232 PHYLUM ANNELIDA. 



wiry land-leeches (Hicinadipsa, etc.), of Ceylon and other parts of the 

 East, move in rapid somersaults along the ground, fasten on to the legs 

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

 leeches are species of Hce/nopis, 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 Nepkelis, predaceous forms with indis- 

 criminating appetites, and the little Clepsine, also common in our ponds, 

 notable for its habit of carrying its young about on its belly. Several 

 marine forms prey upon fishes and other animals, e.g. the "skate-sucker" 

 (Pontobdella wnriiafa), 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 remarkable Branchellion 

 on the Torpedo has numerous leaf-like respiratory plates on the sides 

 of its body. Perhaps the strangest habitat is that of Lophobdella, 

 which lives on the lips and jaws of the crocodile. 

 Classification. 



Family i. Rhynchobdellidre, 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. Gnathobdellidre, 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, fftctnopis, Hic/nadipsa, Aulostoma, Nephelis. 



By itself is the Siberian fish parasite AcctJithobdella,. which has rows of 



setoe on the first five segments, a spacious coelom, and other peculiarities. 



Appendix (i) to Annelid Series. 

 Class CH^TOGNATHA. Arrow-worms. 



There are two little marine " worms," Sagitta and Spadella, 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 Chcetognatha), a 

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

 system consists of a supra-cesophageal ganglion in the head, a sul>- 

 cesophageal about the middle of the body, long commissures between 

 them, and numerous nerves from both; it retains its primitive con- 

 nection with the epidermis. There are two eyes and various patches 



