358 METAMOEPHOSES OF ANIMALS. 



to its early stages, that it then bears a near resemblance to a 

 little crustacean (fig. 370 d). It is only when full grown 

 that it assumes its peculiar mollusk-like covering. 



563. Among the cuttle-fishes there are several, the 

 Loliffo, for example, which are characterized by the form 

 of their tentacles, the two interior being much longer than 

 the others, and of a different form ; whilst, in others, as the 

 Octopus, they are all equal. But if we compare the young, 

 we find that in both animals the tentacles are all equal, though 

 they differ in number. The inequality in the tentacles being 

 the result of a further development. 



564. Among the radiata, the Pentacrinus and the Co- 

 matula exemplify the same point. The two are very different 

 when full grown, the latter being a free-swimming star-fish 

 (fig. 373), while the former is attached to the soil, like a 

 polyp. But we have seen ( 559) that the same is the case 

 with Comatula in its early period ; and that in consequence 

 of a further metamorphosis, it becomes disengaged from its 

 stem, and floats freely in the water. 



565. In the type of thevertebrata,the considerations drawn 

 from metamorphoses acquire still greater importance in re- 

 ference to classification. The sturgeon and the white-fish 

 before mentioned ( 463) are two very different fishes ; yet, 

 taking into consideration their external form and bearing 

 merely, it might be questioned which of the two should take 

 the highest rank ; whereas, the doubt is very easily resolved 

 by an examination of their anatomical structure. The white- 

 fish has a skeleton, and moreover a vertebral column com- 

 posed of firm bone. The sturgeon (fig. 374), on the con- 

 Fig. 374. 



trary, has no bone in the vertebral column, except the spines 

 or apophyses of the vertebrae. The middle part, or body of 

 the vertebra, is cartilaginous ; the mouth is transverse, and 

 underneath the head ; and the caudal fin is unequally forked, 

 while, in the white-fish, it is equally forked. 



