267 



Structure of the Zooids. 



The zooids (fig. 9) are about 5 mm in height and consist of a some- 

 what slender stalk bearing a cup-like anthocodia, the whole being densely 

 covered with a felt-work of minute warty spindles. The tentacles are 

 short and broad, with a single row of pinnules on each side ; their bases 

 are confluent so as to enclose a capacious hollow — the oral disc, over 

 which they can be infolded. They are not retractile, but when at rest, 

 being infolded, the biserial arrangement of the spicules forms a very 

 primitive operculum. 



The wall of the anthocodia is prolonged into eight triangular lobes, 

 on which the spicules are also arranged biserially, so that each pair 

 forms two sides of a triangle, the enclosed angle becoming more and 

 more obtuse towards the base. This arrangement is continued down to 

 the origin of the stalk, forming a series of ridges on the anthocodia. 

 The triangular projections thus function as a protection to the infolded 

 tentacles. The stalk, though narrow, is very elastic, because the zooid 

 when at rest forms an introvert within it, which in turn sinks within the 

 verruca (fig. 10). The zooid is withdrawn by eight strong bands of 

 retractor muscles which thus form eight cavities running upwards, and 

 corresponding to the canals in the octagonal disc. These retractor 

 muscles pass downwards and are continuous with the eight mesenteries 

 of the zooid. 



The oral disc is spacious and circular, containing a rather large 

 elliptical mouth — opening, which leads into a keyhole-shaped, richly- 

 ciliated stomodaeum, in which a very distinct sulcus can be distinguished. 

 The ectodermic cells in this region are more numerous and the cilia are 

 longer (fig. 12). 



The mesenteries are all complete, and the muscle banners on the 

 sulcar aspect are easily discernible. The filaments are continued down 

 the stem canals almost to the very base of the colony, while the asulcar 

 filaments show very markedly the ciliated groove so characteristic of the 

 group (fig. 13a). 



Fig. 10 shows the relation of these parts in longitudinal section: 



a. the outer wall of the verruca. 



b. wall of the zooid stalk. 



c. zooid. 



d. retractor muscle of the zooid. 



e. mesenteries with ova. 



In a cross section through the introverted anthocodia the following 

 parts can be distinguished (fig. 11): 



a. the outer wall of the verruca. 



b. the prolongation of this outer wall, when introverted, forming 

 on its downward course the elastic wall of the zooid stalk. 



