18 ESSAYS OF A BIOLOGIST 



Protozoa have availed themselves. A colonial exist- 

 ence once attained, division of labour, at first be- 

 tween the germinal and the somatic, later between 

 different types of somatic units, will be a further ad- 

 vantage. Such organisms, of which we cannot say 

 definitely whether they are compound aggregates or 

 single wholes, would represent the most natural link 

 between the unicellular Protozoan and the rest of the 

 animal kingdom, the multicellular forms or Metazoa. 

 And indeed such organisms exist at the present day 

 — organisms such as Volvox, Zoothamnium, Protero- 

 spongia, and Myxidium — as adjuvant and confirma- 

 tory of our reasonable faith. 



The multicellular organisms appear to have orig- 

 inated twice over, by divergent routes. There are 

 the true Metazoa, to which belong all the higher 

 types, and the Parazoa or sponges, which have never 

 passed beyond a very primitive type of structure. 

 Both start as simple sacs, whose walls are formed 

 from two primary sheets or layers of cells. Leav- 

 ing sponges out of account, the Hydroid polyps are 

 the simplest representative of this grade of structure, 

 while some of the Jelly-fish and Siphonophores have 

 attained the utmost limit of its inherent possibili- 

 ties. 

 _ The next great step was the intercalation of a 

 third primary layer between the other two. The 

 result of this, the so-called triploblastic type of or- 

 ganization, gives the ground-plan for all subsequent 



