248 CLASSIFICATION OF ANIMALS 



546. Class HI. Hirudinea. The leeches differ considerably 

 from the Chaetopoda. The external segmentation does not 

 correspond to the metamerism. There are usually three or 

 five external rings to one segment. There are 34 metameres. 

 The body cavity is almost obliterated by parenchyma tous tissue. 

 They are all external parasites and are provided with suckers for 

 holding to the host and for locomotion. There are two suckers, 

 a small one at the anterior end enclosing the mouth and a large 

 one on the ventral side at the posterior end. There are neither 

 parapodia nor bristles. Series of lateral pouches render the 

 digestive tract very capacious. The leeches are hermaphrody te. 

 They are found only in the water, or in moist places. 



547. PHYLUN V. Molluscoidea. Under this head are 

 grouped several classes which are sometimes placed under 

 the heading worms. They are aquatic, usually fixed and 

 provided with a cuticular covering in the form of a tube or a 

 two-valved shell. They are unsegmented. The mouth is 

 surrounded by a circle of tentacles covered with cilia. The 

 intestine is usually U-shaped so that the vent lies near the 

 mouth. 



548. Class n. Bryozoa. The Bryozoa, or Polyzoa, are all 

 minute aquatic animals. Most are marine but there are also 

 a few fresh- water forms. They are usually colonial and the 

 colony may spread over considerable areas though the individ- 

 uals are barely visible to the unaided eye. There is a gelatinous, 

 horny or calcareous test into which the animal may completely 

 withdraw. The tests of a colony often form an incrustation over 

 the surface of other objects and again they form branching 

 plant-like structures comparable to moss plants in size and 

 general appearance, hence the name. The test is secreted by 

 the epidermis and forms part of the animal. A pair of strong 

 retractor muscles cause the rapid withdrawal of the body 

 completely within the test on the slightest irritation. The 

 mouth is surrounded by a crown of ciliated tentacles, sometimes 



