i8 ESSAYS OF A BIOLOGIST 



size and greater rapidity of motion, of which many 

 Protozoa have availed themselves. A colonial exist- 

 ence once attained, division of labour, at first between 

 the germinal and the somatic, later between different 

 types of somatic units, will be a further advantage. 

 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 confirmatory 

 of our reasonable faith. 



The multicellular organisms appear to have origin- 

 ated 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. Leaving 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 possibilities. 



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 organ- 

 ization, gives the ground -plan for all subsequent 



