250 CLASSIFICATION OF ANIMALS 



borne on a horseshoe-shaped disc, the lophophore. The vent 

 lies just outside the tentacles. There are no special sense organs. 

 The nervous system consists of a single gangHon and a few 

 nerves leadinp^ from it. The nephridia are very rudimentary or 

 entirely wanting. The colonies are often polymorphic; certain 

 individuals being specialized receptacles for eggs while others 

 form slender, jointed, vibrating whips or a pair of jaws which 

 open and close like the beak of a bird. 



549. Class m. Brachiopoda. — The Brachiopods are not 

 generally well known, although they are not at all rare and are 

 widely distributed. They live only in salt water and may be 

 found not far from shore. They have a shell somewhat like 

 that of a clam but the plane of symmetry cuts the hinge at 

 right angles and one of the valves is dorsal and the other ventral. 

 They are usually unlike. The shell is composed either of chitin 

 or calcareous matter. The animal is attached by a stalk-like 

 extension of the posterior end of the body or by the cementing 

 of the ventral valve to the substratum. On either side of the 

 mouth the body is prolonged into a pair of coiled arms which 

 bear numerous ciliated tentacles. There is a true body cavity 

 in which lie the simple U-shaped digestive tract with a digestive 

 gland, the heart, the gonads, one or two pairs of nephridia and 

 a dorsal and a ventral ganglion. 



550. Phylum VI. Echinodermata. — This is a well-defined 

 group of animals. They are all marine, without exception. 

 The larva are bilateral but by a metamorphosis the adult 

 becomes radially symmetrical and the number of rays is 

 usually five. The name, Echinoderm, means spiny skin and 

 refers to the calcareous bars and plates embedded in the integu- 

 ment. These are not equally well developed in the various 

 classes but they serve in varying degree as an exoskeleton and 

 in most forms some pieces project beyond the general surface 

 in the form of spines. The ambulacral system is the most 

 distinctive anatomical character of the group. It consists of a 



