284 CLASSIFICATION OF ANIMALS 



616. The sub-order Tetrabranchiata is characterized by the four gills, 

 numerous tentacles in place of the arms and a large many-chambered 

 shell of which only the last, largest chamber is occupied by the animal. 

 The ink bag is wanting. There is only one living species, the pearly 

 Nautilus. The Dibranchiata have only two gills and a rudimentary shell. 

 In the section Decapoda there are eight arms and two longer tentacle 

 arms. The suckers are stalked and have a horny rim. There are two 

 lateral fins. In the section Octopoda the tentacles are wanting, the 

 suckers are sessile and without a horny rim. There are usually no fins. 



Cephalopoda: 



Sub-order. Tetrabranchiata, Nautilus. 

 Sub-order. Dibranchiata. 



Section. Decapoda, Squid. 



Section. Octopoda, Octopus. 



617. Phylum IX. — Adelochorda. 



618. Class I. Enteropneusta. — This phylum and class are 

 represented by a few^ genera of v^orm-like animals which are of 

 interest because they form one of the links connecting the in- 

 vertebrates and vertebrates. A representative of the group 

 common on our Atlantic seashore is Dolichoglossus. The 

 animal burrows in the sand and mud along shore. When the 

 tide is out the coiled castings of this animal are often seen form- 

 ing piles several inches in height. The coiled castings and an 

 odor of iodoform are indications of Dolichoglossus. The body 

 is composed of a conical proboscis, a broad band-like ''collar" 

 and a long tapering trunk. Only three points in the anatomy 

 need be mentioned, i. The mouth lies in front of the collar 

 and from here the digestive tract extends directly to the poste- 

 rior end of the body. The anterior part of the digestive tract 

 is (lifferentiated for respiration. It is connected at regular 

 intervals with the body-wall and at these points there are open- 

 ings which form a passage from the enteric cavity to the ex- 

 terior. These openings are called gill slits. The respiratory 

 current enters the mouth and passes out through these sHts. 

 2. The nervous system consists of two ganglionic chains, one 



