Fig. 273. Gastropods
Fig. 274. Cambrian Pteropods
Cephalopods, the most highly organized of the mollusks, started into existence, so far as the record shows, toward, the end of the Cambrian, with the long extinct Orthoceras (straighthorn) and the allied genera of its family. The Orthoceras had a long, straight, and tapering shell, divided by cross partitions into chambers. The animal lived in the “body chamber” at the larger end, and walled off the other chambers from it in succession during the growth of the shell. A central tube, the siphuncle ([s, Fig. 275, B]), passed through from the body chamber to the closed tip of the cone.
Fig. 275. Orthoceras
A, fossil; B, restoration