MOLLUSCA — CEPHALOPODA. 



147 



active predatory animals, or whether they were sedentary Gallery 

 and possibly attached by the shell, is uncertain. But certain 

 it is that at an early period the hump was drawn out into a 

 long visceral cone, and that the shell acquired a similar 

 shape. Then followed a mode of growth very common in 

 sedentary animals that form a tubular shell, and already 

 observed in corals, worms, bryozoans, and gastropods. The 

 mollusc continued to build up the shell around its opening, 

 and thus formed a long tube. As the animal moved along 

 this tube, the visceral cone was pulled away from the shell- 



FiG. 79. — Primitive Cephalopod shells : Encloceras. a, The end of a shell, 

 broken at its apex ; 6, the same cut in half showing the chambers (sc), 

 the swollen end of the wide neck-tube (cc), and calcareous substance 

 deposited by the end of the siphuncle [cd). c. Fragment of another 

 shell cut in half, showing the chambers separated by septa (s), the 

 large neck-tube (sc), a sheath [sh), and the endosiphon {en). Natural 

 size, (a, h, after Holm; c, after Foord.) 



wall; but its skin went on secreting shell-substance, and 

 formed a partition shutting off the space between the visceral 

 cone and the outer wall. In Endoceras, here shown (Fig. 79), 

 the visceral cone remained attached to the end of the outer 

 shell, and shrank at a little distance from the apex, so that 

 the partition or septum does not go right across the shell, but 

 shuts off a chamber at the side. Shrinkage then took place 

 a little higher up, and another chamber was formed. By 

 the continuation of this process there arose a series of 

 chambers entirely shut off by septa, and the down-turned 

 portions or necks of these septa formed a lono^ tube in which 



