62 BALANID^E. 



of the sack. In Xenobalanus, the two folds of corium are 

 united by longitudinal membranous septa, making a series 

 of quite peculiar, square tubes. 



The above-mentioned muscles are attached at their upper 

 ends to the opercular valves, and at their lower ends to the 

 basis. There are, in fact, three pairs, but the pair attached 

 to the basi-carinal angles of the two terga (PL 25, fig. 1 i) t 

 are almost invariably confluent, forming one great bundle; 

 the second pair is attached to the lateral or basi-tergal 

 corners of the two scuta, and are hidden in the figure ; 

 the third pair (/z) is attached also to the scuta, to their 

 rostral angles. These muscles can only act as depressors ; 

 they are often extremely powerful ; they belong to the volun- 

 tary class, for they are transversely striped. By their action, 

 the opercular valves are capable of varied slight movements, 

 within the limit allowed by the width of the flexible oper- 

 cular membrane. By the action of the lateral scutal cle- 

 pressores, the orifice leading into the sack is opened, the 

 movement being generally aided by the protrusion of 

 the cirri. By the sudden contraction of the rostral scutal 

 depressores, the blows which are sometimes given by the 

 beaked terga at the opposite end of the operculum, are 

 probably effected. By the contraction of all three pairs 

 of muscles, the opercular valves are held down with quite 

 surprising force. The valves can be raised only by the ac- 

 tion of the animal's body against the basis. 



In Coronula these muscles are more spread out, and do 

 not extend down to the basis ; their lower portions, as is 

 likewise the case in Tubicinella, do not exhibit transverse 

 striae, and hence tend to pass either into the involuntary 

 class, or into ligament. This condition of the muscles, in 

 the above two genera, accords with the little-developed state 

 of their opercular valves. In Xenobalanus, there is no longer 

 any evidence of the muscles being collected into five or six 

 bundles, for they are thinly and almost uniformly spread 

 out, and show in no part transverse striae. I may add that 

 in much elongated specimens of Balanus balanoides, these 

 muscles become in their lower part ligamentous, and desti- 

 tute of striae. 



