6o8 



MOLLUSCOIDEA. 



the form of tubes, which place the perivisceral cavities of contiguous 

 cells in direct communication, as is seen, for example, in such recent 

 genera as Cellepo?'a among the Cheilostomata, and Domofiora and 

 Hete?-opora among the Cydostomata. These connecting-tubes (fig. 

 452) are usually wide and trumpet-shaped at the points where they 

 open into the cavities of the cells, and are contracted in the middle 

 portion of their course ; and they, doubtless, are simply a further 

 development of the simple pores seen in many other forms. 



Again, in various of the Palaeozoic Polyzoa, and particularly in 

 the family of the Fenestellidce, a portion of the polyzoary consists of 

 dense calcareous tissue which exhibits under the microscope a finely 

 punctated appearance. When sufficiently thin sections of this 

 punctated layer are prepared, and examined with a sufficiently high 

 magnifying power, the tissue is seen to be penetrated by innumer- 

 able exceedingly minute tubuli (fig. 453), which run at right angles 

 to the surface of the polyzoary. Nothing certain has, however, been 

 ascertained as to the nature and function of these tubuli. In some 



types, such as Rhombop07-a 

 (Ceriopora) interporosa, of 

 the Carboniferous rocks, 

 the thickened walls of the 

 cells near their mouths are 

 penetrated by dark rod-like 

 structures which run at 

 right angles to the surface. 

 These structures are of 

 larger size than the tubuli 

 of the Fenestellids, but they 

 sometimes exhibit a clear 

 central space, and it is pro- 

 bable that they are really 

 of the nature of tubes. 



In a great number of the 

 recent Po/yzoa, the poly- 

 zoarium is furnished with 

 singular appendages, which 

 and " vibracula," and which are to be 

 regarded as specially modified zooids. The avicularia or "bird's- 

 head processes" (fig. 454, b and c) differ a good deal in shape, but 

 consist essentially of "a movable mandible and a cup furnished 

 with a horny beak, with which the point of the mandible is capable 

 of being brought into apposition " (Busk). In shape, the avicularia 

 often closely resemble the head of a bird ; and they are in many 

 respects comparable with the " pedicellarise " of the Echinodermata, 

 keeping up a constant snapping movement which continues long 



Fig. 453. — A portion of the punctated layer on the 

 non-poriferous side of a Fenestellid (Pliyllopora sp.), cut 

 parallel with the surface and greatly enlarged. De- 

 vonian, Canada. (Original.) 



are known as " avicularia 



