ON AN INSTANCE OF COMMENSALISM., 987 



tubercles, at first separate, ma,y join together to form single 

 blocks as in G, commensale ; a remarkable example of the former 

 species is shown in fig. 11, where the blocks have developed so 

 enomnously as almost to obliterate the opesia. 



In descriptions, the triangular hollows, spines, tubercles, etc. 

 are usually associated with the distal or oral end of the zooecium^ 

 but they should perhaps rather be associated with the proximal 

 or basal part of a zooecium. On the triple-bud hypothesis it is 

 certainly more I'ational to associate the three buds, no matter 

 what their subsequent history and disguise might be ; and further, 

 the knobs, spines, etc. are often clearly seated on the front of 

 the basal end of a zocecium. Take, for example, the little knobs 

 on the zooecia of Menibranipora savartii Audouin, shown on the 

 base of each cell in Savigny's figure (' Description de I'Egypte,' 

 Atlas, pi. X. fig. 10) *. 



In Thairopora armata MacGillivray (fig. 13) the two club- 

 shaped tubercles are situated one on each side of the oral orifice 

 of a zooBcium, and, it must be admitted, a little imagination is 

 required to idealize that even here the tubercles should perhaps 

 be associated with the proximal end of the following zoo3cium. 

 One might assume that the two lateral buds had been given oK 

 at right angles to the central one, instead of at a forward-opening 

 acute angle. 



The avicularian cell in this species appears beautifully to 

 confirm the theoiy of homology of tubercles and triangular 

 boxes ; for here, in place of tubercles, and on each side of and 

 above the avicularian opening, there is a triangular box with a 

 clear space in the roof (fig. 13). 



Possibly the quincuncial plan of growth so common in Cheilo- 

 stome Polyzoa may be due to adaptation to limited space. 

 Zooecia giving ofl;" three buds at the distal end would not be well 

 able to grow abreast ; accordingly the buds (and their adult 

 equivalents) often alternate transversely. 



The fusion of two lateral hollow knobs into one rectangular 

 hollow box in G. commensale may be connected in some way with 

 reproduction, for in decalcified specimens there can usually be 

 seen a brown body or developing new polypide in the zocecial 

 cavities adjacent to these boxes. 



Discussion. — On two points our description of the new species 

 is open to adverse criticism. Firstly, we attribute specific value 

 to zoarial characters. During an early period of the study of 

 Polyzoa, zoarial characters alone were considered, most of the 

 encrusting and branching calcareous Cheilostomes being relegated 

 to Lepralia and Eschara respectively. Later, zooecial characters 

 became all -important, the zoarial ones being entirely put aside. 

 At the present time the tendency is to take all characters into 



* Tliere is rarely any trace of the knobs in the specimens of this species from 

 tropical seas or from Japan. 



