PATELLA. 245 



be discussed later in dealing with the excretory 

 organs. 



Haller described a pair of coelomic cavities between the 

 visceral hump and the foot, but Pelseneer does not con- 

 firm this, and our results tend to the conclusion that the 

 only epithelium-lined cavity in this region is that of the 

 large kidney. 



The cavity of the gonad, visible only in a very young 

 specimen in which the sex products have not yet been 

 much differentiated, is necessarily a remnant of the 

 coelom. It becomes practically obliterated at a later stage. 



Respiratory Organs. 



With the specialisation of the right side of the 

 branchial chamber as 1 an excurrent channel for waste 

 products, the right ctenidium, we may suppose, dis- 

 appeared at an early stage, in the descent of the 

 Docoglossa. In the less modified members of this group, 

 Acmoea, etc., the work of respiration is, therefore, per- 

 formed mainly by the surviving left gill, but in part also 

 by the mantle skirt, which has increased in importance as 

 the shell became more cup-like, and its projecting edge 

 spread farther out. The mantle skirt in these Mono- 

 branch forms already shows a tendency to the formation 

 of a series of transverse ridges, constituting incipient 

 secondary or mantle gills (Lottia and Scurria). In Patella 

 both primary gills are reduced, being represented only 

 by vestiges (fig. 4). The nuchal chamber, in which these 

 vestiges are contained, is equivalent to the branchial 

 chamber of a Pleurotomaria, Fissurella, or Acmcea in a 

 reduced condition. This chamber, however, still plays 

 a subordinate part in respiration, although that function 

 is mainly effected by the circlet of pallial gills, which 

 have now attained a high degree of development. 



