IMPORTANCE OF THE ANIMAL ENVELOPE. 165 



will have a tolerably accurate conception of the general 

 structure of an Actinia. 



Having given a rough outline of the principal 

 characters of the Actinia's internal structure, sufficient 

 to render intelhgible what will hereafter be referred to, 

 I must direct the student anxious for more precise 

 details, to the Memoire by M. HoUard,* as the latest 

 and best anatomical essay ; noticing, by the way, that 

 there still remains to be written a coinpa7mtive ana- 

 tomy of the various Actinicie, some of which differ in 

 imjDortant characters from the others. In the Anthea, 

 for example, the tentacles have an abundance of round 

 yellow-brown globules, which make them incapable of 

 being retracted under normal conditions ; and the 

 same is noticeable in the two horn-like tentacles in 

 Actinia hellis. What are these globules ? There are 

 several other points of difference to be noted, but I 

 content myself with indicating the desideratum of a 

 careful comparison, to complete our knowledge of the 

 anatomy of this genus. Meanwhile let us endeavour 

 to form some distinct conception of the mode of Ee- 

 production exhibited by the Actiniae. 



Certain general facts must be borne in mind. First, 

 let me call attention to the fact that in all animals, 

 the highest as the lowest, the envelope is of eminent 

 importance, its predominance bearing a precise ratio 

 to the simplicity of the organism. The simplest 

 organisms breathe, exliale, secrete, absorb, and repro- 

 duce by their envelopes alone ; and if the more cora- 



* Annates des Sciences, 1851. 



