310 INVERTEBRATE MORPMOLOGY. 



Pterotrachea and Carinaria, by themselves, the formation of 

 a separate order for their reception would perhaps be justi- 

 fiable, but Atalanta shows their close relationships with the 

 Prosobranchia and indicates their true position as Monoto- 

 cardia. 



3. Order Opisthobranchia. 



The Opisthobranchs are exclusively marine forms, present- 

 ing numerous modifications of shape and structure, but all 

 agreeing in certain important particulars. The rotation of 

 the mantle-cavity has not proceeded quite so far as in the 

 Prosobranchs, the cavity and its organs lying upon the right 

 side of the body, but at the same time the abortion of the 

 organs of the primitively left side of the body has occurred. 

 Thus in those forms which possess respirator^' organs homol- 

 ogous with the branchiae of the Prosobranchs, but one (that 

 of the right side) is present, and with this character is associ- 

 ated the occurrence in the heart of but a single auricle, 

 which lies behind the ventricle. Only one nephridium occurs, 

 and a distinction from the Monotocardiate Prosobranchs is 

 found in the fact that the branchia when present lies as in- 

 dicated by the position of the auricle, behind the heart — the 

 name bestowed upon the order being suggested by this pecul- 

 iarity. 



A more important distinguishing character perhaps is, 

 however, to be found in the arrangement of the nerve-cords. 

 The rotation of the mantle-cavity and its associated parts has 

 not been carried to such an extent as to produce a crossing of 

 the pleuro-visceral connectives, which run more or less par- 

 allel with one another and present what is termed an ortho- 

 neurous arrangement in contradistinction to the chiastoneu- 

 rism of the Prosobranchs. In addition to this character a 

 tendency towards an aggregation of the various ganglia to 

 a complex mass lying behind the pharynx may also be con- 

 sidered a characteristic of the Opisthobranchs. One or both 

 parietal ganglia may disappear, and in some cases where 

 there is a marked concentration of the ganglia the visceral 

 ganglion may also be unrepresented, though usually from one 



