576 BIOLOGY. 



tubes retractile by means of muscles ; the foot is cylindri- 

 form. 



Of the same kind are the Pfahlmuscheln, Teredines, 

 and Sandmuscheln, as also the Solenidse. They typify 

 the Nudibranchiate Snails and the Salpae. 



Fam. 2. Polypary Mussels Klaffmusch eln or Tellinidce. 



Body flat, mostly discoidal; there is a large fissure 

 on the abdominal side of the mantle for the passage of 

 the foot, while posteriorly there are two long respiratory 

 tubes with retractor muscles. Here belong the Telli- 

 nidse and Venusidae. 



I have shown, that they are already to be recognized 

 by the uncinate or hook-shaped groove for the mantle 

 in the shell, and which proceeds from the insertion of 

 the retractor muscles. The foot is usually lancet-shaped. 

 They typify the Patellae and Ascidiae. 



Fam. 3. Accdephan Mussels Cardiacea. 



Body globular in form, mantle having a pedal slit on 

 the abdominal side ; two respiratory apertures without 

 retractor muscles; foot mostly uncinate or strap-shaped. 



T have shown that these creatures are to be recognized 

 by a discoid groove in the shell, which simply depends 

 upon the mantle, the retractor muscles being wanting. 

 Here belong the Cardiacea and gigantic Chamidae, in 

 which last a byssus occurs, and the union also of the two 

 occlusor or sphincter muscles, as in the following order. 

 They antetypify the air-breathing Snails and Cirripedia. 



Order 2. Conchozooid Mussels Spaltmusclieln. 



3494. These animals represent their own circle. 



The pedal slit in the mantle occupies the whole length 

 of the latter, and is confluent with the anterior or also 

 with the posterior respiratory aperture ; so that only one 

 or even no aperture is left, and this when present is 

 besides never elongated in a tubular form ; the retractor 

 muscles are therefore wanting also, and the shell simply 

 exhibits a discoid groove for the mantle. The occlusor 

 muscles approximate and unite at their terminations. 

 Both usually coalesce into one. 



