184 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



as in the crayfish, open into spaces in the ccelom, 

 thence to be taken up again by the vessels which pass 

 to the heart (veins), but they are conveyed through 

 networks of fine hair-like vessels (capillaries), which 

 are completely closed. So, again, the chyle, or the 

 direct result of the products of digestion, is contained 

 in vessels which make their way into the veins (the 

 more or less completely closed lymphatic system). 



In the MoUusca and Arthropoda the blood- 

 vascular system is not completely closed ; in other 

 words, the blood makes its way from the arteries into 

 spaces or cavities in the body cavity, and from these 

 incompletely closed spaces it is again taken up by the 

 veins ; it is clear, therefore, that no closed or proper 

 lymphatic system is here needed ; the results of the 

 process of digestion make their way through the walls 

 of the intestine directly into the ccelom, and thence 

 into the sinuses, and in this way they replenish the 

 store of corpuscles in the blood of the crayfish or the 

 mussel. 



In the Echinodermata the blood-vascular sys- 

 tem would appear to be completely shut off from the 

 ccelorn, and, as it would seem to be connected with the 

 system of water vessels, it is, no doubt, diluted by 

 sea-water ; but the fluid in the coelom contains char- 

 acteristic corpuscles, some of which are coloured. 

 Curiously enough, liEemoglobin has been detected in 

 the water-vascular system of an Ophiuroid (Foettinger). 



In the Protozoa there is, of course, no blood, 

 but even in the Amoeba we observe currents within 

 the protoplasm, and some of these are, no doubt, 

 richer in nutriment than others, so that by their 

 movement the distribution of nutritious material is 

 equalised over the whole organism ; in Paramcecium 

 there is an advance on this, inasmuch as in it currents 

 of definite directions are to be detected. In the 

 Sponge the currents of water that traverse its walls 



