326 ZOOLOGY. 



what after the manner of the atrial folds in A mphioxus, 

 though unlike those they do not fuse in the ventral middle 

 line. Thus there is enclosed by these folds a mantle- 

 chamber, and it is into and from this that the water- 

 currents already mentioned flow. It is a comparatively 

 large chamber, so that the actual body of the animal is 

 much smaller than one might at first imagine, 



3. General Structure of Body, Fig. 168 (transverse 

 sections) will give an idea of the size of the mantle-chamber 

 and of the true body ; while fig. 167 shows a side-view of 

 the body and indicates the chief regions into which it may 

 be divided. Anteriorly we have a region composed entirely 

 of muscle the anterior adductor. Behind this comes the 

 great visceral mass in which most of the digestive and 

 reproductive systems are contained, the cavities of the 

 latter being part of the ccelom. From the ventral surface 

 of the visceral mass there is developed a median muscular 

 mass the foot the only locomotor organ of Anodonta. 

 Postero-dorsally lies the pericardial region, also con- 

 taining a ccelomic cavity. In this region we have part of 

 the alimentary canal, the heart and related blood-vessels, 

 and the excretory organs. Finally, at the extreme posterior 

 end is another muscle the posterior adductor. 



4. Gills and Labial Palps. Certain outgrowths from 

 the body-wall occupy a good part of the mantle-chamber. 

 The disposition of these structures may be understood from 

 a careful comparison of fig. 161, which is a diagrammatic 

 side-view, with the series of transverse sections given in 

 fig. 162. 



There are four * gills in all, distinguished as inner and 

 outer of the right and left side respectively. Each gill is 

 further composed of two plates or lamellae, united along 

 their ventral border and at both ends, so that they form a 

 pouch whose opening is directed dorsalwards. The dorsal 



* Strictly, there is a single pair of ctenidia, each ctenidium having 

 an axis and two plates, which latter are doubled back on them- 

 selves. But in the absence of any other Molluscan type to compare 

 with, the terms given in the text are sufficient. 



