The Morphology of a Solenogastre. 725 
system of muscles in the chitons or for that matter to other classes 
of mollusca. It is possible that a deep seated resemblance exists 
but in view of the fact that there are not more than half a dozen 
species of molluscs whose musculature has been carefully studied 
such data are at present not of great value. 
There appears to be no very strong reason for a disbelief in 
the homology between the mantle cavity and its complex in the 
solenogastres and other molluscs. Without doubt typical ctenidia 
do not exist in the Neomenüdae but in the Chaetodermatide they cer- 
tainly do. In comparing the gill of the present species with one 
from Cryptochiton stelleri it is evident that each has a central axis, 
with ciliated lamellae, those of one side alternating with the others, 
similarly related afferent and efferent sinuses each of which contains 
or is in close proximity to a nerve from the lateral cord, and even 
in the arrangement of the high ciliated epithelial cells along the 
upper and lower surfaces of the axis the two coincide. To consider, 
in the face of such remarkable resemblances, that the branchiae of 
solenogastres and chitons have had an independent origin and that 
therefore those of the first group are pure simulacra is certainly 
taxing our faith in the evidence supplied by comparative anatomy 
without yielding any adequate result. 
Regarding the polybranchiate character of the chitons there 
is very little to determine if it be primitive or not. The hope has 
been expressed that the study of the development of the ctenidia 
might throw some light upon the subject but in the examination of 
three species (Isch. magdalenensis, Isch. regularis and Trachydermon 
raymond) I have found that the most posterior gill arises first at 
a time when the surface of the body becomes too small to supply 
sufficient oxygen. In the above named species this occurs when 
the young are about 1.5 mm in length. Within a very short time 
another gill appears anterior to the first and so on at very definite 
time and space intervals the more anterior ones appear. Each gill 
arises aS a small cone-shaped projection which soon developes two 
or three lamellae to which others are added as the gill increases 
in size. There is nothing from these facts to determine whether 
two gills were present in the primitive state or a larger number. 
The great difference in the size of the foot in the solenogastres 
and chitons and in the relative mass of the mantle is correlated 
with many differences in the plan of the circulatory system and yet 
as the study of Limifossor clearly shows there is an even more 
