12 THE SUB-KINGDOM CCELENTERATA. 



Lastl}^, a wonderful diversity of external pro- 

 cesses, some from the ectoderm, either alone or 

 in gi'eat part, others from the ectoderm and en- 

 doderm combined, are seen to arise ; and these 

 may subsequently multiply to an almost indefinite 

 extent, or, even separating from the primal or- 

 ganism, enjoy a brief but independent existence. 



The student who has perused the account now 

 given of the general structure of the lower animals 

 is warned to be on his guard against the errors 

 which still but too widely prevail on this and 

 other kindred branches of histology. These errors, 

 for the most part, have their origin in the well- 

 known cell-theory of Schleiden and Schwann : a 

 theory which, when first announced by its dis- 

 tinguished promulgators, possessed, indeed, a high 

 dignity and utility, but in the hands of inferior 

 naturalists has tended not a little to check inde- 

 pendent thought, and to render obscure much 

 that, but for it, would have been intelligible. In 

 particular the view that the cell-nuclei, or endo- 

 plasts, constitute special originating centres of 

 vital activity, is worthy of all reprobation ; con- 

 tradicted, as it appears to be, by so many careful 

 observations on the various modes of development. 

 Cells indeed exist, but only as the differentiated 

 products of a primitively homogeneous protoplasm, 

 not as morphological or physiological entities. To 

 borrow the fine metaphor of Professor Huxley 

 (whose views on animal structure we have here 

 more than once sought to interpret and extend), 

 " they are no more the producers of the vital 

 phenomena, than the shells scattered in orderly 

 lines along the sea-beach are the instruments by 

 which the gravitative force of the moon acts upon 



