70 



DIVISION OF TH, ACEPHALA. 



is almost always prolonged into a fleshy mass (p), called the 

 foot, which serves for motion, and sometimes has at its base a 

 bundle of horny threads, called byssus, by means of which the 

 animal fixes itself to submarine bodies. 



3. This class is divided into five great families, which may be 

 recognised by the following characters : 



(FAMILIES.) 

 f having- the mantle open, and without tubes or particular ) n 



\ USTRACEA. 



apertures. 



open in front, and having "j 

 but one separate aperture I 

 for the escape of excre- j 

 ment. J 



closed and pierced by 

 three apertures, the first 

 of which serves for the 

 passage of the foot, the 

 second for respiration, and 

 the third for the exit of 

 excrement. 



open in front and present- ) /^ 



ing two tubes behind. \ CARDUCEA. 



closed, and not having, in"| 



front or below, but a single 



opening for the passage of ^ INCLUSA. 



the foot, and behind, two I 



tubes. 



4. The FAMILY of OSTRACEA has the oyster for its type, 

 and is composed of a great number of mollusks, that are 

 entirely without a foot, or having a very small one only, and 

 which, for the most part, live attached to submarine bodies, 

 either by their shell or by their byssus. The mantle is open 

 be'iind as well as in front, and its two lobes are nowhere united 

 to form particular opening-, as we have remarked in all other 

 acephala. 



5. This group is divided into two tribes, according as there 

 is a single muscle, going from one valve to the other, or two 



3. Into how many families is the class of lamellibranch mollusks divided ? 



4. What are the characters of the family of Ostra'coa ? (From the Latin, 

 ostrea, an oyster.) 



5. What is the distinguishing character of the tribe of Mono'mya'ria ? 

 (Mono'mya'ria. From the Greek, monos, single, and mvon, muscle.) How 

 is the tribe of Di'mya'ria distinguished ? (Di'mya'ria. From the Greek, 

 dis, two, and muon, muscle.) 



