534 LEPADID/E. 



is generally convex, being concave on the under side for the 

 attachment of several muscles presently to be described. The 

 horny layers are in this part usually thicker than elsewhere. 

 The disc is thus upwardly produced, owing apparently to 

 the fissure which leads into the cavity of the shell of the 

 mollusc becoming, during the process of excavation, consi- 

 derably longer than is necessary, — that is longer than the 

 orifice leading into the sack ; and consequently, for the pro- 

 tection of the imbedded animal, the lower and narrow end 

 of the fissure is closed on its under side by this upward 

 production of the horny disc, formed of layers of membrane 

 of unusual thickness. A deposition, also, of lime, hereafter 

 to be mentioned, gives further protection. 



In the bays on each side of the upward production of 

 the horny disc, and likewise a little lower down on its edges, 

 and therefore somewhat protected by lying within the narrow, 

 pointed, lower end of the fissure in the shell of the mollusc, 

 the short-lived Males (PL 22,, fig. 1, m) are attached often 

 in groups of two, three, or more together. 



It may be asked to what part, in other Cirripedes, does 

 the horny disc answer ? Not considering the upward pro- 

 longation, which has been developed for a special purpose, 

 the disc is irregularly circular, — is added to all round, — 

 serves for the attachment of the whole animal to the sup- 

 porting surface, — is covered on the under surface by a 

 conformable and parallel mass of ovarian caeca, and the latter 

 by the inner tunic of the sack ; therefore in every character, 

 and in its relation to the other parts of the animal, the 

 disc answers to the end of the peduncle, or to the basal cup 

 in Lithotrya, or still more closely to the basis in sessile 

 cirripedes, with the important exception that it lies in a 

 line with the longitudinal axis of the whole animal instead 

 of at right angles to this axis. We know that all ordinary 

 cirripedes become first permanently attached in their pupal 

 state by their antennae, which are seated on the ventral or 

 rostral surface, near to the anterior end of the body ; and 

 that from the young cirripede, after the act of metamor- 

 phosis, being turned vertically upwards, and from the ex- 

 treme anterior, now lower, end of the body not being 

 rapidly developed, the surface cemented down, or the basis, 



