616 
DR. T. WILLIAMS ON THE BLOOD-PROPER AND 
position admits, that the peritoneal cavity in these Echinoderms, like that of the 
Asteriadee, does not openly communicate with the exterior. The fluid contents enter 
at the mouth, perform a part in the first stage of digestion, and then carry a portion 
of the product thereof in solution, probably by exosmose, into the open cavity of the 
body, where the fluid is set in motion, determinately, by the agency of vibratile cilia, 
travels round and round the concave of the shell, penetrates the hollow axes of all 
tlie membranous processes of the shell, where it experiences the change of oxygenation, 
conveys the results of this change to the blood-proper, and replenishes the water 
system or ambulacral feet. It contains flattened corpuscles (fig. 11), the largest of 
which are provided with an involucrum, bearing particles of limpid oleine. A cell 
here and there may be seen, the involucrum of which apparently projects out like a 
cilium, and when these are numerous it is easy to mistake such an appearance for 
those characteristic of a sperm-cell. It is really due to the fibrinous contents coagu- 
lating in lines on escaping. When a small test-tube is filled with the peritoneal fluid 
(an experiment which demands the sacrifice of ten or twelve individuals), nitric acid 
will prove clearly the presence of albuminous principles. From the total absence of 
proper-blood-vessels in the membranous structure and processes of the shell, or inte- 
gumentary system of Spatangus and Echinus, from the fact that these latter processes 
are hollow, and openly communicate with the chamber containing the chylaqueous 
fluid, by which they are filled, and that their parietes within and without are pro- 
fusely lined with cilia, the conclusion is not to be disputed, that they constitute the 
real organs of breathing, and that the real subject of the respiratory change is the 
chylaqueous fluid, and not the true-blood, which is limited to the central viscera in 
its circulation. This conclusion, drawn by fair induction from anatomical and phy- 
siological considerations, is corroborative of the view suggested by the results of 
microscopic and chemical inquiries, that the fluid contents of the peritoneal cavity 
in the Echinidm, however nearly in appearance they may resemble sea-water, are 
vitally and organically endowed. In the Ophiuridse and Ophiocomidae (fig. 12) the 
chylaqueous fluid, which occupies the cavities of the body and the hollow axes of the 
arms, is essentially similar in composition to that of the former Echinoderms. In 
these genera also it is unquestionably the fluid which is really oxygenized in respira- 
tion. An exact and faithful account has now been given of the chylaqueous fluid 
of the inferior families of Echinoderms. This distinction must be noted between it 
and that of the superior genera; that in the Asteriadae, Echinidae, Ophiocomidae, and 
Ophiuridae, the morphotic elements have not, taken as a whole, attained to the cha- 
racter of definitively organized cells. They do not in these genera conform so 
obviously to one typical size and figure, as to show that they are produced and 
multiplied under the directive influence of a determinate ^ovcq. They are unclassifiably 
various in form and shape. They are too scanty in number to act an important part 
in vitalizing the fluid in which they are suspended ; though it must again be repeated, 
this fluid is unquestionably an albuminous, living, corpusculated solution. These 
