OR OSSEOUS SYSTEM, 



testaceous coverings are most developed in the balani, where 

 the articulated members of the contained animal are least, 

 and these exterior shells are least in the anatifa, and other 

 pedunculated genera where the feet are largest. The acorn- 

 shells, or balani, form a hollow cone composed of six pyra- 

 midal pieces, ribbed longitudinally with tubular cavities, 

 and overlopping each other in an imbricated manner by their 

 thin margins. These strong pyramidal plates, striated in- 

 ternally, both transversely and longitudinally, rest their broad 

 bases on the margin of a circular fixed calcareous disk, 

 striated in a radiated manner with small hollow tubes which 

 open into those of the pyramidal plates. Between the su- 

 perior separated apices of these larger plates, are interposed 

 six smaller pyramidal pieces with their points directed 

 downwards, so that their bases complete the upper free 

 and smaller margin of this truncated hollow cone. The smaller 

 inverted plates appear to be anchylosed to the larger inferior 

 pieces. Within this fixed cone, which has the means 

 of continued increase by its subdivision into numerous 

 pieces, there are four moveable triangular plates, two larger 

 and two smaller, which form an operculum to open and 

 close at the will of the animal. In the anatifa or pentalasmis, 

 represented in Fig. 13, we have an example of the ordinary 

 form of the skeleton in the barnacles, or lepades, suspended 

 by a fleshy peduncle. The shells here are thin and diapha- 

 nous, and by their striated outer surface, they mark the limits 

 of the successive layers of growth, and the opposite direc- 

 tions in which these lay- FIG. 13. 

 ers are extended in the 

 two lateral pieces of the 

 shell. Fig. 13,1, represents 

 the entire shell with the 

 jointed members of the 

 animal projectingfrom the 

 aperture, and a portion 

 of the contracted pedun- 

 cle. The jointed mem- 

 bers , with their jointed 

 cilia are here very long, and are seen partially extended 

 from the open anterior aperture b, of the shell. The pedun- 

 cle/, is composed of a tough exterior coriaceous tunic, and 



