RxVDIATA. 



31 



expelled when the animal wishes to sink. It is probable that this may be 

 accomplished to a certain extent by the muscular power of the air-vessel. 

 Blainville thinks that this order (which wants the radiated character), with 

 Beroe and Dijihyes, may be allied to the Mollusca, and in the year ISoG lie 

 proposed for them the name Malactinosoaria^ under the impression that 

 they constitute an intermediate division. 



Order 6. Systoligrada. The name Diphyida is derived from that of the 

 genus Diphyes, in allusion to its double nature, each animal being composed 

 of two somewhat conical pieces, the point of one being inserted a short 

 distance into the larger end of the other, and retained by a very slight 

 attachment. See the Penny Cyelopaidia for an extended account, illustrated 

 with figures of this and the preceding orders. 



ISars, a distinguished naturalist of Norway, discovered in 1836 that some 

 of the Acalephfe resemble the Zoophyta, in having a gemmiparous 

 reproduction, lie observed certain projections from the base of the pedicle 

 (or exterior of the stomach), which proved to be budding young, attached 

 by the npper or outside portion of their disk. These young resemble the 

 adult in all essential particulars, and, like the 

 Hydra?, they have an independent action 

 previous to their sej^aration from the parent. 

 This is represented at «, in the annexed figure 

 of Lizla odopwictata of Sars (an animal about 

 one fourth of an inch long), as given by Forbes. 

 The species is named from the eight black 

 ocelli, four of which are large, and towards 

 these the gastric vessels are directed. In 

 Sarsia prolifera^ Forbes, the gemmation 

 takes place at the base of the exterior 

 tentacles. 



We come now to describe a mode of generation whicli has no parallel in 

 the higher animal forms, and to which the Medusre and some other animals 

 are subject. This mode is termed Alternation of generations by the 

 Danish naturalist, Steenstrup, who has the credit of generalizing the fiicts 

 npon which the theory is founded, and of which he is in part the discoverer. 

 An English translation of his work on the subject, by George Busk, M'as 

 published by the Bay Society in 181:5, entitled, '' On the Alternation of 

 Generations; or the Propagation and Development of Animals through 

 Alternate Generations : a peculiar form of fostering the young in the lower 

 classes of animals." Besides this author, the chief observers in this curious 

 branch of science are Chamisso, who published observations on the Salpaj 

 in 1819 ; Sars, on the Medusae, between 1828 and 1841 ; Siebold and 

 Loven in 1837; and Yan Beneden in 1814-7. (See the Cyclop, of Anat. 

 and Phys., Art. Folypifera.) 



This phenomenon is described by Steenstrup as that of " an animal 

 producing an offspring, which at no time resembles its parent, but which, 



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