238 SIDNEY F. HAKMER. 



probable that aviculai'ia and vibracula may conti-ibute to- 

 wards the same result. In the case of Steganoporella, 

 the B opercula, with their formidable teeth, may protect the 

 colony by the destruction of the larvae of Polyzoa or other 

 animals which are fixed in their adult state. The a opercula 

 may also take their share in this work, and would be parti- 

 cularly effective when furnished with teeth. It may be noted 

 that the a opercula are provided with two or four powerful 

 teeth in several of the species in which the b opercula are 

 apparently vestigial or completely absent. The disap- 

 pearance of the B opercula may in fact have been facilitated 

 by the occurrence of teeth on the a opercula. 



The fixation of a Polyzoon larva is a deliberate process, 

 and does not take place until the larva has wandered about 

 over the surface of fixation for some time, rotating by its 

 cilia as it does so. During this process it may easily be sup- 

 posed that it would sooner or later be brought within the 

 reach of an avicularium in genera so provided, or of a b 

 operculum in Steganoporella, one closure of which would 

 be sufficient to destroy any intruding larva. The relations 

 of the oral shelf of the b form are by no means adverse to 

 this idea, particularly in S. alveolata, but the cavity above 

 the shelf, in that species at least, is not adapted for the per- 

 formance of an ocEcial function. 



The nature of the dimorphism of Steganoporella may 

 pei'haps in the future form the subject of direct observation 

 in some locality where suitable species are abundant. 



Descriptions of the Species. 



The species of Steganoporella form an interesting study 

 in evolution. The complicated arrangements connected with 

 the distal part of the cryptocyst can be traced from a simple 

 to a more differentiated condition, and the former can be 

 compared with the arrangements of other genera. Certain 

 forms show a considerable range of variation; but on the 

 whole the characters of each species are well marked, and do 



