CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS. 
625 
only indirectly, either through the blood-proper, which in this species immediately 
receives oxygen from without, or through the alimentary system, which is being con- 
stantly traversed by a current of fluid-sand. These facts are here noticed in historic 
connection with the chylaqueous fluid, because they tend to elucidate its peculiar 
laws. 
Between the corpuscles of the chylaqueous fluid in Nais Jiliformis and those of 
Arenicola, a close resemblance is observable, the same disposition to fibrillate on 
bursting. The typical cell is orbicular in shape, nucleated and granular (fig. 16). 
Another and more embryonic variety presents only a nucleus, the granules being- 
wanting. In Nais the chylaqueous fluid is colourless, and considerable in volume; 
it is the 'bed’ on which the intestine moves. In the several species of the genus 
Nais, these corpuscles present very palpable diversities of shape, though not of struc- 
ture. There are no external organs in this genus, either for the exposure of the chyl- 
aqueous fluid or blood-proper to the aerating element. As the latter is centrally 
situated, and therefore embraced by the former, it is manifest that the chylaqueous 
fluid is the more directly affected of the two by the external oxygen. The peculiar 
disposition of the vessels in Nais renders this inference only the more probable. 
In Sahella vesiculosa (fig. 17), and in a less marked degree in Sabella alveolatu, 
the general cavity of the body is filled with a fluid, the cells floating in which incline 
to a uniform spindle-shaped form. Those which are spherical appear to be only 
the immature phase of the former variety, and between the two there are several 
intermediate grades. They are almost wholly devoid of internal molecules, being- 
filled with a fluid held together by a capsule of determinate form. In this species, 
as in the succeeding, the chylaqueous fluid plays no direct part in the office of respi- 
ration. 
In S. alveolata (fig. 18) the chylaqueous fluid is less marked in volume; its cor- 
puscles are spherical and granular, abounding in non-nuclear oleous cells. The 
blood-system is highly developed. The intestine is tied to the integument by frequent 
septa. 
The splanchnic cavity in Sahella a sang vert of M. Edwards contains a colourless 
corpusculated fluid, the cells floating in which are less uniform in size and figure than 
the corresponding bodies in the preceding species. They are generally observed 
under the form of small orbicular cells, becoming flattened as they grow older, and 
unevenly outlined; many of them exhibit that curious tendency to protrusion of the 
cell-capsule which so frequently characterizes the corpuscles of the chylaqueous 
fluid. They are all more or less charged with molecules of oleine of high refractive 
power (fig. 19). In this species, as first discovered by Milne-Edwards, the true-blood 
is grass-green in colour. The branchial appendages, cephalically situated, and pecti- 
nated in form, are penetrated by the latter fluid, very little, if at all, by the former. 
In this species the chylaqueous fluid is excluded from all direct participation in the 
process of aeration. 
