5^ 



be discussed later in dealing' with the excretory 

 organs. 



Haller described a pair of coelomic cavities between the 

 visceral hump and the foot, but Pelseueer does not cnn- 

 firni 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, visil)le only in a very young 

 specimen iu which the sex products have not yet been 

 mncli differentiated, is necessarily a remnant of the 

 coeloni. It becomes practically obliterated at a later stage. 



liKSPIRATORY < )rGAXS. 



With the specialisati(m of the right side of the 

 branchial chamber as an excurrent channel for waste 

 products, the right ctenidium, we may supj)ose, dis- 

 appeared at an early stage in the descent of the 

 Docoglossa. In the less niodiiied members of this group, 

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

 formed mainly by the surviving' left gill, but iu 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 (Lotfia and Scurria], In Patella 

 both primary gills are reduced, being represented only 

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

 vestiges are containeul, is equivalent to the branchial 

 chamber of a Pleurotomaria, FismreUa, or Acmoca in a 

 reduced condition. This chamber, however, still plays 

 a subordinate part in respiration, although tliat function 

 is mainly effected by the circlet of pallial gills, which 

 have now attained a high degree of development. 



