42 LIFE BY THE SEASHORE. 



members of the colony show the characteristic sea-nettle 

 shape in perhaps its simplest form. Each zooid, as the 

 figure shows, is like a tiny hollow column; it is fixed by 

 one end to the shell, while the other end, with its crown of 

 tentacles, floats freely in the water. Small as the tentacles 

 are, they still bear stinging-cells, which paralyse the prey 

 caught by the tentacles. 



With the help of a lens and patience we can carry our 

 observations considerably beyond this point, and make out 

 that though the little zooids are similar in their broad 

 outlines of structure, there are marked differences in detail. 

 In the majority of the individuals (see diagram) the body 

 is long and cylindrical, ending in a mouth surrounded by 

 twenty to thirty tentacles. These are the "nutritive persons" 

 of the colony, which catch and digest the little particles 

 which constitute the food of the entire colony. Their 

 central cavities are connected with a series of canals which 

 ramify over the surface of the shell on which the colony is 

 placed, and are again connected with the central cavities of 

 the other zooids. By this means the food is conveyed in a 

 digested condition all over the colony. The other zooids 

 are of two kinds. Near the margin of the colony, and 

 overhanging the mouth of the shell, there are peculiar 

 long spiral individuals (marked b in diagram), which are 

 extraordinarily muscular and active, but are without mouth 

 and tentacles. The function of these "sensitive persons" 

 seems to be to warn the other members of the colony of the 

 approach of danger.' Scattered among the nutritive persons 

 are the third set of zooids (marked c in diagram), which are 

 similar to these, but only about half as high, and have 

 rudimentary mouth and tentacles. The special peculiarity 

 of these zooids is that they bear lateral clusters of sporosacs, 

 which are oval bodies containing eggs or male elements. 

 In Hydradinia echinata, as the zoophyte is called, the 

 sporosacs remain permanently attached to the colony, but in 

 very many of the zoophytes minute swimming-bells, or 

 medusoids, are produced instead of sporosacs, and these 

 swimming-bells float away, and carry the eggs to some more 

 or less distant spot. Finally, in Hydradinia there are 

 mingled with the persons a number of spines, which may 

 be aborted persons, and which have some protective function. 



