RADIATA. 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 Diphyes, may be allied to the Mollusca, and in the year 1836 he 

 ]jroposed for them the name MaUictinozoaria^ under the impression that 

 they constitute an intermediate division. 



Okdee 6. Systoligkada, The name Diphyida is derived from that of the 

 genus Diphyes, in allusion t<j 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 Cyclopaedia for an extended account, illustrated 

 with figures of this and the preceding orders. 



Saes, a distinguished naturalist of Norsvay, discovered in 1836 that some 

 of the Acalephse resemble the Zoophyta, in having a gemmiparous 

 reproduction. He observed certain projections from the base of the pedicle 

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

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

 adult in all essential particulars, and, like the 

 Hydrae, they have an independent action 

 previous to their separation from the parent. 

 This is represented at a, in the annexed figure 



of Lizia octopunctata of Sars (an animal about . / / \;>x^>>, \ i.__ a 

 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 which has no parallel in 

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

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

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

 upon 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, was 

 published by the Ray Society in 1845, 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 Salpa* 

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

 Loven in 1837; and Yan Beneden in 1844-T. (See the Cyclop, of Anat. 

 and Phys., Art. Polypifera.) 



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

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



235 



