216 



METAZOAN PHYLA 



suprabranchial chambers unite, and the passage between the two inner 

 plates of the inner gills and the body mass is really narrowly U-shaped, 

 inclosing the body mass behind. The outer suprabranchial chambers 

 enter the chamber formed by the union of the inner suprabranchial 

 chambers and in the wall of the common passage is the anal opening. 

 From the anus onward the passage may be called a cloaca; it opens to the 

 outside through the dorsal siphon. The water which enters through the 

 ventral siphon fills the mantle cavity (Fig. 129), bathes the gills, enters 

 the water tubes, passes up them to the suprabranchial chambers, and by 

 means of the cloaca and dorsal siphon escapes from the body. A current 

 is maintained by cilia which cover the walls of these passages. 



251. Digestive System and Metabolism. — The mouth is an opening 

 on the dorsal wall of the mantle cavity toward the anterior end. On 



Openiha of 



Suprabra/^ch/a/ 

 chambers 



Cloaca 



Dorsa/ 

 siphon 



Ventral 

 siphon 



Labial 



patps 



Mant/e cayj'fy Oufer ^//l 



Fig. 129. — Diagram to illustrate the circulation of water through a fresh-water mussel. 

 The suprabranchial chambers are shown as if cut open. The current is maintained by cilia 

 on all the wall surface of the mantle cavity, aided by movements of the gills and labial 

 palps. The labial palps also direct the food particles into the mouth. 



each side of it is a pair of triangular labial palps (Fig. 128). Cilia on the 

 surface of these palps help carry the food into the mouth. From the 

 mouth a short esophagus leads to a broadly dilated stomach, which receives 

 the secretion of the liver. From the stomach a narrow coiled intestine 

 leads to the cloaca, through which the feces obtain exit from the body. 



The food of fresh-water mussels is made up of particles of organic 

 matter, including microscopic plants and animals, brought in through the 

 ventral siphon. These food particles are strained out as the water passes 

 through the gills and are carried toward the mouth by cilia, the labial 

 palps collecting and directing them into the mouth opening. 



252. Circulatory System. — The circulatory system includes a heart 

 made up of a ventricle and two auricles. It lies in a pericardial cavity 

 in the dorsal part of the body mass (Fig. 128), and a portion of the intes- 

 tine passes through the ventricle. From the ventricle the blood is dis- 



