BRYOZOA. 307 



in the final and most developed condition, they commence with 

 thin and apparently indivisible walls. This I have called the 

 "immature" portion of the tubes; in the ramose, frondescent, 

 and bifoliate forms, it occupies the axial and deeper regions of 

 the zoarium The peripheral or what I have termed the "ma- 

 ture" region commences near the point at which the tubes bend 

 outward and from there extends to the surface. Generally, the 

 two regions differ widely from each other. Thus in the "im- 

 mature" region the tube walls are exceedingly thin and the dia- 

 phragms, if not entirely absent, are always separated by longer 

 intervals than in the "mature" region, while none of the accessory 

 elements, such as the inesopores and acanthopores, are developed 

 until the zootria have passed out of this region. On the other 

 hand, in the "mature" region the walls of the zooecia become 

 more or less extensively thickened, and if at all present, such 

 structures as the cystiphragms and perforated diaphragms, meso- 

 pores and acanthopores are developed. As already mentioned, the 

 diaphragms are also comparatively more numerous and, as I 

 will endeavor to show in the section on diaphragms and oper- 

 cular structures, in some forms at any rate appear to be of a 

 different nature from those which cross the tubes in the "im- 

 mature" region. The thickening of the walls of the tubes by a 

 secondary deposit and a general increase in the deposition of 

 calcareous material is one of the most conspicuous features of 

 the "mature"' region. 



The thin walled basal or prostrate portion of the zooecia of 

 the bifoliate, encrusting and fenestrate species, is equivalent to 

 the "immature" region of the dendroid forms, while the erect por- 

 tion represents the "mature" region. Among massive forms the 

 two regions are often readily distinguished. This is especially so 

 when the zoarium is composed of numerous layers as in Stenopora 

 polymorphfi Prout. In cases of this kind each layer is divisible 

 into a prostrate or immature and an erect or mature region. 

 Less easy is the task with species like Monotrypa? filiosa D'Orb., 

 and Monticulipora mammulatti D'Orb., in which the layers are 

 so intimatelv connected that the tubes form continuous lines 



v 



through the mass. Yet even in these, the two zones are distin- 

 guishable, the walls being slightly thicker and the diaphragms 

 more closely arranged in the "mature" region than in the im- 



