TUBARIUM OF CEPHALODISCUS H0D6S0NI. 51 



apparently lateral ostia are explicable od the ground that a lateral branch is iu each 

 of these cases in the initial stages of its growth. The openings are oval, sometimes 

 circular, and average between 3 by 2 and 3 • 5 by 2-5 mm. across. Each opening has 

 four or five spines around its margin. Five representative figures are given of the 

 ostia and their spines, since the number and distribution of the spines of the test of 

 Cephalodiscus will prolmbly prove to be among the features useful for distinguishing 

 the various species (figs. 16-20, plate 4). 



The spines are simple, or forked, or even trifid. The length of the free part 

 varies considerably, usually within the limits 5 to 15 mm. The width varies from 

 • G to 1 mm. The central axis of a spine is darker than the general test, and can be 

 traced for some distance into the latter. The total length of a spine, including this 

 embedded base, varies from 12 to 25 mm. Most of the spines arise from the edges 

 of the ostia. 



The successive increments by which the spines are Imilt up remain clearly 

 visible, and a study of the fine dark lines which mark oft' the difterent strata 

 shows that the forking of a spine is not due to dichotomy, but that one of the 

 two limbs of the fork is the primary spine and the other one, although it may be 

 equal to it in size, or even greater, has been secondarily attached to its side (see 

 fig. 21, plate 4). 



In connection with the probable freedom of locomotion possessed liy the 

 polypides of Cephalodiscus and the bearing which this freedom has upon the mode 

 of increase of the test, it may be noted here that in specimen C a few pieces of 

 Flustra (Carhasea) have been caught up among the branches of the colony. In 

 some places the flattened branches of the coenoecium of the polyzoon have been 

 completely buried in the test, but in others the strata of the test are continued 

 along one surface, or along one edge, as a thin film, or as several films, with here 

 and there a spine of the usual structure. This coating of the foreign object 

 extends to a distance of 20 or 30 mm. away from the nearest hollow part of the 

 tubarium occupied by polypides. It seems clear from this that not only have the 

 polypides the power of moving a1)out over the surface of their own test — one 

 cannot conceive how the terminal increments of the spines are put on if they 

 cannot do so — but they are also able to roam about upon neighbouring foreign 

 objects, which they smear over with their profuse secretion. 



Specimen C is further of interest in that in certain parts of the colony a 

 stoloniferous polyzoon allied to Bowerhankia is found embedded in the test. The 

 relations of the zooids of the polyzoon to the Ceplialodiscus tubarium are such as to 

 suggest that when they were flooded by the not yet solidified material of the test, 

 they had time to dispose themselves so that their tentaculated ends were flush 

 with the surface before the material hardened. Some of the zooids are normal to 

 the surface of the tubarium, others are oblicjue, l»ut all have their openings flush with 

 the surface. 



