ECHINODERMATA SEA-STARS. 95 



until within a few years they were confounded with the infuso'ria 

 rotato'ria, the structure of which is very different. Their body, 

 sometimes round, sometimes long and flat, is often covered with 

 little cilise, and contains ordinarily a considerable number of 

 cavities, which seem to discharge the functions of so many 

 stomachs. The above figure will give an idea of the most com- 

 mon species of these creatures. The movements of the poly- 

 gastrica, when seen under the microscope, are exceedingly viva- 

 cious ; and although many of them inhabit a space not larger 

 than the point of a needle, they swim about with great activity, 

 avoiding each other as they pass in their rapid dance, and 

 evidently directing their motions with wonderful precision and 

 accuracy. 



13. The ECHINODER'MATA or Echi'noderms (from the Greek, 

 echinus, a hedge-hog, and derma, skin) are formed for crawling 

 at the bottom of the sea, and are ordinarily provided with a mul- 

 titude of retractile appendages, by means of which they attach 

 themselves to bodies they touch ; in general the skin is covered 

 with spines, and their organization is more complicated than that 

 of most Zoophytes. They often 

 possess a kind of skeleton, vessels 

 for circulation, special organs for 

 respiration, and a separate intes- 

 tinal canal furnished with two 

 openings. 



14. The sea-stars Asteria 

 (Jig> 85) belong to this division. 

 Also, the sea hedge-hogs or sea 

 eggs, which have the appearance 

 of balls covered with spines ; in 



some ports of the Mediterranean Fig, 85. SEA-STAR. 



they are used for food. 



15. The ACALE'PHA or Acale'phans (from the Greek, acalephe, 

 a nettle), commonly called sea-nettles, on account of the irritation 

 contact with them produces on the skin, are of a gelatinous 

 consistence; they always float on the sea, and are essentially 

 organized for swimming. Their organization is very simple; 



Explanation of Fig. 84. Infu'soria polygas'trica as seen under a micro- 

 scope ; 1, monad ; 2, trachelius anas; 3, enchelis or flask animalcule ; 

 4, paramecium ; 5, kolpoda ; 6, trachelius fasciolarius as seen walking on 

 microscopic plants. 



13. What are the characters of echi'noderms ? 



14. What are sea-stars ? 



15. How are acale'phans characterized? What are the characters <&' 

 medusae ? 



