244 CLASSIFICATION OF ANIMALS 



the pear-shaped Pleurobrachia which is also comparable to a 

 pear in size. The mouth is located at the small end and 

 opposite it there lies a single statocyst. Extending about two- 

 thirds of the distance from pole to pole and at about equal 

 distances from the two poles are eight bands of vibratile plates 

 which are regarded as rows of cilia fused together. These are 

 the locomotor organs. In place of nettling cells the two 

 long tentacles are covered with adhesive cells to which the 

 prey adheres. 



532. The Coelomata. — In none of the Ccelenterates are the fundamental 

 animal characteristics strongly developed. Sense organs and the organs 

 of locomotion are in no case highly developed and the symmetry of the 

 body is always primarily radial though in some cases a tendency toward 

 secondary bilateral symmetry may be observed. In the Coelomata there 

 is always a well-developed mesoderm. This makes possible a more highly 

 developed muscular system and consequent greater locomotor activity. 

 With this go also more highly differentiated sense organs and bilateral 

 symmetry. The mesoderm is derived from the entoderm and encloses a 

 paired series of cavities, the coelomic, or body, cavity. 



533. Phylum III. Scolecida. — Several classes of animals 

 more or less resembhng worms in the form of the body but with 

 no evidence of metameric segmentation are often called the 

 unsegmented worms. In this group the true body cavity is 

 Hmited to small spaces connected with the excretory and re- 

 productive organs. 



534. Class I. Platyhelminthes.- — The animals of this group 

 have a flattened body. The digestive tract is sack-like, with- 

 out vent, or wholly wanting. The space between the intestine 

 and body-wall is filled with a parenchyma of contractile fibres. 

 The nervous system consists of a paired supra-oesophageal 

 ganglion and a pair of ventral longitudinal nerves. Two other 

 pairs of longitudinal nerves are sometimes present. The 

 excretory system consists of a branched system of protoneph- 

 ridia, also called a water vascular system. The proximal 



