76 MARINE ANIMALS OF MASSACHUSETTS BAY. 



immense number of these animals, with which the .sea actually 

 swarms at times, when we know that as fast as they are 

 dropped, and it takes but a few days to complete their de 

 velopment, they each begin the same process ; so that in the 

 course of a week or ten days one such Medusa, supposing it to 

 have produced six buds only, will have given rise to forty-two 

 Jelly-fishes, thirty-six of which may be equally prolific in the 

 same short period. These Medusa3 budding thus, and swimming 

 about, carrying their young with them, bear such a close resem 

 blance to the floating communities of Hydroids formerly known as 

 Siphonophoras, that did we not know that some of them arise 

 from Tubularians, it would be natural to associate them with 

 the Siphonophora3. 



Nanomia. (Nanomia cam A. AG.) 



The Nanomia (Fig. 115), our free floating Hydroid, consists, 

 when first formed, of a single Hydra containing an oblong oil 

 bubble (Fig. 107). The whole organization of such a Hydra is 



limited to a simple digestive cav 

 ity ; it has, in fact, but one organ, 

 and one function, and consists of 

 an alimentary sac resembling the 

 probdscis of a Medusa (Fig. 107) ; 

 the oil bubble is separated from 

 it by a transverse partition, and 

 has no connection with the cavity. 

 Presently, between the oil bubble 

 and the cavity arise a number of 

 buds of various character (Fig. 108), which we will describe one 

 by one, beginning with those nearest the oil bubble, since these 

 upper members of the little swimming community bear a very im 

 portant part in its history. The infant community (Fig. 108) 

 passes rapidly into the stage represented in Fig. 109, and then 

 through all the stages intermediate between this and the adult, 

 shown in its natural size in Fig. 115. The upper buds en- 



Fig. 107. Young Nanomia ; magnified. 



Fig. 108. Young Nanomia with rudimentary Medusas. 



