THE SESSILE BAEITACLES. 



125 



In addition to those recorded below there are numerous lots in the 

 Jeffreys collection without definite locality, probably all British. 

 The long series from Exmouth, chiefly seated on sandstone, com- 

 prises some small, nearly white examples, with the sheath white, on 

 Lepas, Pecten, and Fissuridea. The var. cranchii is well represented, 

 and there are specimens of var. mirahilis from E-ochelle. The ma- 

 terial is not sufficient for an opmion on the status of the named 

 varieties. 



Series of Balanus gregarius. 



Large, solid barnacles, in which the basis consists of a thick disk 

 with very few pores, and attached only at one pomt; this disk 

 typically is surmounted by a cyhndric portion with thick walls 

 permeated by many minute pores, and with the cavity filled with a 

 mass of large, thin-walled cells, in old individuals; waU-compart- 

 ments having porous parietes. Opercular vqilves unknown. 



Tlie structure of the walls and of the cellular filling of the basis 

 in the single species of this group is strictly similar to that of B. 

 Isevis coquimhensis; but that form has far thuiner walls, and the lower 

 part of the basis differs, being attached throughout, while in this 

 species the very thick basis seems to be fixed only by a small initial 

 attachment in all the specimens seen which show the basis; sub- 

 sequent growth being free. Whether this condition is solely due to 

 attacliment to very small objects is a point to be determined. 



The cellular filhng of the cavity of the base in B. Isevis coquimbensis 

 and B. gregarius is probably due to the production of gas by the 

 mantle lining the floor of the cavity. Bubbles are formed between 

 the solid basis and the mantle, elevating the latter locally. A cal- 

 careous film is then secreted on the lower face of the mantle, impris- 

 oning the bubbles of gas. Similar production of gas is known in many 

 aquatic animals, such as fishes and Nautilus; but in Balanus the 



