MODE OF GROWTH OF THE RADIATES. 169 



Coming now to the Portuguese man-of-war (Physalia) which have 

 so much occupied the attention of the best naturalists, it would seem 

 at first well nigh impossible to trace their relationship with the or- 

 dinary Hydroids. A Physalia may, however, be compared to a fixed 

 colony of Hydractinia or Coryne, for example. Each Physalia is 

 either male or female, and consists of four kinds of zooids ; viz., 

 nutritive and reproductive, with medusa buds, which, by their 

 contractions and dilatations propel the colony onward ; and the 

 "feeders," a set of digestive tubes which nourish the entire colony. 



The Siphonophores (as observed by Gegenbaur, Kowalevsky, 

 Hseckel and MetschnikofT, in Agalma and several other genera) 

 arise from eggs which pass through a morula stage, into a ciliated 

 planula, whose body consists of an ectoderm and endoderm. The 

 gastro-vascular cavity in the Siphonophores, as in the lower Hy- 

 droids so far as observed, is formed by a fold of the endoderm, 

 while, as we shall see farther on, in the Discophorous jelly fishes it 

 is formed by an infolding of the ectoderm. 



The further development of Nanomia, a Siphonophore native to 

 our northern shores, from the larval state, has been described and 

 figured by Mr. A. Agassiz. To use nearly his own words, the 

 Nanomia consists, when first formed, of an oblong oil bubble, 

 with but one organ, a simple digestive cavity. Soon between the 

 oil bubble and the cavity arise a number of m< 1 s 1 Is tl o gh 

 without any "proboscis" (manubrium), as these medusa buds, 

 vi mining bells," are destined to " serve the purpose of 

 locomotion only, having no share in the function of feeding the 

 community." After these swimming buds, three kinds of Hydra- 

 like zooids arise. In one set the Hydra is open-mouthed, and is 

 in fact a digestive tube, its gastro-vascular cavity connects with 

 that of the stem, and thus the food taken in is circulated through- 

 out the community. These are the so-called "feeders." The 

 second set of Hydras differ only from the "feeders" in having 

 shorter tentacles twisted like a corkscrew. In the third and last 

 set of Hydras the mouth is closed, and they differ from the others 

 in having a single tentacle instead of a cluster. Their function 

 has not yet been clearly explained. Gradually new individuals are 

 added, until a long chain of Ilydmids is formed, which move 

 gracefully through the water, with the oil globule upj < nim.t . 

 which serves as a float and is identical with the larg»-ere>ted 

 "float" of the Physalia. 



