live in moist places on land. The Leeches are characterised 

 by the fact that the body is destitute of 

 lateral bristles or foot-tubercles, but is 

 provided with a sucking-disc at one or 

 both extremities. In the typical forms, as 

 in the common Medicinal Leech (fig. 82), 

 there are sucking-discs at both ends of the 

 body, and in those in which only the hin- 

 der sucker is present, the head can be 

 converted into a suctorial cavity. Loco- 

 motion is effected either by means of the 

 alternate iixation and detachment of the 

 suckers, or by a serpentine bending of the 

 body. 



The body is obviously ringed or annu- 

 lated, but none of the rings carry lateral 

 appendages of any kind. The mouth is 

 sometimes destitute of teeth, but is some- 

 times armed with complex jaws. The 

 alimentary canal is short, with lateral 

 ]iouches, and united to the skin by means 

 of a spongy vascular tissue, so that the 

 body-cavity is obliterated. The pseudo- 

 h;emal system is well developed, and con- 

 sists essentially of four great longitudinal 

 vessels. The segmental organs are par- 

 tially tubular and partially saccular, and 

 open on the lower surface by minute 

 pores, while they terminate blindly internally. The nervous sys- 

 tem has its usual form, and the ganglia in front of the gullet 

 {"■ prm-cesophageal " ganglia) give off branches to a number of simple 



Fig. 82,— Hirudinea. a Tlie 

 Medicinal Leet'li {Sani/uisu- 

 ga officLiLah^), natural size; h 

 Anterior extremity of the 

 same magnified, showing the 

 sucker and triradiate jaws ; 

 c One of the jaws detached, 

 showing the semicircular 

 toothed margin. 



