Metazoa Obelia. 223 



be distinguished, like the tentacular zooid in general appear- 

 ance, but bearing two-thirds of the way up a number of 

 swellings. This is the reproductive zooid. 1 Lastly, there 

 are to be found scattered here and there throughout the 

 colony upright stiff branches, incapable of movement and 

 tapering to a point. The perisarc in this type of zooid is 

 largely developed. This we may term a protective zooil 

 Before proceeding to the comparison of Hydractinia and 

 Obelia we must endeavour to explain this peculiar arrange- 

 ment and modification of parts. 



We have often had occasion to refer to division of labour 

 as the key to the understanding of the evolution of highly 

 developed from lowly developed types, briefly sketched out 

 in Chapters II. and III. We must again employ this expla- 

 nation in the present instance. Examining into the func- 

 tions of the several zooids in the colony, we find that the 

 alimentary zooid digests food not only for itself but for the 

 whole colony ; the tentacular zooid works towards the same 

 end by catching and killing small animals floating in the 

 water ; the reproductive zooid, fed like the others through 

 the stolon by the nourishment prepared by the alimentary 

 zooid, confines its attention to the production of gonophores 

 (p. 227) capable of forming sexual elements ; whilst lastly, 

 on the approach of danger, all three varieties of zooid rapidly 

 contract under shelter of the strong pointed protecting 

 zooids, which stand up firm and erect to shield the others 

 from harm. The tentacular zooid no doubt also serves to 

 warn off enemies by its active movements on the border of 

 the colony. In other words, we have in a Hydractinia 

 colony a number of persons all reducible to the same type 

 but each modified in some peculiar way to perform one or 

 other of the functions necessary to the maintenance of indi- 

 vidual and tribal life. We have morphological differentia- 

 tion accompanied by physiological specialisation or division 



1 Though termed the reproductive zooid, it is to be noted that it 

 does not produce sexual cells (p. 227). 



