48 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Oncidiidas, but not in tho Opisthobranchiata, while there is no blood- 
gland, which is very frequently present in Opisthobranchs. Though of 
less significance, the enteric canal speaks also to the Pulmonate nature 
of the Oncidiidaa. 
If we allow their Pulmonate nature, we find that the forms under 
discussion are much nearer the Basommatophora than the Stylommato- 
phora. After pointing out the characters in which they agree with the 
former, he argues against the value of the points in which they resemble 
the latter. 
He concludes that the tectibranchiate stem-forms of the Pulmonata 
were shore-dwellers, which, in consequence of the ebb and flow of the 
tide, took to an amphibious mode of life, and converted the gill-cavity 
into a lung. They were thus, though still retaining their shell, 
Oncidium-like, and further differentiation followed, according as they 
remained on the sea-coast, wandered into fresh water, or became purely 
terrestrial animals. 
At the same time Dr. Plate allows that the Oncidiid® have so many 
points of resemblance to the Opisthobranchiata that the similarity 
cannot be supposed to be due to chance convergence, but must be 
regarded as the consequence of a common phyletic origin. These points 
of resemblance justify us in considering the Oncidiidae as archaic forms 
which stand closer to the Opisthobranchlate stem-form of the Pulmonata 
than any group of Pulmonates now known. Thus, the Oncidiidae are 
opistliopneumonous, the liver is tripartite, there are only three ganglia in 
the visceral chain, and the penis is armed with teeth, as it often is 
among Opisthobranchs. 
Although, then, the Oncidiidae exhibit the structural characters of 
the tectibranchiate primitive form of the Pulmonata more truly than 
any other division, it is equally true that in several points they have 
undergone secondary modification. In the first place they have lost the 
shell and the columellar muscle, as well as the olfactory organ and 
the jaw ; the kidney projects so far into the lung-chamber that it becomes 
partly fused with the base thereof ; the anus lies outside the lung- 
chamber ; the aorta does not divide until some distance from its root, 
and the posterior aorta breaks up into two chief branches, a visceral and 
a genito-pulmonary artery ; the back has developed special eyes, and in 
some cases arborescent appendages. 
The author sums up his results in the following table : — 
Tectibranchia 
