CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS. 
619 
There exists no difficulty whatever, in the case of the Sipunculidse, in proving- be- 
yond doubt, that in them, at all events, the sea-water (which in these orders, as in 
the inferior Echinoderms, constitutes the bulk and basis of the chylaqueous fluid) is 
NOT derived directly from without. I have repeatedly shown, by variously contrived 
injections, that no fluid whatever will escape externally through the skin or integu- 
ment, and that the hollow membranous appendages at the head, and which the 
animal exserts hy distending or injecting them with the fluid of the peritoneal cavity, 
proving their hollowness, are coecal at their distal extremities. As far as it is possible 
to arrive at certainty in anatomical demonstrations by negative proofs, it may now 
be held as established, that in all Echinoderms, to which these proofs apply, i. e. the 
Asteriadee, Echinidse, Holothuriadae and Sipunculidse, there exist no perforations or 
orifices in the integuments in any part, or under any form, through which the external 
water can directly gain admission into the peritoneal cavity. The inference is then 
irresistible, that it enters at the mouth in form of alimentary material, reaches the 
digestive canal, and thence passes into the great cavity which surrounds the viscera. 
In the Echinodermata, as a class, it is impossible to dispute the importance of the 
functions enacted by the fluid contained in the peritoneal cavity (profusely ciliated 
as are its walls), when regarded especially in connection with the peculiar structure 
and situation (when distributed universally over the integumentary surface of the 
body, as in the Asteriadee and Echinidae, or in part centralized at the head, as in the 
Sipunculidse and Holothuriadae) of the organs of respiration. 
I have already lamented that my opportunities of examining the Holothuridan Echi- 
noderms, in the living state, have been few. Several points of sur()assing interest, as 
the climax and triumph too of the preceding inquiries, in the anatomy of these genera 
demand scrupulous revision. 1st. Does the water admitted into the “respiratory 
tree” serve to aerate the blood-proper or the chylaqueous fluid? This question can 
only be answered by first determining the exact situation of the blood-vessels in rela- 
tion to the parietes of this unprecedented organ. 2nd. Are the tentacles the scene 
of a double respiratory process, by which the blood-proper and the chylaqueous fluid 
are aerated simultaneously ? Analogy renders it certain that the integuments of these 
genera, like the Sipunculidans, are ‘ fenestrated,’ and that with express view to the 
aeration of the contents of the visceral cavity. These inquiries have thus placed in 
clear light the interesting fact, that there prevails but one essential type or plan of struc- 
ture in the integumentary system of all Echinoderms, and that the blood-proper may 
have its own respiratory apparatus, or that it may be aerated through the medium of 
or by the chylaqueous fluid, itself having first received oxygen from the surrounding- 
element. 
The Entozoa constitute, reallv, the true commencement of the Annelida in the 
rest. At present I will only commit myself to the statement, that in the fluid of the peritoneal cavity of the 
Sipunculidae the germ-cell-W^Q bodies are veritable ova, and the sperm-cells are veritable spermatozoa. 
4 L 
MDCCCLII. 
