524 



POLYZOA 



calcareous forms is often marked by pores (Fig. 239, C, p), which 

 are vacuities in the calcareous wall, closed externally by mem- 

 brane. A special median yore (Fig. 241, A, m.p) may occur, and 

 is in some cases at least a complete perforation through the 

 body-wall. 



The tentacles are protruded through the orifice, which in 

 Cheilostomata is usually guarded by a movable chitinous lid, or 

 operculum (Fig. 256, A, o). Should the ectocyst be thickened or 

 raised into a ridge surrounding the orifice, a tubular passage 

 results, known as the secondary orifice (Fig. 255), at the deeper 

 end of which is the true orifice. The peristome (Fig. 255, C, 



FIG. 257. A, Cribrilina an- 

 nulata Fabr., Norway, x 

 33 ; c, calcareous bars 

 concealing the membran- 

 ous aperture : B, Mem- 

 braniporella nitida 

 Johnst., ' Plymouth, x 

 45 ; a, calcareous bars 

 growing up round the 

 margin of the aperture ; 

 b, the same, further de- 

 veloped ; c, the same, 

 completely formed (as in 

 A) ; av t avicularium ; o, 

 immature, and o', ma- 

 ture, ovicell ; s, marginal 

 spines. 



pr~) is the raised or thickened part which gives rise to the 

 secondary orifice. Should the zooecium be outlined by a raised 

 ridge, the part so enclosed is known as the area (Fig. 256, C, ), 

 if calcareous. The aperture or opesia (Fig. 256, A, B, ap) is a 

 membranous part of the front surface; and may consist of the 

 whole or part of the area. The orifice or the aperture is 

 commonly provided with spines (Fig. 256, B, s). 



The avicularium and the vibraculum are specially modified 

 zooecia (see p. 482), which occur in a great variety of forms, in 

 certain Cheilostomata only. The operculum of the ordinary 

 zooecium is represented by the mandible (Fig. 239, B, tn) in the 

 avicularium, and by the seta (Fig. 242, s) in the vibraculum. 

 The representative of the zooecium itself is known as the avicu- 

 larian (Fig. 239, A, a.z) or vibracular zooecium (Fig. 242, v.z). 



