OF THE ACALEPHiE OF NORTH AMERICA. 249 



larger than the common four-rayed specimens, the disk measuring about half an inch ; and 

 I for a moment suspected this to he a distinct species ; but, upon close examination, I 

 found that every part was so perfectly identical with the corresponding parts of the fonr- 

 rayed individuals, that I not only failed to discover the slightest specific distinction, but 

 should even feel unwilling to recognize this as a variety. I should rather view it as a 

 mere accidental modification of the number of parts, of no more importance in the range 

 of specimens 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 ; — perhaps more striking here, as 

 it ran through all the systems and influenced the general appearance of the whole body. 

 But wherever we have an additional number of parts, we see everywhere that those 

 tissues and systems of organs which belong to such parts are naturally developed in it ; 

 and so it was in this six-rayed Sarsia. 



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 presented the same arrangement 

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

 But if these six-rayed Sarsise had no importance whatever with reference to specific dis- 

 tinction, they were none the less of great interest with reference to the value of the 

 number of parts in different genera of the same family of naked-eyed Medusae. For, if 

 a change in number, such as was noticed in this species, can occur without a modifica- 

 tion of the specific character, we shall be prepared by this example to consider those 

 genera in which the number of rays, or tubes, or ovaries, or eye-specks differs, as more 

 closely allied than would otherwise appear. A variation in the number of parts in this 

 family, when the parts are otherwise identical in structure and adaptation, will no longer 

 be considered as a natural foundation for distinguishing families. They may indicate 

 distinct genera, if the difl'erences in number are combined with some modification in 

 adaptation. But, however constant the differences in the number of parts may be, if 

 they are not combined with some special adaptation in one or the other of the systems, 

 I should not consider them even as warranting generic distinction, as we see in the 

 case before us that such differences do not even warrant specific distinction. 



This case of Medusae with different numbers of rays is precisely parallel to the case 

 of star-fishes with a variable number of rays, such as have been described by the older 

 Linck, who, unfortunately for himself and the progress of science, considered each varia- 

 tion in this respect as indicating generic distinctions ; when he might easily have as- 

 certained that several species vary greatly in this respect. 



