LABIAL PALPS. 
191 
Though a very constant characteristic of the Pelecypoda, they 
are apparently quite absent in some few marine genera, but in 
others are so greatly developed as 
to be even larger than the branchipe; 
they have been termed the lesser 
and the accessory branchife by 
Swammerdam and other authors, 
as their gveat vascularity and posi- 
tion, each in connection with a large 
pallial sinus, suggests some connec- 
tion with the respiratory function, their ctenidiform appearance being 
materially increased by the jiresence of numerous transverse folds or 
ridges, which are most pronounced to- 
wards the distal margin, becoming less 
distinct or even deficient as the base of 
attachment is approached. These trans- 
verse ridges also vary considerably in 
number and relative size, according to 
the species, those of Unio being propor- 
tionately larger than those of Anodonta, 
and much more so than Dreissensia, in which there are thirty-five of 
these ridges on each palp, six times finer than the branchial filaments 
In Anodonta there are sixty-five ridges on each palp, while in 
Spluvrium corneum var. nucleus there are only twelve. 
The bases of the palps are practically almost joined to and continuous 
with the lines of attachment of the gill-lamelhe, the anterior palps 
being apparently continuous or coterminous with the outer gill- 
Fig. 375. — Anodo7ita cygnca, with, the palps 
and gill-lamellae removed to show the apparent 
continuity of the labial palps and the gills (after 
Lankester). 
a. anus ; a. ad. anterior adductor ; a.p. line of 
attachment of anterior palp, practically continuous 
with line of attachment of outer gill ; f. foot ; 
g.o. left ureter ; l.c. axis of the left ctenidium or 
gill, showing lines of attachment of inner and outer 
gills; m. mouth ; n.o. orifice of left reproductive 
gland ; p.ad. posterior adductor ; p.p. line of 
attachment of posterior palp, practically con- 
tinuous with line of attachment of inner gill. 
laminm and the posterior palps being similarly continuous with the 
line of attachment of the inner branchim, leading Prof. Lankester to 
suggest that the branchim and labial palps may be modifications of a 
double horse-shoe shaped area of ciliated filamentous processes which 
existed in the more primitive mollusk. 
Fig. 374. — Four ridges of the 
labial palps of Afiodonta cygnca 
highly magnified (after Moquin- 
Tandon). 
Fig. 373. — Three ridges of the labial palps 
of Afiodonta cygnea, to show the arterial 
vessels, highly magnified (after Keferstein). 
