ZOOLOGY. 521 



The shells of Bivalve Mollusca are a calcareo-thoracic 

 box, open in front, inverted behind, and rnoveable like ribs. 



3198. The two sphincter or occlusor muscles signify 

 shoulder and haunch. 



3199. These animals begin for the first time to exhibit 

 bilaterality or symmetry, because in them there is stirring 

 the idea of the osseous, formation. From the branchiae 

 being situated symmetrically upon both sides, the cardiac 

 chambers are also symmetrical. 



3200. The pectoral tunic (mantle) usually elongates at 

 the anal extremity into two tubes, respiratory tubes, 

 through which the water is drawn in and thrust out or 

 expelled. Such is the case in the highest Worms or Holo- 

 thuriae, only there the respiratory tube leads into the body 

 itself. A similar arrangement is found in the Echini. 

 Many aquatic larvae from all kinds of Insects or out of 

 different classes breathe through anal tubes. All these 

 animals consequently repeat the Mussels, and this forma- 

 tion admits of being followed out up to Man, where it is 

 left as allantois and primordial kidneys. The thorax of 

 the Mussel thus opens by the anus. But, since the tho- 

 rax is that which is here preponderating and is nearly the 

 whole animal, so does the anus open into the posterior 

 respiratory tube. 



3201. The Mussel can be regarded as an animal con- 

 sisting of three cysts or sacs inclosed within each other, 

 namely, of intestine, around this the ventral pouch, and 

 around this again the thoracic sac or mantle. If, more- 

 over, the shells be regarded as a cyst, the animal then 

 consists of four sacs. The heart and branchiae lie within 

 the thoracic cavity ; intestine, liver, and ovariuni within 

 the abdominal or ventral cavity. The Mussel is thus a 

 doubled Acalephan. 



What is termed foot in the Bivalve Mollusca is nothing 

 else than the abdominal tegument dilated in front to 

 form a muscular ridge. 



3202. The Mussels are embryos, in whom the liver 

 originates, and whose chorion acquires a placenta. Then 

 again, as the embryo is nearly all liver, and hangs sur- 



