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projects all round in the form of a flap (the mantle skirt) 

 overhanging the head and the sides of the foot. The 

 space between the mantle skirt and the sides of the body 

 is the subpallial chamber, and into this the anus and the 

 nephridia and generative apertures open. From each side 

 of the body, rather far back, there grows out into the 

 subpallial chamber a process of the body-wall, consisting 

 of an axis (containing two blood vessels) from which a 

 double row of filaments hang down. These organs are the 

 gills or ctenidia (fig. 15, B and C). Near the base of the 

 ctenidium on the body-wall there is a sense organ (the 

 osphradium) composed of modified epithelium and having a 

 special ganglion and nerve in connection with it. Its probable 

 use is to test the quality of the water flowing over the gills. 

 The mouth leads into the stomodseum, an anterior portion 

 of the alimentary canal which is epiblastic in origin, and 

 which forms a muscular bulb. At the posterior end of the 

 intestine there is a short proctodaeum, which is also derived 

 from the epiblast. The alimentary canal between these two 

 terminal parts is the remains of the archenteron and is 

 therefore hypoblastic. An outgrowth from this region on each 

 side forms the liver. Around the alimentary canal is a cavity 

 (the coelom), which has been formed from diverticula of the 

 archenteron, corresponding probably to the intermesenteric 

 spaces in the Actinozoa, and in which the blood flows. 

 The ccelom is more or less broken up into small spaces, 

 but a large open region is found dorsally to the alimentary 

 canal, and in this (the pericardium) lies a muscular tube, the 

 heart, which propels the blood through ill-defined vessels and 

 lacunae, which are merely parts of the coelom. The heart 

 consists of two lateral dilated thin-walled sacs, the auricles, 

 which receive the purified blood brought back from the gills 

 and pour it into the median thick-walled muscular tube, the 

 ventricle, which on contracting drives the blood out through 



