CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS. 
611 
peculiar to, and strikingly characteristic of, the nutritive fluids of the Sipunculidse. 
In different species they exhibit slight variations of form and size. They are the 
proper and invariable morphous elements of the fluids in these genera. In many 
instances however there exist with them in the chylaqueous fluid, sperm-cells and 
ova in variable proportions. 
These corpuscles, when observed in the chylaqueous fluid, are not mutually repul- 
sive, but when examined while yet in the blood-channel, they manifest the most 
extraordinary restlessness. The motion of the individual particles is singularly irre- 
gular, as though due to attractive and repulsive forces acting among themselves. 
The compression of the object under the microscope increases the confusedness of the 
motion. But what is the real explanation of a phenomenon so unusual in the history 
of blood-vessels? Are the corpuscles self-motive, or is their agitation excited by an 
external cause ? The former question is at once set at rest by the observation that 
when the corpuscles escape out of the vessel, their motion instantly ceases. The 
cause is not therefore in themselves. The real exciting force is derived from the 
vibratile epithelium by which the interior of the vessel is lined. But is not this 
extraordinary fact opposed at once by every dictate of analogy ? It is, though not by 
that of the analogy of the blood-system in other genera of Echinoderms. Never 
before has the physiologist recognised the anatomical incipiency of the blood-vascular 
system of tliis class. It is the first grade of a novel apparatus in the living organism. 
Viewed in tliis relation of imperfect tievelopment, the presence of cilia on the interior 
of its conduits occasions no surprise. From the utter insignificance in magnitude 
and distribution of the blood-proper system in the economy of the Echinoderms as 
compared with the chylaqueous, it is beyond dispute that it is upon the latter that 
devolve the vital offices of nourishing the solids. I have lately discovered that the 
integuments of all the nude Sipuncles is remarkably fenestrated, not with open per- 
forations, but with elliptical spots closed only by a thin ciliated epidermis ; nothing 
therefore but a delicate cuticle intervenes between the chylaqueous fluid contained 
in the visceral cavity and the aerating medium without. No mechanism can be better 
adapted to favour the interchange of gases between the fluids divided by such a par- 
tition. In the Sipunculidse the tentacles are hollow appendages, ciliated within and 
without, and penetrated freely by the chylaqueous fluid. It seems hopelessly im- 
practicable to demonstrate any traces whatever of blood-vessels proper in the parietes 
of these tentacles. They appear therefore to be designed to expose, for the purposes 
of respiration, the chylaqueous fluid rather than the true-blood. It is however pro- 
bable that in the Holothuridan genera, as in the majority of Annelids, the true-blood 
has its respiratory apparatus, while the chylaqueous fluid, in the same individual, 
has distinctly another. With reference, then, to the hlood-proper of the Echinoderms, 
it may be stated that, in the genevsL Asteriadw and Echinidoe, including the Ophiuridue 
and Ophiocomid(e, the morphotic elements are almost unformed, the fluid is almost 
a non-corpusculated albuminous solution. In the Sipunculidan and Holothuridan 
4 K 
MDCCCLII. 
