374 E. W. MACBRIDE. 
the intercellular jelly or plasma has acquired staining properties. 
To Leipoldt (9) is due the credit, in a careful paper on the 
anatomy of *‘ the so-called excretory organ of the sea-urchin,” 
of emphasising the fact that the ovoid gland and the oral blood- 
ring are of totally different nature; he describes branches from the 
blood-ring ramifying on the external surface of the ovoid gland. 
The question arises, what is the true nature of this blood- 
ring? Cuénot (8) answers that it is a lymphatic gland, or 
centre for the formation of amcebocytes; and there is a great 
deal to be said for this view. We must, however, remember 
that structures of similar nature are found accompanying the 
alimentary canal in Echinids and Holothurids. Ludwig (18) 
has given a splendid description of their arrangement in the 
last group. He brings out with great clearness that they are 
tracts of connective tissue in which the fibres are sparse. The 
close relation of these “ vessels ” to the alimentary canal suggests 
forcibly that we may have here the first attempt at forming 
blood-vessels. There is certainly no propulsive organ or proper 
circulation, but the staining properties of the plasma show that 
it has been chemically altered, and the idea is suggested of 
some secretion from the gut-cells propelling itself along these 
tracts by the vis a tergo force of secretion. In the Asterid 
no close connection with the gut is observable,—the oral ccelom, 
in fact, intervenes between the cesophagus and the ring, as we 
have seen (p. 365); but the altered character of the plasma 
suggests that perhaps here some substance is formed necessary 
for the well-being of the organism, which then diffuses out into 
the neighbouring coelomic spaces. The blood-spaces of the 
higher animals are known in many cases to be remnants of the 
blastoccele or segmentation cavity of the embryo; this has been 
shown in the case of Balanoglossus with great clearness by 
Spengel (21). Strictly speaking, therefore, the blood and 
lymph spaces of other forms are represented in 
Echinodermata by all the spaces in the body-wall 
unoccupied by fibrous tissue and dermal ossicles, and 
traversed by amcebocytes; but the blood-ring, gut vessels, 
&c., may be a first attempt at specialisation. 
