18 MARINE ANIMALS OF MASSACHUSETTS BAY. 



nity, when it has attained any size, forms a little white mound on 

 the rock, higher in the centre, where the older members have 

 attained their whole height and solidity, and thinning out toward 

 the margin, where the younger ones may be just beginning life, 

 and hardly rise above the surface of the rock. These communi- 

 ties rarely grow to be more than two or three inches in diameter, 

 and about quarter of an inch in height at the centre where the 

 individuals have reached their maximum size. When the ani- 

 mals are fully expanded (Fig. 18), with all their tentacles spread, 



the surface of every such mound 

 becomes covered with downy 

 white fringes, and what seemed 

 before a hard, calcareous mass 

 upon the rock, changes to a soft 

 fleecy tuft, waving gently to and 

 fro in the water. The tentacles 

 are thickly covered with small wart-like appendages, which, on 

 examination, prove to be clusters of lasso-cells, the terminal 

 cluster of the tentacle being quite prominent. These lasso-cells 

 are very formidable weapons, judging both from their appearance 

 when magnified (Fig. 19), and from the terrible effect of their 

 bristling lash upon any small crustacean, or worm, that 

 may be so unfortunate as to come within its reach. 



The description of the internal arrangement of parts 

 in the Actinia applies in every particular to these corals, 

 with the exception of the hard deposit in the lower part 

 of the body. As in all the Polyps, radiating partitions 

 divide the main cavity of the body into distinct separate 

 chambers, and the tentacles increasing by multiples of 

 six, numbering six in the first set, six in the second, and 

 twelve in the third, are hollow, and open into the cham- 

 bers. But the feature which distinguishes them from 

 the soft Actiniae, and unites them with the corals, re- 

 quires a somewhat more accurate description. In each 

 individual, a hard deposit is formed (Fig. 20), beginning 

 at the base of every chamber, and rising from its floor to about 



Fig. 18. Single individual of Astrangia, fully expanded. 

 Fig. 19. Magnified lasso-cell of Astrangia. 



