22 



MARINE ANIMALS OF MASSACHUSETTS BAY. 



Fig. 24. 



munities, or remain in that condition through life. These Hy- 

 droid stocks, as they are sometimes called, give rise to buds ; 

 these buds are transformed into Jelly-fishes, which in some in- 

 stances break off when mature and swim away as free animals, 

 while in others they remain permanent members of the Hydroid 

 stock, never assuming a free mode of life. All these buds when 

 mature, whether free or fixed, lay eggs in their turn, from which 

 a fresh stock arises to renew this singular cycle of growth, known 

 among naturalists as " alternate generations." 



The Hydroids are not all attached to the ground, some 

 like the Physalia (Portuguese man-of-war), or the Nanomia, that 

 pretty floating Hydroid of our own waters, move, about with as 

 much freedom as if they enjoyed an individual independent ex- 

 istence. As all these orders have their representatives on our 

 coast, to be described hereafter in detail, we need only allude 

 here to their characteristic features. But we must not leave un- 

 noticed one very remarkable Hydroid Acaleph (Fig. 24), not 

 found in our waters, and resembling the 

 Polyps so much, that it has long been asso- 

 ciated with them. The Millepore is a coral, 

 and was therefore the more easily confounded 

 with the Polyps, so large a proportion of 

 which build coral stocks ; but a more mi- 

 nute investigation of its structure (Figs. 25, 

 26) has recently shown that it belongs with 

 the Acalephs.* This discovery is the more 

 important, not only as explaining the true po- 

 sition of this animal in the Animal Kingdom, 

 but as proving also the presence of Acalephs 

 in the earliest periods of creation, since it re- 

 fers a large number of fossil corals, whose 

 affinities with the millepores are well under- 

 stood, to that class, instead of to the class of 

 Polyps with which they had hitherto been associated. But for 

 this we should have no positive evidence of the existence of 



Fig. 24. Branch of Millepora alcicornis; natural size. (Agassiz.) 



Fig. 25. Animals of M. alcicornis expanded; magnified, aaa small Hydroid, b larger Hydroid, 

 I tentacles, m mouth. (Agassiz.) 



* See " Methods of Study," by Prof. Agassiz. 



Fig. 25. 



