68 BALANID^S. 



Attachment of the Body to the Shell. Muscular System. 



The prosoma which carries the posterior thoracic segments, 

 and in appearance the mouth, is the only part of the body 

 which is attached to the general covering (PI. 25, fig. 1), 

 namely, to the opercular valves. Except through the con- 

 tinuity of the lining membranes of the sack, the body lies free 

 within the walls of the shell. The area of attachment (shown 

 by a sinuous broken line round a and b) extends from about 

 the middle of the two scuta down to their basal margins. As 

 these valves lie obliquely across the orifice of the shell, the 

 animal's body comes to be suspended almost in the middle 

 of the sack. The two scuta, as we have seen, have the 

 power of opening and shutting a little ; and are brought 

 together by the adductor scutorum muscle (a), which is 

 generally very powerful. The body is attached to these 

 valves, round and beneath the adductor, so as to hide it 

 until one of the valves be removed. The attachment is 

 chiefly effected by three pairs of widely expanded, super- 

 ficial muscles, two pairs of which are spread over the 

 flanks of the prosoma, and the third pair over its rounded 

 (properly dorsal) surface, which lies close to the rostral 

 compartment (a, fig. 1) of the shell. I should have stated, 

 that my chief examination of the attachment of the body 

 to the scutal valves, has been made on Coronula balcenaris, 

 and less closely on Balanus tintinnabulum. Within these 

 three pairs of superficial muscles, there are (besides the 

 adductor) no less than five other pairs; of these one 

 long pair is attached at one end to the basal margin of 

 the labrum (e), and at the other end, to the under side, 

 near to the basal margin of the scuta : two other, shorter, 

 parallel pairs of muscles are attached at one end to the 

 interspace of membrane between the basal edge of the 

 labrum and the adductor scutorum muscle, and at the other 

 end, to the under side of the scuta, above the attachment of 

 the first pair : the fourth and shortest pair curls close under 

 the adductor, and is there attached at both ends beneath it. 

 The action of these four pairs of muscles must be to draw 

 back, from the orifice of the shell, the mouth, and that inter- 



