260 TEXTBOOK OF ZOOLOGY 



four auricles; ink sac absent. This suborder reached its 

 peak of development in the Silurian and Devonian periods 

 and is one of the most clear-cut examples of evolutionary 

 development in the invertebrates. 



During the Ordovician period the cephalopods constituted one of the chief 

 groups of marine animals. Even though at that time cephalopods with coiled 

 shells existed, the predominant ones were the orthocones (those with straight 

 conical shells). This latter group in all probability gave rise to the entire 

 series of coiled shells, culminating in Nautilus. In all nautiloids a series of 

 partitions, termed septa, extend the full length of the shell. The point of union 

 with the septa and sides of the shell may appear as a straight, curved, angulate 

 or highly complex line. This line is called the suture and in fossil shells whose 

 outer shell coating is lost, it stands out rather conspicuously. The suture line 

 is used as a taxonomic character for the group. 



Order 2. Dibranchiata 



Octopods and squids are representative types. Shell internal 

 or absent; two gills and two primitive kidneys; ink sac pres- 

 ent; mouth surrounded by 8 to 10 tentacles which are 

 furnished with suckers. This order includes the largest of 

 all moUusks, the giant squid (Architeuthis princepsj which 

 may attain a total length, including arms, of over fifty feet. | 



The squids and octopods are noted for their ability to change 

 color by the rapid contraction or expansion of chromatophores 

 in their skin. Their juovements are rapid and are produced 

 by expelling water from the mantle cavity through the mus- 

 cular siphon with such force that the animal is jerked back- 

 ward. In the squids, fins along the sides of the body 

 facilitate locomotion. 



Loligo hreviyennis is the small squid found along the Gulf coast. 

 Wheu taken out of the water it is usually a mottled red or tan. 

 The visceral mass and mantle cavity are enclosed by a thick mus- 

 cular mantle. Beneath the skin along the back is a primitive endo- 

 skeleton in the form of a feather-shaped shell. The squid is preda- 

 tory, feeding on almost any animal it can capture. Within the 

 pharynx are two large jaws moved by powerful muscles. The 

 pharynx connects with an esophagus which in turn terminates in a 

 muscular stomach. Digestive juices from the liver and pancreas 

 are emptied into the stomach, and after the food is partially di- 

 gested, it is passed into a thin-walled cecum where digestion is com- 

 pleted and absorption takes place. Wastes are discharged through 

 the anus which opens near the base of the siphonal fold. The blood 



