244 PHYLUM ANNELIDA. 



East, move very rapidly along the ground, fasten on to the legs ot 

 man or beast, and gorge themselves with blood. The hungry horse- 

 leeches are species of Hcemopis, 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 Aulostoma, which are carnivorous in habit. Other 

 common leeches are species of Nephelis^ predaceous forms with indis- 

 criminating appetites, and the little Clepsine^ 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 Lophobdella, on the lips and 

 jaws of the crocodile. 

 Classification. 



Family I. Rhynchobdellidoe, 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. 



e.g. Clepsine, Pontobdella^ Branchellion. 



Family 2. Gnathobdellidse, 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. 



e.g. Hirudo, Hczmopis^ ffamadipsa, Aulostoma, Nephelis. 

 Family 3. Acanthobdellidae. By itself is the Siberian fish parasite 

 Acanlhobdella, which has rows of setae on the first five segments, 

 a spacious ccelom, and other peculiarities. 



Appendix ( I ) to Annelid Series 



Class CH^ETOGNATHA. Arrow-worms 



There are two little pelagic "worms," Sagitta and S-badella, 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 Chsetognatha), a 

 median region with lateral fins, and a trowel-like tail. The nervous 

 system consists of a supra-oesophageal ganglion in the head, a sub- 



