THE OUTER AND INNER TISSUES OF ANIMALS. 301 



wide opening of the cup be supposed to contract, thus 

 becoming a narrow opening, there will result something like 

 the gastrula form. Manifestly that part of the original layer 

 which has become internal is differently conditioned from 

 the rest which remains external : the one continuing to hold 

 converse with the forces of the environment, while the other 

 begins to hold converse with the nutritive matters taken 

 into the sac-formed chamber the archenteron or primitive 

 stomach. Interesting evidence of the primitive externality 

 of the digestive cavity is yielded by the fact that whereas 

 the blastula consisted of ciliated cells, and whereas the cilia- 

 tion persists throughout life on the outer layer, or parts of it, 

 in sundry low types even in some Chaetopods it persists 

 also on the alimentary tract of sundry low types : not only 

 in the Hydra but commonly in Nemertines, in some Platy- 

 helminthes, and even in some leeches. 



Besides being enabled thus to understand how an aggre 

 gate of Amoeba-form units, originally consisting of a single 

 layer, may pass into an aggregate consisting of a double 

 layer; we may also understand under what influences the 

 transition takes place. If the habit which some of the 

 primary aggregates have, of wrapping themselves round 

 masses of nutriment, is followed by a secondary aggregate, 

 there will naturally arise just that re-differentiation which 

 the Hydra shows us. 



289. This account of the primary differentiation carries 

 us only half-way towards a true conception of the distinction 

 between outer and inner tissues. Though, using words in 

 their current senses, this introverted part of the primitive 

 layer has become internal in contrast with the remainder, 

 which continues external, yet this introverted part has not 

 become internal in the strict physiological sense. For it 

 remains subject to the actions of those environing matters 

 which are taken in as food : such environing matters, when 

 they happen to be moving prey, acting upon it much as they 



