POLYZOAN CORALS 



i6i 



are replaced by thin horny joints. It is extremely interest- 

 ing to find in this group the same " admirable contrivance of 

 Nature " of hard and soft joints for resisting the violent 

 motions of the sea that has already been mentioned as 

 occurring in some of the Alcyonaria (p. 121), and will also be 

 recorded in the Gymnolaemata (p. 172) and in the chapter 

 on Coral Algae (p. 207). It cannot for a moment be suggested 

 that the Polyzoa are genetically related to the Alcyonaria 

 or to the coral Algae, and therefore we must consider 

 that this admirable contrivance 

 has been attained independ- 

 ently in the course of evolution 

 and forms a fine example of 

 the principle of " convergence " 

 in Nature. 



There is just one more 

 feature of interest in the OV 

 structure of the Crisia 

 colony to which reference may 

 be made in passing, as it is 

 characteristic of the Cyclosto- 

 matous Polyzoa. 



On some of the branches 

 of the colony a swollen pear- 

 shaped body may be seen 

 which has the appearance of 

 a distorted or abnormal 

 zooecium (Fig. 75, OV). This 



is an " ooecium " or " ovicell," and is formed for the 

 protection of the embryos. Ovicells also occur in the 

 Cheilostomatous Polyzoa, but they are not usually so con- 

 spicuous as they are in the Cyclostomata. 



In the family Tubuliporidae the colonies usually form 

 little encrusting masses and spreading branches adherent 

 to foreign objects, but, if erect, as some of them are, 

 they do not exhibit the horny nodes seen in the genus 

 Crisia. 



The delicate fragile branches and the small size of most 

 of the genera of the Cyclostomata give them an appearance 



M 



Fig. 75. — Crisia eburnea. A small 

 fragment of a colony. OV, an 

 ooecium. x 25 diams. 



