THE MORPHOLOGY OF THE CHBILOSTOMATA . 307 



The characteristic horns of this species are probably to be 

 regarded as modified avicularian zooecia. In certain zooecia 

 one of the horns is replaced by a large normal avicularium. 



Catenicella alata, Wyv. Thomson^ (fig*. 53). 



The zooecial structnre of Catenicella has been described 

 by Busk (1852, p. 4), and by Waters (1883, p. 428). The 

 zooecium develops on each side three sti-uctures, which to- 

 gether form the great wings so characteristic of this species. 

 Taking a case which is not complicated by the presence of 

 the twin zooecium (cf. fig. 56) which occurs just before a 

 bifurcation of the branch,^ the zooecium is found to be some- 

 what spindle-shaped, each end passing into a tubular sheath 

 which surrounds the chitinous joint by which it is connected 

 with its neighbours. The basal surface of the zooecium is 

 extremely gibbous, and projects in this direction far beyond 

 the wings. The frontal surface is nearly flat. The large 

 orifice has, on its proximal side, a scutiform calcareous region 

 bearing five fenestrge closed by membrane. The zooecium is 

 considerably wider than this scutiform plate, its convex, 

 lateral surfaces being overlapped by parts of the wings. 



Each wing consists of three parts : (1) a large proximal 

 cavity (infra-aviculariau -f- pedal compartments of Waters), 

 which is almost two thirds of the length of the zooecium, and 

 is provided on its frontal surface with two large membranous 

 fenestrfB, one at each end (fig. 53, inf. avic.) ; (2) a transverse 

 cylindrical cavity, the free outer end of which has a mem- 

 branous vacuity {avic.) ; (3) a large distal cavity with a single 

 large fenestra, its distal point being uncalcified {su2). avic). 

 The second of these cavities is morphologically an avicularian 

 zooecium, as is proved by comparisiou with other species of 

 Catenicella. 



The five suboral fenestrse perforate the entire thickness of 

 the calcareous frontal wall. At the level of their internal 



1 'Nat. Hist. Review,' v, 1858, " Proc. of Societies," p. 137. 



2 Waters terms the solitary zo(j'.ciuin a "!,Mobulus," and the two associated 

 zooecia a " bii^lobulus." 



