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MOLLUSCA. 



CLASSES. 



A. With univalve shells, or none. 

 1. Cephalopoda. 



Body in the form of a A head covered with Sexes separate. 



bag, open before, con- large, long, fleshy pro- 



taining the branchiae. ductions, serving for lo- 

 comotion and prehen- 

 sion. 



2. Pteropoda. 



Body entirely closed. 



■ 



Appendages of the 

 head small or none ; or- 

 gans of movement two 

 wings or membranous 

 fins on the side of the 

 neck, and frequently 

 bearing the branchial 

 tissue . 



Hermaphrodite. 



3. Gasteropoda. 



Creep on the fleshy disk A head distinct and Hermaphrodite and 

 of the belly, sometimes anterior, or none. sexes separate, 



compressed into a fin (2). 



(1) These are the only Mollusca in which organs of hearing have been disco- 

 vered, and which have the brain (sending forth innumerable optic nerves from the 

 two ganglions) within a cartilaginous box : they are called Cephalopoda because 

 they have the feet on the head. They have three hearts ; they respire in water by 

 branchiae ; their mouth is placed in the centre of their feet, and resembles a beak ; 

 the head is also distinguished by very large eyes, and has the ears placed inter- 

 nally ; the stomach is muscular like a gizzard, the liver very voluminous. A par- 

 ticular gland secretes a black liquor, which they throw out, and which darkens the 

 water around them whenever they wish to conceal themselves. They swim with 

 their head behind ; and walk in all directions with the head below and the body 

 above. Aristotle remarked, that certain herbs, which have a strong odour, were 

 avoided by cuttle-fishes and the octopus. 



(2) They are so called because they crawl on their bellies ; the head is move- 

 able, and frequently provided with tentacula ; the heart is single. 



When the name of the genus appears in the following tables without any cha- 

 racteristic particulars, the animal is very imperfectly known. 



