12 MARINE ANIMALS OF MASSACHUSETTS BAY. 



summit to base, and we have two Actinias where there was orim- 



o 



iially but one. Another and a far more common mode of re- 

 production among these animals is that of budding like corals. 

 A slight swelling arises on the side of the body or at its base ; 

 it enlarges gradually, a digestive cavity is formed within it, tenta- 

 cles arise around its summit, and it finally drops oif from the 

 parent and leads an independent existence. As a number of 

 these buds are frequently formed at once, such an Actinia, sur- 

 rounded by its little family, still attached to the parent, may ap- 

 pear for a time like a compound stock, though their normal mode 

 of existence is individual and distinct. 



The Actinia is exceedingly sensitive, contracting the body and 

 drawing in the tentacles almost instantaneously at the slightest 

 touch. These sudden movements are produced by two powerful 

 sets of muscles, running at right angles with each other through 

 the thickness of the body wall ; the one straight and vertical, ex- 

 tending from the base of the wall to its summit ; the other cir- 

 cular and horizontal, stretching concentrically around it. By 

 the contraction of the former, the body is of course shortened ; by 

 the contraction of the latter, the body is, on the contrary, length- 

 ened in proportion to the compression of its circumference. Both 

 sets can easily be traced by the vertical and horizontal lines cross- 

 ing each other on the external wall of the body, as in Fig. 5. 

 Each tentacle is in like manner furnished with a double set of 

 muscles, having an action similar to that described above. In 

 consequence of these violent muscular contractions, the water im- 

 bibed by the animal, and by which all its parts are distended to 

 the utmost, is forced, not only out of the mouth, but also through 

 small openings in the body wall scarcely perceptible under ordi- 

 nary circumstances, but at such times emitting little fountains in 

 every direction. 



Notwithstanding its extraordinary sensitiveness, the organs of 

 the senses in the Actinia are very inferior, consisting only of a 

 few pigment cells accumulated at the base of the tentacles. The 

 two sets of muscles meet at the base of the body, forming a disk, 

 or kind of foot, by which the animal can fix itself so firmly to 

 the ground, that it is very difficult to remove it without in- 

 jury. It is nevertheless capable of a very limited degree of 



