SYSTEMATIC ACCOUNT 



229 



cause these differences. Between these distinct forms there are colonies which are difficult 

 to assign to either type. The existence of such intermediate forms proves that the external 

 differences are the result of the amount of coenoecial material secreted by the animal. 

 The coenoecium and spines are formed by the secretion of the coenoecial substance, 

 laid down in thin lamellae. Increase in size is brought about by the deposition of lamellae 

 over existing layers. Owing to differences in nutrition, and other causes, the amount 

 of coenoecial substance secreted varies and this may account for differences which are 

 observed between colonies dredged from different places. 



In the absence of zooids in the colonies obtained from stations 167 and 187, the 

 structure of the coenoecium is the only evidence that these colonies are modified forms 

 of C. hodgsoni. The colour of the 

 coenoecium, the thickness and length 

 of spines and the disposition of short 

 branches are sufficient differences to 

 separate them from Forms A and B 

 (see below). 



Owing to subtle variations in the 

 coenoecium of C. hodgsoni any attempt 

 to give a comprehensive account of the 

 structure may prove to be unsatisfac- 

 tory. Generalization is rendered difficult 

 by intermediate forms between the ex- 

 treme types ; but since the differences 

 are only due to the amount of coenoecial 

 substance secreted by the colony they 

 do not affect the diagnostic character 

 of the coenoecium as a whole. 



The colony is formed of a main stem 

 which has many lateral branches. The 



stem and branches are hollow and the pig. i. C. hodgsoni showing the disposition in form A of 

 continuous cavity opens externally by ostia and spines on a short hiteral branch. o,ostium;c, cavity 

 numerous ostia. The sides of the lateral °^ coenoecium. 



branches and the edges of the ostia bear varying number of spines. These characters are 

 constant in all the forms, and variations occur only in the disposition of the lateral 

 branches, the spines, the number of ostia and the colour of the colony. Ridewood (1918 a, 

 p. 51) defined two forms based on these variations, which he termed Forms A and B 

 respectively. The present account follows the same line with a few new observations. 



Form ^ (PI. XXXIII, fig. i). This form is represented only by a few small pieces, mostly 

 of a rich amber colour. The thickness of the coenoecium and width of the cavity varies 

 between different parts. At the base, the cavity is small and circular with a diameter of 

 1-75 mm. and the coenoecium is about 1-5 mm. in thickness. Farther up, the cavity 

 gradually widens and the walls become thinner. Towards the tips of the lateral branches 



