294, SIR Cy ELIOL AND! Ts, EVANS: 
penetrating to mantle margin; cnidocysts present. One 
spermatotheca : hermaphrodite gland a single mass, separate 
from the liver. Most of the species are small. Some attain 
a length of 35 mm.! 
Gill-less molluscs are on the whole smaller than the corre- 
sponding forms with gills, and the genera less numerous in 
species and probably in individuals ;? but except for this the 
presence or absence of gills seems to make no difference, 
similar forms being found with and without them. A re- 
markable instance of thisis seenin the Docoglossa. Acmea 
has a ctenidium only; Scurria a ctenidium and a circle of 
accessory pallial branchiz ; Patellide the circle of pallial — 
branchiz only; Lepetide and Bathysciadium no 
branchizee whatever. Yet all these forms seem to lead the 
same kind of existence, and to thrive equally well. Simi- 
larly in the Pteropoda we find Dexiobrancheza with only 
a lateral gill, corresponding to a ctenidium; Pneumo- 
derma with this lateral gill and secondary terminal gills as 
well; Notobranchea with these secondary branchie only ; 
and Clione with no branchie at all. 
In the other Opisthobranchiata the combination of a 
ctenidium with accessory branchizw has not been recorded, 
but we find, though not within the limits of one family, 
Pleurobranchea with a fully developed ctenidium, 
Tritonia with no ctenidium but with secondary pallial 
branchie, and Tritonidoxa with no branchie at all. The 
ctenidium in the Pleurobranchide does not show any sign 
of becoming vestigial® preliminary to its disappearance, and 
the suddenness of the change from the Tectibranchs to the 
1 On the hypothesis that the Hlysioidea are derived from the Lophocercide, 
it is noticeable that gill-less forms make their appearance near what may be 
supposed to be the point of origin. In the Limapontiide the liver is not 
much ramified, and there is no trace of gills. In Klysia a number of ridges 
radiate from the pericardium, but hardly form branchial lamellae; the liver is 
ramified in the wide lateral wings, much as in Doridoeides. - 
* But the Pteropod Clione is found in large shoals. 
3 In the Peltide (which appear not to be derived from the Pleurobranchide) 
the gill is small and simple. 
