216 HYDROIDiE. Part IV. 



tube opens into the vertical tube on the side of the bulb {Fig. 17, h^), towards 

 its lower margin, and so far behind its edge as scarcely to appear connected with 

 it, when seen in front. 



The free Medusae of this species are very sensitive to the density of the 

 medium in which they live, and the mere change arising from the difference in 

 density between freshwater and saltrwater is sufficient to kill them almost instan- 

 taneously. Taking up in a spoonful of sea-water a fresh Sarsia, in full activity, 

 when swimming most energetically, and emptying it into a tumbler-full of fresh- 

 water of the same temperature, the little animal will at once drop like a ball 

 to the bottom of the glass, and remain forever motionless, killed instantaneously 

 by the mere difference of density of the two media. This experiment, which I 

 have often repeated, has led me to notice that the total disappeai'ance of our 

 small Medusae uniformly coincides with heavy rainfalls, while the larger species 

 survive. These little Medusa? occur in large numbers along our wharves, during 

 the spring and summer, and as they swarm near the surface of the sea, they 

 are particularly exposed to the action of rain-water. They move rapidly in all 

 directions with the greatest freedom and energy. They are exceedingly voracious, 

 and feed wpow any kind of marine animals, not sparing their own species. 



I have observed an interesting anomaly in this species, in the number of its 

 parts. Though 1 have examined many thousand specimens of our Sarsia, I have 

 always found it to present the most uniform arrangement of its parts, the speci- 

 mens having, in every instance, shown four tentacles, four eye-specks, four radiating 

 chymiferous tubes, and four main bundles of muscles. But, in one instance, two 

 specimens were noticed, among many others, in which the parts were arranged in 

 sixes ; there were six tentacles, six eye-specks, six radiating chymiferous tubes, and 

 six bundles of muscles. The specimens were somewhat larger than the common 

 four-rayed specimens, the disk measuring about half an inch ; and I for a moment 

 suspected this to be a distinct species; but, upon close examination, I found that 

 every part was so perfectly identical with the corresponding parts of the four- 

 rayed individuals, that 1 failed to discover the slightest specific distinction. I would, 

 therefore, view this case as a mere accidental modification of the number of parts, 

 of no more importance than the accidental development of an additional spur on 

 the foot of a cock, or an additional finger to the hand or paw of an animal. 

 It was, perhaps, more striking here, as it ran through all the systems and influ- 

 enced the general appearance of the whole body, but the six eye-specks were all 

 identical in the details of their structure, and identical with those of the four- 

 rayed ones. The connection between the circular tube and the radiating ones 

 was the same, and the muscular bundles jjresented the same arrangement in relation 

 to the lower margin, and intervening radiating tubes, as in common specimens. 



