358 



COELENTERATA ANTHOZOA 



the Verrucella by preying upon other organisms that might be 

 injurious. An interesting feature of the association is that the 

 Brittle stars are of the same colour as the host, and the knob- 

 like plates on their aboral surface have a close resemblance to 

 the verrucae (Fig. 157). 



Order V. Pennatulacea. 



The Sea-pens form a very distinct order of the Alcyonaria. 

 They are the only Alcyonarians that are not strictly sedentary 

 in habit, that are capable of independent movement as a whole, 



and exhibit a bilateral symmetry 

 of the colony. No genera have 

 yet been discovered that can be 

 regarded as connecting links between 

 the Pennatulacea and the other 

 orders of the Alcyonaria. Their 

 position, therefore, is an isolated one, 

 and their relationships obscure. 



The peculiarities of the order are 

 due to the great growth and modi- 

 fication in structure of the first 

 formed zooid of the colony. This 

 zooid (Oozooid, Hauptpolyp, or Axial 

 zooid) increases greatly in length, 

 develops very thick fleshy walls, 

 usually loses its tentacles, digestive 

 organs, and frequently its mouth, 

 exhibits profound modification of its 

 system of mesenteries, and in other 

 ways becomes adapted to its func- 

 tion of supporting the whole colony. 

 The axial zooid shows from an 

 early stage of development a division 

 into two regions : a distal region 

 which produces by gemmation on 

 the body-wall numerous secondary 

 zooids, and becomes the rachis of the colony ; and a proximal 

 region which becomes the stalk or peduncle, and does not pro- 

 duce buds (Fig. 158). The secondary zooids are of two kinds: 



Fig. 158. — Diagram of a Sea-pen. L, 

 leaves composed of a row of auto- 

 zooids ; R, rachis ; St, stalk ; T, 

 aiithocodia of the axial zooid, 

 usually suppressed. (After Juii- 

 gersen.) 



