244 PHYLUM ANNELIDA. 
East, move very rapidly along the ground, fasten on to the legs of 
man or beast, and gorge themselves with blood. The hungry horse- 
leeches are species of Memopis, greedily suctorial, though the teeth, 
which occur in two rows, are too small and irregular to be useful in 
medicinal blood-letting ; but the name is also applied to species of the 
common genus Az/ostoma, which are carnivorous in habit. Other 
common leeches are species of Wephelzs, predaceous forms with indis- 
criminating appetites, and the little C/epsize, also common in our 
ponds, notable for carrying its young about on its ventral surface. 
Several marine forms prey upon fishes and other animals, e.g. the 
‘¢skate-sucker ” (Pontobdella muricata), with a leathery skin rough with 
knobs. This form lays velvety eggs in empty mollusc shells, and 
mounts guard over them for more than a hundred days. The remark- 
able Branchellion on the Torpedo has eleven pairs of leaf-like res- 
piratory plates on the sides of its body, and so has the related 
Ozobranchus jantseanus, a parasite of a river turtle in the Jantsekiang. 
One of the strangest habitat is that of Lophoddel/a, on the lips and 
jaws of the crocodile. 
Classification. — 
Family 1. Rhynchobdellide, in which the fore part of the 
pharynx can be protruded as a proboscis. There is an anterior 
as well as a posterior sucker. The blood plasma is colourless. 
The ova are large and rich in yolk; the embryos are hatched 
at an advanced stage, and soon leave the cocoon, which contains 
no albuminous fluid. 
2g. Clepsine, Pontobdella, Branchellion. 
Family 2. Gnathobdellidz, in which there is no proboscis, but 
the pharynx usually bears three tooth-plates. The mouth is 
suctorial. The blood plasma is red. The ova are small 
and without much yolk; the embryos are hatched at an 
early stage, and swim about in the nutritive albuminous fluid 
of the cocoon. ; 
2g. Hirudo, Hemopis, Hemadipsa, Aulostoma, Nephelis. 
Family 3. Acanthobdellide. By itself is the Siberian fish parasite 
Acanthobdella, which has rows of setz on the first five segments, 
a spacious ccelom, and other peculiarities, 
Appendix (1) to Annelid Sertes 
Class CHa@aToGNaTHA. Arrow-worms 
There are two little pelagic ‘‘ worms,” Sagz¢/a and Séadel/a, which 
are so different from all others, that they have been placed in a class 
by themselves, It is possible to regard them as Annelids with three 
segments, 
The translucent body, which may be nearly 3 in. long, but is 
usually much less, has three distinct regions, —a head bearing a ventral 
mouth with spines and bristles (whence the name Chetognatha), a 
median region with lateral fins, and « trowel-like tail. The nervous 
system consists of a supra-cesophageal ganglion in the head, a sub- 
