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ON THE MORPHOLOGY OF THE MEDUSA. 



SINCE it has been ascertained that the Medusas pass through differ- 

 ent forms in their various metamorphoses, and that their changes are 

 further complicated by an alternation of generations, it is a matter 

 of importance to learn how far the Polypoid stems from which free 

 Medusae arise are themselves allied to Medusae, or to Polypi. As 

 long as the so-called Hydroid Polypi were supposed to be perfect 

 animals, propagating themselves under the same forms, it was natural, 

 from their general appearance, that they should be considered as a 

 peculiar type among Polypi, and that the question of their structural 

 relations to Medusae should not even be raised. But now it is a 

 matter of the first importance to ascertain whether in this Polypoid 

 form their structure is more closely related to Medusae than to 

 Polypi, and whether their relation to Polypi is merely analogical, 

 and not truly structural. I have investigated this subject at some 

 length, and satisfied myself that even in an anatomical point of view 

 the so-called Hydroid Polypi should be referred to the class of Me- 

 dusae, and that their resemblance to Polypi is simply the result of a 

 close analogy, and not of true affinity, as the Morphology of their 

 parts indicates a very close affinity to Medusae. 



In order to appreciate fully this statement, it is necessary to 

 remember that true Polypi have a distinct digestive sac hanging into 

 the large main cavity of the body, and that this cavity is sub-divided 

 into more or less distinct compartments by partitions projecting 

 inwardly. Again, the tentacles arise from the upper margin of the 

 main cavity, and are in no instance appendages of the margin of the 

 mouth. In Medusae, on the contrary, the main cavity is undivided, 

 and where there is a peripheric prolongation it assumes the shape 

 of circumscribed tubes, penetrating into the substance of the body. 

 Again, the tentacles are, at least, of two kinds those, which arise 

 from the margin of the body, and others which are prolongations of 

 the margin of the mouth. 



If, with these facts before our minds, we now attempt a comparison 

 of the so-called Hydroid Polypi with either Medusae or true Polypi, 

 we shall find, in the first place, that the main cavity in Hydroid 

 Polypi is not divided by radiating partitions into distinct compart- 

 ments, as is the case in true Polypi, and that their buds at least, if 

 not the main cavity of the system, have radiating tubes arising from 

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