THE MOLLUSCA AND THE MOLLUSCOIDA 115 



CHAPTEE VI. 



THE ORDERS INTO WHICH THE CLASSES OF THE INVERTEBRATA 

 ARE DIVISIBLE. 



I. THE MOLLUSCA. 



THE CEPHALOPODA are divided into two orders, the Dibran- 

 chiata and the Tetrabranchiata. 



1. In the DIBRANCHIATA, the processes (" Arms ") into which 

 the margins of the foot are divided are not more than ten in 

 number, and they are provided with acetabula, or suckers. 



The funnel is a complete tube. When a pallial shell is 

 developed, it is internal, or enveloped by a prolongation of the 

 mantle, and does not lodge the body. The beaks are horny. 

 There are only two gills ; and there is an ink-bag. 



2. In the TETRABRANCHIATA, represented by the Pearly 

 Nautilus, the processes into which the margins of the foot are 

 divided are very much more numerous. They bear no acetabula, 

 but each contains a retractile tentacle. The funnel is open below. 

 The shell is pallial, external, chambered, and siphunculated. 

 The beaks are more or less covered with calcareous matter. 

 There are four gills, and no ink bag. 



I am not satisfied that any good divisions having the value 

 of orders have at present been formed among the Pteropoda, 

 Pulmogasteropoda, Branchiogasteropoda, and Lamellibranchiata. 



. II. THE MOLLUSCOIDA. 



The ASCIDIOIDA are divisible into three orders 



1. The BRANCHIALIA, in which the branchial sac is very 



large, in proportion to the intestine and generative viscera, so 



that these usually lie on one side of it. 



i 2 



