BRYOZOA. 303 



We now pass on to the consideration of other sets of minute 

 tubuli, which, though often strongly resembling the "acantho- 

 pores'', must have had very different functions. Until they are 

 better understood I propose to designate them by the term 

 ''median tubuli", the name having reference to their position in 

 the zoarium. 



V. Median tubuli. In its strictest sense the term applies to 

 certain exceedingly slender longitudinal tubes which are present 

 between the two median laminae in many of the STICTOPORID.E. 

 In Stictopora and P achy diet y a they are of sufficient size to be 

 recognized with ease in transverse sections. So far as observed 

 here, they do not seem to be provided with distinct walls, but 

 appear to have been produced by an equal longitudinal grooving 

 of the laminae, so that half of the tube is on each side of the 

 divisional line between the laminae. Another set, which I con- 

 sider as essentially of the same nature as those just described, 

 traverses the walls of the zooecia (or the interspaces between 

 them) in a vertical direction. These are arranged in crowded 

 series and very much resemble small "acanthopores''. In Sticto- 

 pora and some species of Pachydictya they are usually arranged 

 in a single series along the middle of the walls between the 

 zooecia. In certain species of Stenopora, Rhombopora and 

 Anisotrypa they are also arranged in a single series, but in 

 these they are confined to a distinct linear space which sepa- 

 rated the walls of adjoining zooecia. Structures of probably the 

 same type, but slightly modified, occur in the walls of Idiotrypa 

 parasitica and Nicholsonella ponderosa. 



The spine-like structures of Fistulipora utricula Rominger, F. 

 spinulifera Rominger. and other species of the genus, which 

 Nicholson and Foord identified with the "acanthopores" of 

 Dekayia, are, most probably, of a veiy different nature. That 

 in tangential sections they resemble small acanthopores is true, 

 yet certain peculiarities, which they often show in an unmis- 

 takable manner, indicate functions decidedly different from those 

 here attributed to the true acanthopores. On plates 4(5 and 48 

 I have figured sections of various species of Fistulipora, some 

 of which are provided with the structures in question. Upon 

 close examination we find that the cover of the interstitial vesi- 

 cles is perforated by one or more minute openings that look 



