130 



INVERTEBRATE ANIMALS. 



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

 them pass through a metamorphosis. 



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

 Earth-worms, in which respiration is canied on principally through 

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

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

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

 Annelides. 



Order I. Hirudinea. 



The Hirudinea or Discopliora are well known under the name of 

 Leeches, some foinis lieing maiine, while others inhabit fresh water, 

 anil a few live in moist ]ilaces on laud. 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 liin- 

 der sucker is ]iresent, the head can be 

 Converted into a, suctorial cavity. Loco- 

 motion is effectcil either by means of the 

 alternate fixation and detachment of the 

 suckers, or by a serpentine bending of the 

 l)ody. 



The body is oliviouslj' ringed or annu- 

 lated, but none of the rings cai-ry 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 

 pouche.s, and xmited to the skin by means 

 of a sjiongy vascular tissue, so that the 

 body-cavity is obliterated. The jiseudo- 

 li;emal system is well develo])ed, and con- 

 sists essentially of four great longitudinal 

 vessels. The segmental organs are par- 

 tially tubular and p;irtially saccnlar, and 

 open on the lowi'r surface by minute 

 |)ori«, while they terminate lilindly inteiiiall}-. The nervous sys- 

 ti'iii lias its u.sual form, and the ganglia in front of the gullet 

 {'^ ljr<i:-i>-xiiph(t(jeal" ganglia) give otf branches to a number of simple 



Tlie 



^ig. S'2. — ninulini'a. 



Mi-(lH-iri;il Liicjrli (.S,. 

 f/.i njlirinalix). iKilnial 

 Anterior fxtrcNiity 

 Kaiiic liiai^liilii'il, slinw 

 siifkcr anil triiailiali^ Jaw.s ; 

 c One of tlia jawM .li'taeli.^il, 

 stinwiTri; lilt; .si.aiiiL'in'uIar 

 tuoLlital laaTgin. 



IIKtlK 



