528 



THE INVERTEBRATA 



The Tetrabranchiata are divided into two groups, the nautiloids 

 and the ammonoids. The first of these contains Nautilus and other 

 forms which agree with it in the position of the siphuncle and the 

 shape of the septum. They reach their maximum development in the 

 early Palaeozoic, where the dominant forms have straight shells like 

 Orthoceras and Actinoceras, which were sometimes as much as 

 8 feet long. It is difficult to suppose that shelled animals of this size 

 were anything other than sedentary organisms. There is a tendency 

 for the shell to become coiled in later forms, exhibiting itself first in 



o.Un. 



Fig. 387. Fig. 388. 



Fig. 387. Nautilus macromphalus adhering to the substratum by means of its 

 tentacles in a vertical position. It usually lies horizontally. After Willey. 

 The shell shows alternate light and dark bands which resemble "ripple- 

 marking", d.fn. dorsal muscular attachments of the funnel ; e. eye ; hd. hood ; 

 mt. mantle; o.ten. ophthalmic tentacles. 



Fig. 388. A, Phylloceras heterophyllum, from the Lias : a part of the shell has 

 been removed to expose the sutures, x j. B, Suture line of Phylloceras 

 heterophyllum, from the Lias : the arrow indicates the position of the siphuncle 

 and points towards the aperture of the shell. From Woodward. Natural size. 



slightly curved forms like Cyrtoceras, then in loosely coiled forms like 

 Gyroceras and finally in the closely coiled Nautilus. There is also the 

 reverse tendency, and in Lituites the young shell is closely coiled but 

 in adult life it straightens out completely. 



The ammonoids appeared first of all in the middle of the Palaeozoic 

 but reached their zenith in the Mesozoic. From the beginning of the 

 Trias onward new families, genera and species are ceaselessly evolved. 



