570 ORDEIl ABDOMINALIA. 



there becoming thinner and ligamentous, gives attachment 

 to some powerful muscles. 



At each exuviation, the external membrane with the den- 

 tated hardened orifice, the lateral bars, the inner tunic of the 

 sack with its bars, are all moulted, together with the usual 

 integuments of the animal's body. New and sharp triturating 

 points are thus periodically formed for the work of exca- 

 vation. The whole animal increases during growth in every 

 direction, and therefore, at its lower or basal end, as was 

 the case with Alcippe. The disc or surface of attach- 

 ment, is added to by new underlying layers, extending be- 

 yond the old layers at the lower end and on the sides, but not 

 at the upper end, where, as in the case of the calcareous discs 

 of Lithotrya, the old layers are deserted and worn away. I 

 saw what I believed to be little globules or patches of cement; 

 but I was not able to discover any cement-ducts. 



Muscles of Sack and Orifice. — The animal is surrounded 

 by rather strong longitudinal muscles, not running up close 

 to the orifice : these muscles exhibited, to my surprise, dis- 

 tinct traces of transverse striae : there are no external trans- 

 verse muscles, as in all the Lepadidae. Attached to both sides 

 of the inward fold or hinge, at the posterior end of the 

 orifice, some striated or voluntary muscles run for a short 

 distance downwards, diverging like a fan : their contraction 

 would cause the dentated rim to open : a strictly homo- 

 logous muscle occurs only in Alcippe. At the opposite end 

 of the orifice, a remarkably powerful voluntary muscle is 

 attached to the ligamentous bar above described as pro- 

 ceeding from the rostro- medial end of the toothed rim 

 (c"); and at its lower expanded end, it is attached under 

 rather above the middle of the disc : this muscle cor- 

 responds with a similar one in Alcippe, and with some 

 much weaker muscles in other Lepadidge. Its action would 

 be to draw down within the shell-cavity the whole den- 

 tated rim, and likewise to close the orifice ; and here, I 

 believe, come into use the lateral elastic horny bars with 

 their curious basal projecting plates, furnished with ex- 

 panded points, for much friction would thus be caused by, 

 yet some play be allowed for, the several movements ; the 

 elasticity of the bar bringing up the dentated orifice, when 



