56 INTRODUCTION. 



certain Crustacea, called cirri. The stomach is provided with 

 little cells which appear to perform the office of a liver; the in- 

 testine is simple; there is a dorsal heart and lateral branchiae, 

 also a double ovary or a mass of internal buds and a double 

 serpentine canal for the exit of the young. These animals are 

 sessile or pediculated, but always fixed; their nervous system 

 is a series of ganglions under the belly. 



49. The acephalous or conchyferous mollusca, have the 

 body deprived of a head, containing all the viscera and are 

 completely enveloped, (like a book by its cover) by the man- 

 tle doubled in two, and by a calcarious shell, generally bivalve, 

 sometimes multivalve. The mouth is armed with tentacular 

 leaflets concealed under the mantle; the anus is hidden in the 

 same manner; at the other extremity there are four very large 

 branchial leaflets; the liver is very voluminous, embracing the 

 stomach and a part of the intestine which greatly varies. The 

 foot when it exists is attached between the four branchiae, and 

 consists of a fleshy mass, which moves like the tongue of the 

 mammalia. The heart is commonly single, aortic, and placed 

 near the back. They have one or two muscles for closing the 

 shell and an elastic ligament which opens it; they have also a 

 principal ganglion situated above the mouth, united to another 

 opposite by two nervous cords, and some other nerves and gan- 

 glions. They produce living young ones without copulation. 



The branchiopoda, are other acephala, but few in number, 

 which in place of ieet have two fleshy arms; they appear to 

 have two aortic hearts, and a convoluted intestine surrounded 

 by the liver; neither their generation nor their nervous system 

 is well understood. 



50 The gasteropoda are cephalous mollusca that generally 

 crawl on a fleshy disk placed under the belly, the back being 

 covered by the mantle that varies in length and figure, and 

 producing most commonly a univalve or multivalve shell. In 

 this class are to be found some mollusca whose organs of res- 

 piration and shell are not symmetrical. The head, placed for- 

 ward and more or less disengaged from under the mantle, pos- 

 sesses generally two, four, or six tentacula, situated above the 

 mouth, that perform the offices of feeling, seeing, and perhaps 



