234 DESCRIPTIONS OF PREPARATIONS. 



48. BROAD-LEAFED HORN WRACK (Flustra foliacea], 



With Figure lo. 



A seaweed-like Polyzoon, universal in European seas and widely 

 spread over the world. It is colonial, like the vast majority of Polyzoa, 

 and the colony or zoarium forms erect fronds which spring from a small 

 basal portion, spread over some foreign object in the fashion of a Membrani- 

 pora. The fronds vary in outline in different specimens : they are flexible 

 owing to the small amount of calcareous matter deposited in the chitinous 

 walls of the zooecium, i.e. the resistent part of the ectocyst of the individuals 

 making up the colony. They are composed of two lamellae, in which the 

 zooecia are placed back to back. Some species of the genus, however, have the 

 zooecia disposed in only one lamella. The zooecia themselves are arranged in 

 parallel longitudinal rows, and the zooecia in one row alternate in position 

 with the zooecia of the adjoining rows to either side. An individual 

 zooecium is elongate in shape, with one, the distal, end curved or rounded, 

 and the other, the proximal, end somewhat contracted. The margin of the 

 rounded distal end bears two spines to either side the middle line, and in 

 some cases a fifth single spine placed centrally. A whitish spot near this 

 end, readily visible under a lens, marks the position of the retracted fore- 

 part of the individual or tentacle-sheath which bears the circlet of tentacles. 

 New zooecia are added to the colony at the curved edges of the fronds. 



Flustra foliacea generally grows upon stones or shells at moderate 

 depths, but may occur in deeper water (sixty-two to seventy fathoms). It 

 possesses a strong and peculiar smell when first taken from the water, and 

 its fronds form a favourite resting place for other Polyzoa and various 

 Hydroids. 



The anatomy of Membranipora (Flustra) membranacea has been well studied by 

 Nitsche, and is illustrated in Fig. 10 A. It is abundant and universal on our coasts. 

 The colony is composed of a single lamella in which the zooecia are disposed in 

 parallel series, alternating as in Flustra foliacea. The zoarium is attached to the 

 fronds of Laminaria and Fucus, and may attain the length of several feet. 



The zooecium or cell is in outline a parallelepiped. One surface, the lower, is 

 attached ; the other, the upper, is free. It has two elongated sides and two short 

 ends. Of the two ends, one, that to the left in the figure, is proximal, i. e. near to 

 the original parent zooid ; the other distal, i. e. near to the growing free edge of the 

 colony. A spine (Sp.} projects from the free surface at each of the two proximal 

 angles. The mouth so-called, or aperture made by the retraction of the flexible 

 fore-part or tentacle-sheath of the zooecium, is placed distally on the free surface. 

 It is crescentic in outline, and its proximal edge or lip (op.} is thickened, forming 

 the operculum, a structure from which the suborder Cheilostomata takes its name. 



