A REVISION OP THE GENUS STEGANOPORELLA. 237 



idea that one of the principal dangers to which they are 

 subjected is the competition of other fixed organisms. An 

 excellent example of this is afforded by a specimen of the 

 encrusting form of S. neozelanica, which I have found 

 growing over another Cheilostome with a very uneven surface 

 in such a way that the basal wall of the Steganoporella 

 had moulded itself over every irregularity of the form on 

 which it was growing, thereby completely closing every 

 zooecial orifice. Many Polyzoa are particularly subject to the 

 attacks of encrusting Polyzon, — as, for instance, Lepralia 

 foliacea of our own coasts. In some cases these attacks 

 probably do little harm, because the basal parts of the colony 

 become covered by a calcareous thickening of its own which 

 occludes the zooecial orifices, and the zooecia with functional 

 polypides are those in the younger parts of the colony. 



Although Steganoporella grows into large flat surfaces 

 which might form an excellent fixing-point for other Polyzoa , 

 I have never seen any overgrowth of this genus by encrust- 

 ing Polyzoa in specimens which were in good condition, with 

 their full complement of opercula. I have, however, exa- 

 mined a colony of S. truncata which was largely covered 

 by Membranipora., Cellepora, and Smittia; and it was 

 obvious that some of these were growing on parts of the 

 Steganoporella which had lost their opercula. The others 

 were in parts in which the opercula were wanting here and 

 there, and it seems to me not unlikely that the zooecia had 

 really been dead, although they had not yet lost their 

 opercula. Another case of Cellepora growing on the Vin- 

 cularian form of S. neozelanica is capable of being ex- 

 plained in a similar way. 



I venture to suggest that certain of the external features 

 of a Polyzoon colony may be correlated with the discourage- 

 ment of the fixation of the larvaj either of Polyzoa or of 

 other encrusting animals. Many of the irregularities of the 

 surface, such as the armature of oral or marginal spines, may 

 act in this way by making the surface irregular and unfit for 

 the attachment of larvse ; and in particular it seems to me 



