THE STUDY OP SHORE ANIMALS. 29 



originally simple individual. Very many sponges are in 

 this way colonial. 



Above Sponges, but still forms of very simple structure, 

 are the CCELENTERA, or hollow-bodied animals sea-anemones, 

 corals, sea-firs, "dead men's fingers," jelly-fish, and many 

 others, almost all beautiful in form and colour, and with a 

 delicacy and fragility which makes it essential that they 

 should be studied in the living condition. They agree with 

 Sponges and differ from higher animals in 

 containing one central cavity only, instead 

 of having an alimentary canal inclosed 

 within a general body-cavity. They have, 

 however, a true mouth surrounded by 

 tentacles, instead of the numerous pores 

 of the Sponges; the skin, especially on 

 the tentacles, contains offensive and de- 

 fensive stinging-cells which can be ejected; 

 there is often a skeleton of lime or some 

 other substance; their symmetry is radiate, 

 like that of a flower. Many of the Cos- FIG 9. 

 lentera are colonial, and it is such colonial Hi*^d"k' Ooeofttie 

 forms which build up the coral reefs of sea-ihs. 

 warm seas. 



Above the Coelentera we come to the UNSEGMENTED 

 WORMS, animals not nearly related to one another, but all 

 differing from the Coelentera in having distinct anterior and 

 posterior regions a distinction of head and tail, in having 

 a separate alimentary canal within the general body-cavity, 

 which may, however, be largely filled up, and in the greater 

 complexity of their structure. Among these we shall be 

 concerned only with certain little flat-worms called Turbel- 

 laria, and with the ribbon-worms (Nemertea). 



Much more highly differentiated, but sometimes loosely 

 included under the heading of " worms," we have the 

 SEGMENTED WORMS, or ANNELIDS. In them the body is 

 divided into successive rings, or segments, of similar struc- 

 ture, which usually bear locomotor organs furnished with 

 bristles. There is a well-developed body-cavity, which 

 opens to the exterior by little coiled tubes, or nephridia, 

 structures of much importance to the morphologist. The 

 Annelids which are especially adapted for a marine life 



