256 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 ? Cuenot (3) answers that it is a lymphatic gland, or 

 centre for the formation of amoebocytes; 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 (13) 

 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 coelom, 

 in fact, intervenes between the oesophagus and the ring, as we 

 have seen (p. 247); 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 amoebocytes; but the blood-ring, gut vessels, 

 &c., may be a first attempt at specialisation. 



