138 MEMBRANIPORID^. 



of lines of cells arranged in single series, which some- 

 times anastomose so as to form a rude network. 

 Var. 7. Zoarium rising into frond-like expansions, and 

 frequently much branched [Pallas] . 



Polypides with 11-14 rather thick tentacles, each of which 

 is furnished with a number of very delicate setiform 

 appendages (tactile hairs), ranged along the back. 



Range of Variation. The appearance of this species is 

 much affected by the presence of a greater or smaller 

 number of spines, and the degree in which the corneous 

 vibraculoid appendage is developed. One variety, which 

 usually spreads over stone and shell, forms a delicate 

 silvery crust ; while another, investing the fronds of Algae, 

 has the appearance of a coarse brown coating, which 

 bristles with the tall setae. In the former the aperture is 

 almost completely surrounded by slender, acuminate, in- 

 curved spines, of which one, placed in the centre of the 

 lower margin, exceeds the rest in size. In other cases the 

 marginal spines are much fewer in number, and often 

 short, subconical, and erect. In one variety (Plate XXIII. 

 fig. 3) they seldom exceed three, of which two are situa- 

 ted at the upper part of the cell, and one below the aper- 

 ture. The horny appendage rises from a kind of socket, 

 and frequently attains an enormous size. In many cases 

 it is aborted ; in others it is present in abnormal pro- 

 fusion. Sometimes there are two, sometimes three 

 together; and I have seen a cell round which no less than 

 six were developed, the small marginal spines being also 

 present. Occasionally one or two of the tall horny spines 

 are intercalated amongst the latter. 



There are also differences in the size of the aperture 

 and the proportion which it bears to the inferior portion 

 of the cell. In one marked variety it is much elongated. 



