70 THE GENUS MONTICULIPORA. 



tubes by their size, but that these two sets of tubes are in no 

 way distinguished from one another in point of structure, while 

 their dimensions are often very nearly the same. The tubes 

 are all rounded ; and their walls are very thick, and are com- 

 posed of delicate calcareous laminae arranged concentrically 

 around the cavity, and not showing any line of demarcation 

 between each other. In this respect the walls have the struc- 

 ture of such species oi Montictilipoi^a as M. ramosa, M. Jamesi, 

 &c., and differ altogether from such other species as M. pul- 

 chella. The most remarkable feature, however, in the struc- 

 ture of the wall consists in the presence of numerous canaliculi, 

 which pass transversely across the thickened wall (fig. 9, a) and 

 open at both ends by wide funnel-shaped apertures into the 

 cavities of the tubes. In this way the zooecia and interstitial 

 tubes are placed in direct and free communication with one 

 another throughout the entire colony. Sections of this nature 

 also prove with absolute certainty that these canaliculi are 

 strictly confined to the lualls of the tubes — a point upon which, 

 as will be seen, longitudinal sections might leave us in some 

 doubt. 



Another very interesting and important point brought out 

 by tangential sections is, that both the zooecia and cancelli are 

 provided In this part of their course with numerous delicate 

 radiating spines, which spring from the wall (fig. 9, a) and 

 are directed inwards for a longer or shorter distance, usually 

 falling short of the centre. I am not aware that the presence 

 of these radiating spinules has hitherto been recognised as 

 occurring in the zooecia of the recent HeteroporcB, or in the 

 extinct forms, except in H. clavata, Goldf (according to Busk), 

 and then only at the mouths of the interstitial tubes. ^ \r\ H. 

 ncozclaiiica they are very slender and delicate, and often break 

 up in thin sections, so that they may appear to be wanting in a 

 greater or less number of the tubes ; but I have never failed to 



1 Mr Waters has pointed out to me that he has described delicate radiating spines 

 as occurring in the " cancelli" oi Discoporella radiata, as well as in other species ot 

 the same genus. (" On the Bryozoa of the Bay of Naples," Ann. Nat. Hist., ser. 5, 

 vol. iii. p. 276. 1879). 



