MOLLTJSCA: PTEKOPODA CEPHALOPODA. 269 



united, or forming a spiral, but in some cases the body is 

 naked. The head is rudimentary, and bears the mouth, which 

 is occasionally tentaculate, and which is furnished with an 

 odontophore. There is a muscular stomach, and a well deve- 

 loped liver, and the flexure of the intestine is neural, so that 

 the anus is situated on the ventral surface of the body. 



The heart consists of an auricle and ventricle. The respira- 

 tory organ is very rudimentary, and consists of a ciliated 

 surface, which is either entirely unprotected, or may be 

 contained in a branchial chamber. 



The ganglia of the nervous system * are concentrated . into 

 a mass below the oesophagus ' (Woodward), and the eyes are 

 rudimentary. 



The sexes are united in all the Pteropods. 



The Pteropoda are divided into two orders, termed Thecoso- 

 mata and Gymnosomata ; the former characterised by possess- 

 ing an external shell, and an indistinct head ; the latter by 

 being devoid of a shell, and by having a distinct head, with 

 fins attached to the neck. 



SYNOPSIS OF THE FAMILIES OF THE PTEROPODA. (AFTER WOODWARD.) 



ORDER I. THECOSOMATA. 



Animal with an external shell ; head indistinct ; foot and ten- 

 tacles rudimentary ; mouth situated in a cavity formed by the union 

 of the locomotive organs. Eespiratory organs contained within a 

 mantle-cavity. 

 Fam. 1. Hyaleidce. 



Shell symmetrical, straight or curved, globular, or needle-shaped. 

 111. Gen. Hyalea, Cleodora, Theca, Conularia. 

 Fam. 2. Limacinidce. 



Shell minute, spiral, sometimes operculate. 111. Gen. Limacina, 

 Spirialis. 

 ORDER II. GYMNOSOMATA. 



Animal naked, without mantle or shell, head distinct; fins at- 

 tached to the sides of the neck ; gill indistinct. 

 Fam. 3. Cliidce. 



Body fusiform, foot distinct, with a central and posterior lobe ; 

 head with tentacles. 111. Gen. Clio, Pneumodermon. 



CHAPTER L. 



CEPHALOPODA. 



CLASS IV. CEPHALOPODA. The members of this class are 

 denned by the possession of eight or more arms placed in a 

 circle round the mouth ; the body is enclosed in a muscular 



