

PATELLA. 247 



and, from this apex, the third side runs straight up to 

 the mantle (fig. 8a). A pallial gill is merely a down- 

 growth from the mantle skirt, and both, therefore, have 

 an essentially similar structure. It is covered by 

 epithelium, with many cells ciliated, some glandular, and 

 some sensory (fig. 9b) ; beneath this is a nerve plexus, 

 with a few multipolar ganglion cells (fig. 9b). Within 

 this we find a comparatively small amount of muscle 

 fibre and other tissue enclosing a large blood space sub- 

 divided by fibrous trabecular (fig. 9a). Blood enters the 

 inner border of the gill and leaves from its outer border, 

 and in these two positions we, therefore, find rather large 

 and distinct blood spaces. 



We may now shortly summarise what has been said 

 concerning respiration and the circulation of the blood 

 in the mantle. 



Blood comes in from lacunas running between the 

 fasciculi of the shell muscle ; it is then distri- 

 buted both to the pallial gills (through a blood 

 channel running ventrally and reaching the inner 

 border of the gill) and to the mantle proper (through 

 lacunae running further dorsally). In the latter set of 

 lacunae, the blood seems mostly to reach the mantle edge 

 whence it returns through channels which project like 

 veinlets on the ventral surface of the mantle. In the 

 former set of lacunae it is distributed over the gills and 

 is then collected into ventrally placed efferent lacunae 

 (fig. 7), which leave the outer border of the gill 

 and join the veinlets from the mantle edge. The united 

 sinus opens into the inner side of the great pallial vein 

 (fig. 7). Though the pallial gills and the nuchal roof are 

 the only distinctly respiratory organs, it must not be 

 supposed that they are the only places in which blood is 

 oxygenated. The skin in several parts seems to be 



