THEIR CLASSES. 101 



corpus mucosum of vertebrates. This muscular envelope 

 contains the viscera, among which the nervous system lies 

 unseparated, the principal ganglion or brain, as some call it, 

 being placed under the gullet and encircling it with a fila- 

 ment as with a collar. Of the proper senses the mollusca 

 possess those of taste and sight only, and the latter often 

 fails. A single class has organs of hearing.* There is in 

 all a complete system for the circulation of a white serous 

 blood ; and respiration is performed by special organs. The 

 organs of digestion, and of the secretions, are nearly as com- 

 plicated as they are in the vertebrated animals, and not less 

 remarkable for their variety and curious adaptations. Such 

 are the leading characters of the mollusca ; and although 

 this type of organization is not distinctly revealed by a very 

 uniform correspondency between it and exterior configura- 

 tion, yet there is such a degree of harmony and mutual de- 

 pendency between the inner anatomy and the outward show, 

 that only a little experience in their investigation is wanting 

 to make us perceive and acknowledge it. 



The mollusca defined in this general manner form a sub- 

 kingdom, which is next divided into six classes, -j- their dis- 

 tinctions being founded on modifications of the organs of 

 progression. For reasons already assigned one of these 

 classes (Cirrhopods) is now considered as properly pertaining 

 to the annulose sub-kingdom, and the five remaining may be 

 shortly characterized as follows : 



I. Cephalopods. — Body enclosed in a muscular sac con- 

 taining the branchiae, and open superiorly, where the head 

 projects. This is well developed, and is surmounted with a 

 circle of eight or ten subulate cotyligerous appendages (feet) 

 subservient to progression and the capture of their prey. 



II. Pteropods. — Body not sacciform ; the head has no 

 appendages, or only very small ones ; the principal organs of 

 locomotion are two wings, or compressed fins, situated at the 

 sides of the neck, and which, in many species, perforin the 

 function of branchiae. 



III. Gasteropods. — These are also cephalous,but less dis- 

 tinctly so than the preceding: they crawl on the belly or on a 

 fleshy flattened disk, which is sometimes, though very rarely, 

 compressed into the form of a fin, different, however, from those 

 of the Pteropods, and distinguished by its ventral position. 



IV. Acephales. — Headless mollusca in which the mouth 

 lies concealed in the base of the mantle, which also envelopes 



* It will be remembered that I am almost translating from Cuvier. The 

 sense of hearing is not so limited in the class. 

 t Reg. Animal, iii. 6, 7. 



