354 ESCHARID^. 



Range of Variation. Specimens of this species exhibit 

 very striking diversities in appearance ; and there is also a 

 considerable amount of variability in certain characters. 

 The zooecia present two very distinct shapes^ being in some 

 cases ovate and moderately convex, in others rectangular, 

 with a flat surface. The orifice wears a very difi'erent 

 appearance according to the degree in which the peristome 

 is developed. In the young cell it is not at all elevated ; 

 the mouth is a simple, suborbicular opening, with a flat 

 rim, arched above, almost straight below, and slightly com- 

 pressed laterally. In this condition, which seems to be 

 permanent in the Arctic variety a, the denticle on the 

 lower lip is a conspicuous feature. Usually the peristome 

 rises in the adult cell and forms a thin wall round the 

 mouth, which is prolonged in front into a kind of spout. 

 In the fertile cells, this raised border is much developed ; 

 and in some cases it forms a continuous ring round the 

 mouth, in front of the ovicell. The oral spines vary in 

 number from two to four. 



As usual, the avicularium is the most inconstant member 

 of the colony. It presents itself under two forms : one of 

 these (and the only one which has been noticed by pre- 

 vious writers) is of large size, at times gigantic, elongate, 

 with a triangular mandible ; the other is much smaller, 

 oval, with a rounded mandible. In their position and 

 direction there is no constancy; but the larger form seems 

 to be most commonly placed close to the mouth, on one 

 side of the central sinus, with the mandible directed up- 

 wards, while the smaller is usually developed near the top 

 of the cell, on one side of the mouth, or on both sides, with 

 the mandible pointing downwards ; but the exceptions 

 arc numerous. The large avicularia, which are often 

 wanting altogether, are generally single; but sometimes 

 two occur on the same cell, in very variable positions. 



