DORIDOBIDES GARDINERI. 297 
the leaves of seaweeds, are devoid of gills makes it likely 
that these gill-less forms are more ancient than Pleuro- 
phyllidia, and not retrograde. 
Though Doridoeides is superficially not unlike Corambe 
(Doridella, Hypobranchiea) it is not nearly allied to 
either ‘the Corambidz or the Phyllidiide. Both these 
families are holohepatic, and have a totally different arrange- 
ment of the mouth parts: branchial lamellee situated beneath 
the mantle edge are found in all the genera comprised in 
them. 
The heart and circulatory system of Doridceides offer 
some points of interest. ‘The heart lies somewhat to the 
right of a median line drawn longitudinally through the 
viscera, and the auricle adheres to the right wall of 
the pericardium. This may be a reminiscence of an earlier 
arrangement in which there wasactenidium on the right hand 
side communicating with the auricle. The walls of the heart 
are thin, and in many sections the organ has an unsubstantial 
and shadowy appearance. The arteries also are thin, and 
hardly extend beyond the middle fifth of the body either 
backwards or forwards. They are developed most fully in 
the smaller specimens, and seem to atrophy as the animal 
grows. A similarly feeble development of the heart and 
circulatory system seems to occur in other gill-less nudi- 
branchs. The Scaphopoda have neither gills nor heart. 
Kovalevsky! had some difficulty in seeing the heart in 
Pseudovermis and Hedyle, and could find it only in one 
species of the latter. In Tritoniella the heart lies to the 
right of the median line, and Bergh says of Pleuroleura 
ornata, “die aorta konnte nicht verforgt werden.”? It 
would seem that in a gill-less mollusc the heart has no power of 
collecting purified blood and distributing it over the body, 
for the purification takes place all over the surface, not in a 
special organ. A strong pulsating machine and an extended 
1 «Mémoires de l’Acad. Imp. des Sciences de St. Pétersburg,’ vol. xii, 
Nos. 4.and 6. _ 
_ 2 © Mal. Unters.,” in Semper’s ‘ Reisen.,’ Heft vi, p. 284. 
