CHAPTER VIII 

 SEVENTH EACE OR PHYLUM OF THE ANIMAL KINGDOM 



ECHINODERMATA. 



The Echinodermata are, as a rule, essentially radiate in structure. 

 They, however, always deviate from strict radial symmetry in minor 

 l^oints, both in the skeletal system and in the arrangement of 

 the inner organs ; sometimes they may become almost bilaterally 

 symmetrical. The Echinodermata possess a skeleton of calcare- 

 ous matter deposited in the deeper connective tissue layers of 

 the integument. This skeleton is in texture a fine rigid sponge- 

 work. It consists either of microscopically small isolated calcareous 

 bodies (Holothurioidea) or of larger plates which often carry spines, and 

 are connected togethei' either movably or immovably (other Echino- 

 derms). The coelom is spacious. There is a blood vascular system. 

 The intestine, which is provided with a mouth and anus, is completely 

 separated from the ccelom. The Echinodermata possess a peculiar sys- 

 tem of canals or tubes — the water vascular system. This system, on 

 the one hand, takes in water from the exterior through a stone canal 

 (sometimes several such canals are present), which primitively opens 

 outwards, and, on the other hand, sends out terminal canals to ex- 

 ternal extensible appendages arranged in the radii or ambulacra. 

 These are the ambulacral feet or tentacles, which in free forms serve 

 principally for locomotion, but also for respiration ; in attached forms, 

 for respiration, and also perhaps for conducting food. The sexes are 

 almost always separate. Development is accompanied by metamor- 

 phosis. The larvse are free-swimming and pelagic ; they are bilater- 

 ally symmetrical, with ciliated bands, general^ produced on processes. 

 The Echinodermata are exclusively marine, and contain a great number 

 of fossil forms ; certain extinct types attained a great development 

 during the palaeozoic age. 



The race of the Echinodermata is divided into five classes — Holo- 

 thurioidea, Eehinoidea, Asteroidea, Ophiuroidea, and Pelmatozoa. 



