214 ZOOLOGY. 



there are several hundred species, are free echinoderms with a central 

 disc and usually five arms. The latter are large and contain liberal 

 coelomic spaces in which are lodged outgrowths of the digestive system 

 and other organs. There is a distinct oral and aboral surface. The anus 

 and madreporic body are on the latter. Distinct ambulacral grooves lie 

 on the oral surface of the arms. Adult star-fish may vary in size from 

 a few inches to two feet or more in diameter. 



Class V. Ophiuroidea (brittle-stars}. These are fragile, free echino- 

 derms in which the arms are small and much more distinct from the 

 disc than in the asteroids. The organs of the disc are not continued 

 into the arms. There is no anus, no ambulacral grooves, and the madre- 

 poric body is on the oral surface. Their slender arms are useful in cling- 

 ing to supports or to prey, and are used in locomotion. 



Class VI. Echinoidea (sea-urchins, sand-dollars'). These are free 

 echinoderms without free arms. The arms are connected by the develop- 

 ment of interradial plates. The calcareous rods are united into plates 

 which produce a complete external skeleton varying from flat dome-shape 

 (as in sand-dollars) to a globular form (Echinus or Arbacia). The 

 mouth is usually in the centre of the oral surface and the anus near the 

 centre of the aboral, yet one or both may come to have an excentric posi- 

 tion. In this way the bilateral symmetry is accentuated at the expense of 

 the underlying radial symmetry. The madreporic body is aboral and there 

 are no ambulacral grooves. The spines of the urchins are usually well 

 developed and may be used to scour out rounded pockets in rock in which 

 the animals are sometimes found. 



Class VII. Holothuroidea (sea-cucumbers'). These are soft, free 

 echinoderms, elongated, cylindrical or flat, with mouth and anus at 

 opposite poles of the horizontal long axis. The skeleton is not well- 

 developed, usually being represented merely by scattered spicules. The 

 water-vascular system in most forms communicates with the body cavity 

 instead of the exterior. Well-developed tentacles occur about the mouth. 

 Most holothurians burrow in the sand or mud, but others cling to rocks 

 near the surface of the water, and still others occur at great depths in 

 the ocean. 



251. Suggestive Studies for the Library or Laboratory. 



1. Read and report on the metamorphosis of the various 

 members of the group. 



2. Study from dry and moist material and report on the 

 structure and mode of action of " Aristotle's lantern " in 

 Echinus. 



3. Construct a table of parallel columns one for each of 

 the five living classes and contrast them as to: (i) general 

 form of body including symmetry, (2) manner of motion, 



