132 INVERTEBRATE ANIMALS, 
be converted into a suctorial cavity. Locomotion is effected 
either by means of the alternate fixation and detachment of the 
suckers, or by a serpentine bending of the body. 
The body is obviously ringed or annulated, but none of the 
rings carry lateral appendages of any kind. The mouth is 
sometimes destitute of teeth, but is sometimes armed with 
complex jaws. The alimentary canal 
is short, with lateral dilations, and 
united to the skin by means of a spongy 
vascular tissue, so that the body-cavity 
is obliterated. The pseudo - hemal 
system is well developed, and consists 
essentially of four great longitudinal 
vessels. Respiration appears to be 
effected, in part at any rate, by means 
of the segmental organs, which have 
the form of little sacs which open 
externally by minute apertures. The 
nervous system has its usual form, and 
the ganglia in front of the gullet (“ pr@- 
esophageal” ganglia) give off branches 
to a number of simple eyes which are 
placed on the head. The sexes are 
united in the same individual. 
Fig. 59. — Hirudinea. @The The most familiar of the leeches are 
medicinal leech (Sanguisuga 
officinalis), natural size; 4 the common horse-leech (Hemopsis), 
Anterior extremity of the and the medicinal leech (Sanguisuga 
same magnified, showing the Digit 
sucker and triradiate jaws; officinalis, hig. 59). The former has no 
c One of the jaws detached, ‘teeth, but the latter is provided with 
eMC ep a eae three semicircular toothed jaws (fig. 
59, 4, ¢), which meet in a point, and 
are sufficiently powerful to cut through the human skin. The 
medicinal leech is a native of fresh waters throughout the 
south and east of Europe, and it is imported in large numbers 
from Hungary, Bohemia, and Russia. 
ORDER II. OLiGocHatTa.—In this order are included the 
earth-worms (Lumbricide), and the water-worms (Naidide). 
They are all distinguished from the preceding by the fact that 
the body is furnished with rows of bristles which take the place 
of the foot-tubercles of the higher Annelida, and which are 
