330 



PRINCIPLES OF ANIMAL BIOLOGY 



merely a mass of fat varying in size with the state of nutrition of the 

 animal, and is not correlated with any pecuUarity of the skeleton. 



Evolution of the Cephalopods. — An excellent fossil record among the 

 invertebrates has been established for the tetrabranchiate (four-gilled) 

 Cephalopoda (Mollusca) . This branch of the cephalopods is represented 

 today by Nautilus, which lives in a coiled shell, externally resembling a 

 snail shell. However, the animal lives in only a small portion of the shell 

 near the aperture. The rest of the shell is divided by partitions into a 

 number of chambers, from whioh the animal is excluded. These parti- 

 tions, or septa, represent the positions occupied by the animal earlier in 



A 



Fig. 227. — Diagrams of sutures of cephalopods, represented as spread oiit with the 

 ventral point in the middle. A, orthocone; B, nautiloid; C, goniatite; D, ceratite; E, 

 ammonite. 



its life. As th^ body grows, it moves periodically forward into the wider 

 part of the shell and secretes a partition behind itself each time it moves. 



Tetrabranchiate cephalopods have been found, as fossils, in Cambrian 

 rocks. They became fairly abundant in the early Ordovician time. At 

 that time, unlike the modern Nautilus, their shells were straight cones 

 (orthocones) . All later forms appear to have descended from these 

 orthocones. Fossils of cephalopods are found in the strata of all periods 

 from the Ordovician to the Cretaceous, and as stated above a few mem- 

 bers of the group are still living. 



The course of evolution was as follows. The shell soon began to 

 bend, and in many forms became closely coiled in flat spiral form like 

 the shell of some snails. Owing to their resemblance to Nautilus these 

 animals are called nautiloids. They were very abundant in Silurian 



