Cumings — Development of some Paleozoic Bryozoa. 65 



basal plate opposite the aperture of each zooecium indicate very 

 clearly the radius of action of each corresponding polypide 

 (figs. 47, 51, 53, 54). It will be seen at once that the carinse, 

 without interfering noticeably with the movements of the 

 exserted polypides, afford a very efficient means of protection 

 against the snipping off, by some obnoxious visitor, of the 

 tentacles of the polypides. That the danger of such rude 

 treatment is by no means imaginary is abundantly proved by 

 the fact that recent Bryozoa are frequently found with their 

 tentacles lost or in various stages of regeneration. The carinse 

 also function as strengthening structures, especially in the 

 basal portion of the zoarinm, and may, by secondary deposits 

 during later growth stages of the colony, be greatly increased 

 in height and breadth. The completed nepiastic stage, with 

 its strong carinse, is strikingly suggestive, both in external 

 appearance and in sections, of a segment of Arthroclema. 

 (Cf. the axial region of iig. 55 with fig. 83, left.) 



Neanastic Stage. 



This stage begins with the assumption of the conical habit 

 of growth. A plane passing through the apex of the cone (p, 

 figs. 55 and 56), therefore, separates the nepiastic from the 

 neanastic region of the zoarium. Fig. 52 shows a Fenestella 

 just entering upon the neanastic stage. The nepiastic stage in 

 this individual comprises three tiers of zooeeia or thirty zooecia 

 in all, including the protoecium. The first branching of one 

 of the primary carinse is shown at di, 0I2. Fig. 60 shows the 

 same phenomenon in transverse section. Fig. 59 shows a 

 transverse section cutting the neanastic region of a slightly 

 smaller zoarium, at a somewhat higher level. The vesicular 

 tissue occupying the apex of the cone is of altogether second- 

 ary origin. It is deposited mainly during the gerontastic 

 stages. 



It will be noticed that the zooecia of Fenestella lie always on 

 the outside of the expanding cone, i. e., they face away from 

 the axis of the zoarium. It is also evident that this habit of 

 growth is impressed upon the zoarium from the very outset. 

 In any case where the initial circle of zooecia (metanepiasty) is 

 completed, the resulting zoarium will be a cone with the zooe- 

 cia on its outer surface. Where, for any reason, the initial 

 circle is incomplete or abnormal, the resulting zoarium will, as 

 has been pointed out above, be flabellate, or produce a cone by 

 modifications arising during the neanastic stage. Such abnor- 

 mal cones may, by reversal of the normal curvature, bring the 

 zooecia on the inside, This fact does not, however, modify the 



Am. Jour. Scl— Fourth Series, Vol. XVII, No. 9 V.— January, 1904. 

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