290 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



(ii.) A thin plate of connective tissue may also be 

 traced downward below the intestine, as a sort of ventral 

 mesentery, which connects the intestine to the integument 

 of the roof of the mantle cavity, and thus divides the 

 body cavity into halves. 



2. On each side of this mesentery, notice the sections 

 of the foot-retractor muscles (Fig. 147, s). 



3. The remainder of the body cavity is filled, on each 

 side of the partition, by a dark-colored, glandular organ, 

 with very thick, delicate, plicated walls, enclosing an ir- 

 regular cavity (Fig. 147, t). This structure is the gland- 

 ular portion of the organ of Bojanus. Notice that the 

 halves of this organ are entirely separated by a partition. 



4. The body cavity is limited below, as in the preced- 

 ing section, by the ciliated layer of the integument of 

 the mantle. 



c. The mantle cavity. 



This contains the gills, and is now divided, by the at- 

 tachment of the gills to the body, into four chambers 

 (Fig. 147, h, t, f, &)- 



1. The lower or branchial chamber (h) presents sub- 

 stantially the same features as before. 



2. The cloacal chamber (d, of Fig. 146), is now divided 

 into three chambers. 



(i.) A central chamber (&) which lies above the two 

 inner gills, on the median line. 



(ii.) Two lateral chambers (i, i) which lie above the 

 outer gills, and which may be called the cloacal tubes of 

 the outer gills. 



d. The gills. 



1. Note that the upper edge of the outer lamella (n) of 

 rach outer gill (I) is attached as before, to the mantle. 



2. The inner lamellae (r) of the inner gills (m) are united 

 to each other, but not to the roof of the mantle cavity. 



