416 DR S. F. HARMER AND DR W. G. RIDEWOOD ON THE 



ANDERSSON has actually seen living zooids creeping over the surface of the coenoecium 

 (07, p. 7). 



In the case of C. agglutinans the state of preservation of the material does not 

 enable us to conclude with certainty that each superficial tube opening to the surface 

 was inhabited by a single zooid and its set of buds, but the probabilities are much in 

 favour of this. The even calibre of each tube that opens to the surface points to its 

 gradual growth outwards, as the branch thickens, being the work of a single zooid, as 

 in the case of species of Idiothecia and Orthoecus. There would, therefore, be in any 

 one piece of colony approximately as many fully formed zooids as there are ostia. 

 What occurs in the middle part of the tubular system it is not easy to conjecture ; 

 these parts can scarcely be inhabited in the ordinary sense of the term, since there is 

 probably in the deeper tubes no circulation of sea-water such as would suffice for 

 nutrition and respiration of zooids living there. One must conclude that these internal 

 tubes are rather in the nature of deep retreats for the zooids in time of danger, and 

 that when conditions of life are favourable and all the full-grown zooids have their 

 proboscis and arms, and perhaps the whole of their body except the stalk, projecting 

 out of the mouth of the tube, the inner part of the tubular system is empty, or serves 

 only as a safe situation in which the young buds may grow. 



When a bud is so far grown that it severs its connection with the parent and 

 wanders off on. its own account towards the apex of the branch to settle down as an 

 independent zooid with a peripheral tube of its own, it may possibly reach the apex 

 by the internal tubes, where, as suggested above, it would meet with but little resistance 

 from other zooids ; but it is much more likely that the young zooid takes a superficial 

 route to the end of the branch, as one must suppose a young zooid of C. nigrescens 

 does. But in the present case the young zooid on arriving at the apex does not com- 

 mence to form a new short csecal tube for itself, as is the case in C. nigrescens, but, 

 in conjunction with other young zooids like itself, elaborates the tubular labyrinth 

 already existing. At any one time one must suppose that every tube that opens upon 

 the apex has in it a single young zooid which is adding to the mouth of the tube to 

 increase its length, and is filling up with softer coenoecial material and shelly fragments 

 the interval between its tube and those around. A newly arrived young zooid would 

 therefore have to contend with some other zooid for the possession of a tube, and 

 what seems to happen is that a modus vivendi is arrived at by the two agreeing upon 

 a branching in an apical direction of what is at present a single tube. This, of course, 

 is but speculation, but it seems to be the most plausible conclusion from a careful study 

 of the form and disposition of the tubes at the free extremity of a branch. 



In the older parts of a branch it would be interesting to know what happens to a 

 peripheral tube when the adult zooid dies. The probability is that the vacant tube 

 would soon be appropriated by some newly liberated bud or young zooid, either from 

 within or from without. In some cases, as is shown on p. 418, the vacant tube is 

 sealed up, but this is very exceptional. 



(BOY. soc. EDIN. TRANS., VOL. XLIX., 540.) 



