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A PLAINLY WORDED BIOLOGICAL QUARTERLY 

 Vol. II. No. 5. MARCH, 1895. 



THE LIFE-HISTOEY OF THE EOCK BAENACLE, 



(BALANITS). 



BY THEO. T. GROOM, F.Z.S. 

 (Part II). 



THE question of the meaning of the various larval forms known, has, 

 for many years, engaged the attention of naturalists, and, thanks 

 to the writings of Darwin, Haeckel, Fritz Mtiller, Hatschek, Lang, 

 Lankester, Balfour, Sedgwick, and others, we are gradually getting 

 some idea of the true meaning of the facts of development. In the 

 following pages, I do not pretend to set forth anything essentially 

 new, but merely to illustrate some of the points which arise when 

 one considers the development of animals belonging to such a group 

 as the Cirripedia. 



In comparing the anatomy of two animal forms, zoologists 

 very commonly restrict themselves to a consideration of the adult 

 forms alone. It is found, however, that in some cases the adult of a 

 certain form resembles the larva of another form more than the final 

 or adult stage of the latter, or more frequently that two forms widely 

 different in adult structure have very similar larvae. The structure 

 and development of the curious parasitic Cirripede Sacculina, bears 

 out these statements. The adult differs greatly from such a form as 

 Balanus, though the Cypris-stages of the two show great agreement. 

 In order to ascertain the true points of agreement of two forms, 

 it is necessary, then, to compare not only the adult but also the 

 young forms : hence the importance of embryology from a morpho- 

 logical point of view. 



We may represent an animal form by the symbols ABCD, 

 D being the adult, and A, B, and C successive stages in its deve- 

 lopment : now in the production of a higher form from a lower, we 



