BRYOZOA. 305 



just like the supposed acanthopores; and, when the section 

 passes through the zoarium just beneath the surface of a fully 

 matured example, where the vesicles are filled with a dense 

 deposit of sclerenchyma, they cannot be distinguished from each 

 other. Precisely the same conditions prevail in Coscinium, 

 Eractinopora, Phyllodictya, Pachydictya. Idiotrypa, Nichol- 

 sonella and Constellaria, and, with the addition of true acantho- 

 pores, in species of Trematopora. On account of their excessive 

 minuteness and consequent liability to obliteration, I have found 

 it extremely difficult, if not impossible, to determine whether these 

 structures are ever actually tubular (i. e., like the acanthopores, 

 passing in a continuous line through successive stages of the 

 zoarium). The evidence afforded by vertical sections is generally 

 opposed to this view, excepting in some sections of Cystodictya, 

 and other genera, in which the secondary deposit that filled the 

 interstitial vesicles exhibits a finely lined appearance, making it 

 not unlikely that calcification was arrested by the pores and 

 only extended around them, so that with the continuation of 

 the deposit, they gradually formed tubular shafts*. Taking all 

 the evidence at hand into consideration, the only plausible ex- 

 planation would seem to be that we are dealing with small per- 

 forations in the calcareous covers of the mesopores and inter- 

 stitial vesicles. Whether these were closed at the termination 

 of each stage, or remained open so as to maintain communica- 

 tion throughout the zoarium, is a problem we may never be 

 able to solve. 



In species of Ptilodictya of the type of P. pavonia D'Orb., 

 and P. falciformis Nicholson, and Tseniodictya, (n. gen.) and 

 Cyclopora fungia Prout, the intra-mural space is crossed by 

 numerous dark lines, which tangential sections, taken from ex- 

 ceptionally preserved specimens, show to be composed of closely 

 arranged spots. Whether these spots represent extremely min- 

 ute vertical tubes or moniliform tubuli, which traversed the 



*In the Ann. & Mag. Nat Hist. 5 ser. vol. 13, p. 213, Rev. T. Hincks, the eminent 

 authority on recent Bryozoa, describes a very similar condition in the development of 

 his Mitcronella spinosissima var. major. After stating that in the young state there are 

 two or three rows of pores around the margin of the zocecia, he says, "As calcification 

 proceeds it is arrested by the pores and only extends around them and not over them; 

 so that they continue open and form at last tubular shafts piercing the stony cru.s 

 which has been piled up about them," 



--38 



