CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS, 
607 
photically identical with those of the water-vascular system and with those again of 
the visceral cavity. 
I’he unity of the two last systems of fluids has been recently advocated by Milne- 
Edwards. — “ M. Milne-Edwards s’est assure qu’une communication semblable existe 
entre la cavite generale du corps et les coecums exsertiles des Echinodermes. Ces 
coecums, bien distincts des pieds a ventouses, sont distendus non pas par le sang, mais 
par le liquide de la cavite generale. Quelques observations personnelles me portent a 
penser qu’il en est de meme des pieds a ventouses eux-memes*.” 
M. Quatrefages, by whom this opinion of Milne-Edwards is expressed, does not 
state the grounds upon which this latter physiologist rests his conclusions. I have 
been independently induced to ground the same belief upon the following demonstra- 
tions ; — 
1st. Injection thrown into the water-vascular system in the Asteriadae will Jirst Jill 
the sand-canal, and then, by continuation of the injecting force, escape into the peri- 
toneal cavity. 
It is by this route, I infer thence, that a direct communication takes place between 
the fluid contained in the visceral cavity and that of the water-vascular system. 
This office implies an adequate function to the sand-canal. It is a simple filterer 
of the fluid passing from the visceral chamber into the water-vascular system, and 
conversely. Muscular compression exerted upon the fluid in the water-vascular 
system (the walls of which are remarkably muscular) will force it hack into the 
visceral cavity; a ‘diastole’ of the vesicles of the feet occurring coincidently with 
the compression of integumentary parietes of the general cavity, will cause a return 
of it again into the blood-vascular system. This is a beautiful and perfect mechanism. 
The grounds whereon the identity of the fluid contained (in the Echinoderms) in 
the system of the blood-proper with that which fills the water-vascular system and 
the abdominal cavity is maintained, may be briefly stated as follows: — In Uraster 
pajypnm the oral membranous disc exceeds very much in diameter that of the common 
Asterias. This circumstance renders the circular central blood-vessel in the former 
much more conspicuous and accessible than in the latter. It was this fact which led 
me to make choice of Uraster as the subject of my researches. In this species the 
circular channel forming the centre of the blood-system coincides with the circum- 
ference of the oral membranous disc. The vascular channel (scarcely to be called 
a vessel) rests upon the hard edge of the calcareous framework. It is so closely and 
intimately adherent to the hard unyielding surface beneath, that ‘the conditions’ 
seem to be destroyed which might permit the contractions of this vessel as a cir- 
culating centre. No instance is known in which a contractile heart is thus anatomi- 
cally connected : nor are the coats of this circular vessel, in Uraster papposa, endowed 
with muscular fibres. It is lined internally as well as externally by vibratile epithelium. 
* Quoted by M. Quatrefages in his Memoire, ‘ Sur la cavite generale du corps des Invertebres,’ Annal. des 
Sc. Nat. 1850. 
