ANNELIDA 30I 



up into a dorsal and ventral part in the clitellar region. The so-called 

 testes (really vesiculae seminales) are tubes running through several 

 segments, filled with developing spermatozoa and their epithelial wall 

 is continuous with that of the perivisceral coelom, another primitive 

 feature. The vasa deferentia, moreover, open into the testes by typical 

 sperm funnels. 



It is interesting to find that in the Branchiobdellidae, a family of 

 the Oligochaeta, parasitic on crayfish, there is the same sort of leech- 

 like structure: a posterior sucker, annulated segments, absence of 

 chaetae and presence of jaws. But the condition of the coelom, 

 nephridia and generative organs is so like that of the Oligochaeta that 

 the family must remain in that group. 



Family Rhynchobdellidae. 



Pontohdella, parasitic on elasmobranch fishes. 



Glossiphonia (Fig. 206), a freshwater leech feeding on molluscs like 

 Limnaea and Planorbis and on the larvae of Chironomus ; body ovate 

 and flattened ; hind gut with four pairs of lateral coeca ; eggs laid in 

 the spring, the young when hatched attaching themselves to the 

 ventral surface of the body of the mother. 



Family Gnathobdellidae. 



Hirudo, the medicinal leech, at one time a common British species 

 but now extinct ; jaws armed with sharp teeth. 



Haemopis^ the horseleech, common in streams and ponds, which it 

 leaves to deposit its cocoons and in pursuit of prey ; jaws armed with 

 blunt teeth, which cannot pierce the human skin; a single pair of 

 coeca in the mid gut. 



This leech is carnivorous, devouring earthworms, aquatic larvae of. 

 insects, tadpoles and small fish. The land leeches of the tropics, of 

 which Haemadipsa may serve as an example, live in forests and swamps 

 and, mounted on leaves and branches, wait until a suitable mam- 

 malian prey presents itself. 



The following classes, the Echiuroidea and the Sipunculoidea, 

 were formerly classed together as the Gephyrea. There is, however, 

 good reason for separating them in spite of their general similarity, 

 which is possibly due to the fact that they are both composed of 

 burrowing animals and have lost their segmentation. 



Class ECHIUROIDEA 



Annelids which show few signs of segmentation, with a spacious 

 coelomic cavity, a well-developed prostomium, a terminal anus, a 

 single pair of ventral chaetae, sometimes several pairs of segmental 

 organs, and in Echiurus a trochosphere larva in the nervous system of 



