22 



OVUM. 



and may remain for a long time in that state ; 

 continuing to multiply by budding into others 

 of the same kind, and occasionally giving rise 

 by the process of fission to its Medusa progeny. 

 The observations of J. Reid * have shown 

 that the Polype or polypoid stock may remain 

 for a very long time in this condition without 

 forming any Medusa progeny; and these obser- 

 vations, as well as those of Steenstrup and of 

 Desor, appear to show that these Polypes bear 

 a considerable resemblance in their internal 

 structure to the Medusae which they produce 

 by gemmation. The latter author, indeed, is 

 inclined to believe that the new Medusa ani- 

 mals are produced not by a mere transverse 

 fission of the Polype, but by successive gem- 

 mation on its summit, that is, round its mouth 

 and within the tentacula; and he states that 

 he has observed the Polype remaining with 

 its tentacles at the base of the Strobila of 

 Medusa?. The observations of Dalzell and 



Fig. 18. 



Medusa larva. (From J. Held.) 



A, Polype before it has undergone any gemma- 

 tion of Medusa;, showing the mouth and four canal 

 openings. 



B, the strobila or larva forming Medusae. 



c, lower surface of one of the young Medusa;, 

 after separation. 



J. Reid appear, however, inconsistent with 

 this view ; but it is possible that there may be 

 varieties in respect to the mode of formation 

 of the Medusa progeny, so that in one set the 

 tentacles of the Polype may be included in the 

 upper Medusa, and when all the progeny is 

 separated, new tentacles may be formed on the 

 Polype stock at the base, while in others 

 the budding Medusa may be within the circle 

 of the tentacula of the Polype. 



It appears from recent investigations that 



* Ann. and Mag. of Nat. Hist. 1848. 



others of the Acalephaj also undergo remark- 

 able processes of non-sexual multiplication. 

 According to Huxley's recent most interesting 

 researches *, the Physsophoridse, Diphydae, 

 and Physalia, are to be regarded as compound 

 organisms in which the floating processes 

 of most various form are analogous to Polype 

 or attached Medusa individuals, which are the 

 bearers of sexual organs, in some of one kind, 

 in others of both, and others of which are 

 neuter, on the same compound stock. ] These 

 are probably a progeny developed by budding 

 from a single individual, which is the parent 

 stem. 



By these discoveries a remarkable relation 

 is shown to exist between the medusoid and 

 polypoid animals. Some we have been ac- 

 customed to see principally in their largest 

 and most developed condition as Medusae, 

 others are best known in that polypoid condi- 

 tion in which they remain for the longest 

 time; but we must regard that condition in 

 which sexual reproduction takes place as the 

 complete one, and this we have seen is in 

 both the Acaleph or Medusa form, while the 

 Polvpe or polypoid state, however permanent 

 it may appear, is to be looked upon as a pre- 

 paratory stage, in which, it is true, multiplica- 

 tion of its own kind may occur by gemmation, 

 but which can only effect the true reproduc- 

 tion of the species by forming its progene of 

 Medusans to which is committed the offic of 

 producing the fecundated ova. This, there- 

 fore, is another example of multiple metage- 

 nesis, or alternating generation. J 



Mollusca. Among the Mollusca the only 

 examples of alternate generation that are yet 

 known have been observed in the Tunicated 

 Acephala : and among these, three modifica- 

 tions of the reproductive process are known 

 in the Bryozoa, Ascidia, and Salpidas. 



The Bryozoa, or so-called Ciliobrachiate 

 Pohpes, long ranked with the Polypes on 

 account of their union in branched groups, 

 their radiated arms, and retractile body, but 

 now regarded as more nearly allied by their 

 internal organisation to the Tunicated Mol- 

 lusca, present a very marked example of the 

 multiplication by budding of the progeny of a 

 single ovum. These animals never continue 

 for any considerable time as single or distinct 

 individuals, but, multiplying by gemmation, 

 form numerous colonies, in which the new 

 individuals remain connected with the pri- 

 mitive one from which they have proceeded 

 and with each other. They thus always con- 

 stitute compound groups spreading from the 

 first individual as from a centre. All the in- 

 dividuals of the group may acquire sexual 

 completeness, and the male and female organs 

 are united in each individual : the ova are 

 fecundated within the cavity of the mantle; 



* Phil. Trans. 1840, Pt. ii. 



f Professor Goodsir has informed me that his ob- 

 servations on Stqihanoinaia are quite confirmatory 

 of this view. 



J See also on this subject the interesting treatise 

 hy Prof. E. Forbes, on the Naked-eyed Medusae, in 

 Itay Soc. Pub. 1848. 



