3462 



CCELENTERA TES 



not branched and feathered is Hydractinia echinata, found in the North Sea and 

 on the English and Norwegian coasts, where it attaches itself, to gastropod shells, 

 inhabited by hermit crabs. The polyp probably profits by changes of place for 

 feeding, or else for some other reason adapts itself to the restless life of the crab. 

 The part of the stock common to all the individuals is the skin-like portion which 

 adheres to the surface of the shell or other object to which it is attached. This 

 skin is raised up into spiny prominences, as shown in the accompanying figure. A 

 horny layer occurs at this integument, similar to that of which the single tubes con- 

 sist. The nutritive canals running down the stems of the polyps are continued into 

 this membrane, promoting its life and growth. In such a stock there are never 

 more than two kinds of individuals; namely, the nutritive individuals, distin- 

 guished by their long tentacles, mouths, and digestive cavities; and the repro- 

 ductive individuals, male or 

 female. These latter have no 

 mouths, and are supplied with 

 food through the system of 

 canals running to them from the 

 nutritive individuals. These 

 reproductive individuals, in- 

 stead of tentacles, carry at 

 their tips a circle of stinging 

 batteries for the protection of 

 their eggs, which are inclosed in 

 capsules clustered together round 

 the stalk a little way below the 

 tips. The ciliated larvae emerg- 

 ing from the eggs swim away, 

 and eventually become attached 

 and found new colonies. The 

 egg capsules in no way recall 

 medusae, but all medusae which 

 pass through a polyp-like inter- 

 mediate stage also pass through 

 the simple capsule stage. 



Two more of the hydroid 

 stocks are worth mention, since 

 they secrete masses of carbonate 

 of lime out of which the animals 

 protrude like corals, which indeed 

 they were thought to be. This 

 error was made because only the 

 massive skeletons of the Hydro- 

 corallia as they are called 

 GROUP OF A FEMALE STOCK OF Hydractinia echinata. and not the living animals were 



a. Nutritive individuals; b. Female individuals. (Enlarged.) knOWU. Instead of the homy, 



