OR OSSEOUS SYSTEM. 



53 



FIG. 25. 



several chambers of these polythalamous cones communi- 

 cate with each other, sometimes by means of a prolonged 

 calcareous syphon, as in the spirula and nautilus, some- 

 times by one or more simple foramina in the partitions. 

 As the cephalopods do not creep upon a muscular foot, 

 like the slow-moving gasteropods, but for the most part 

 swim freely through the sea, their shells are thin and 

 light, and sometimes xconsist of simple straight laminae 

 unconsolidated by calcareous matter. The form and struc- 

 ture of the external polythalamous shells of the cepha- 

 lopods is seen in the section of the nautilus pompilius, 

 represented in Fig. 25. 2, where (0,) shows the interior of 

 the last formed chamber, in which the animal is fixed 

 by two muscles of attachment. The syphons (,) by which 

 all the posterior chambers (c,) communicate with each 

 other, are seen to extend for a short 

 distance, tapering from before back- 

 wards, into each of the succeeding 

 chambers. These separated partitions 

 and chambers of this convoluted cone 

 have nearly the same relations to 

 each other and to the contained ani- 

 mal, as the successive contiguous 

 layers of the convoluted shell of a 

 gasteropod, and they are formed in 

 the same manner by periodical exuda- 

 tions of calcareous matter from the 

 exterior surface of the mantle. In 

 the spirula., the long calcareous syphon 

 of each septum extends through the 

 whole of the chamber, and into the commencement of 

 the next succeeding syphon ; so that there is a continuous 

 calcareous tube passing through the whole shell on the 

 inner concave side of its convolutions. The shell of the 

 sepia, (Fig. 25. 1. 0,) affords an example of an internal 

 shell belonging to this class. It is contained within the 

 substance of the dorsal part of the mantle, and consists 

 of numerous nearly flat layers, placed within each other, 

 the first formed being at the outer part and posterior termi- 

 nation of the shell, and the succeeding new layers ex- 

 tending always more forwards than the edges of the old. 



