CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS. 
613 
interior of these processes, if not for the purposes of aeration ? In the solid parietes 
of these membranous processes no vestige of a blood-system can be discovered. 
Thejluid of the peritoneal cavity in all the inferior Echinoderrns is charged with 
globules or cells, which, however, are less determinately organized, and fewer in 
number, than those of the chylaqueous fluid of the superior genera. That of the 
Asteriadse and Echinida is less corpusculated than that of the Ophiocomidse and 
Ophiuridfie, the arms of which are unoccupied by the prolongations of any of the 
viscera ; that, on the other hand, which is found in the peritoneal space of the Holo- 
thuriadse, is probably charged with highly organized cells, and that too in presence 
of a complex blood-system, and of a peculiar organ expressly designed for the intro- 
duction of water into the interior of the body. If the contents of the peritoneal 
cavity consist of pure sea-water, whence the necessity for the superaddition of this 
peculiar and unparalleled organ, ‘the respiratory tree’? It is not the homologon of 
the peritoneal space of the Asteriadse, as maintained by physiologists, for this space 
as well as this organ exists in Holothiiria. It will be afterwards proved, unquestion- 
ably, that if the office of this ‘ tree’ be respiratory at all, it can discharge this function 
only in a secondary and incidental manner. 
In the peritoneal space of the SipuncuUdce there exists a large volume of opalescent 
fluid, holding in suspension corpuscles of definitive organization ; and yet the bulk 
and basis of this Jiuid also is salt water (figs. 6 and 7). They exhibit a pink hue, like 
those of the blood-proper, figs. 4 and 5 of the Sipuncles. 
They are flattened, irregularly oval cells, bearing a single minute nucleus. These 
cells, in fact, correspond in every minute particular of colour, structure, figure and 
dimensions, to those found in the blood-vessels of the same Sipuncle. Wherefore this 
identity? The question cannot be eluded. They be one and the same. Evapo- 
rated, chloride of sodium is disclosed in a rich crop of cubes and octahedrons. 
The morphotic elements of the fluid of the peritoneal cavity of Asterias ruhens, 
which in number and amount vary at different seasons, consist of irregular cells 
(fig. 8), jagged and broken in many instances, nucleated and perfectly organized in 
others; some cells are compounded of several minutely granulated secondary cells, 
the group being enveloped in an involucrum, from the circumference of which 
thready appendages project. These latter cells are not sperm-cells ; the thready 
processes are accidental formations, depending upon the fibrillation of the contents of 
the cells ; in some examples the threads occur under the character of a flattened pro- 
jection or bulging of the involucrum. A “ molecular basis ” may also be observed 
in form of minute cells and granules. If the specimen of Asterias, from which the 
fluid is taken, be allowed to remain some time out of the water before the examina- 
tion is made, these corpuscles may be obtained in far greater number. The cor- 
puscles in the water-vascular system or feet, are always found to be identical with 
those of the peritoneal f idd. The contents of the digestive caeca are identical in com- 
position with the peritoneal fluid ; it differs only in the absence of the largest cells of 
4 K 2 
