280 TADPOLES OF THE SEA 



oxygen to the blood, and as a flushing, purifying current, 

 carrying away the refuse of the gut. I have mentioned 

 that the Ascidian has blood. Its blood is colourless, but 

 is contained in a fine branching system of blood-vessels, 

 which pass, as two branching trees, one from each end of a 

 muscular, regularly beating little heart. There is a curious 

 thing about the Ascidian's heart, it receives the blood 

 from the set of vessels at one end of it, and pumps it on 

 into the vessels at the other end. After some twenty or 

 thirty beats it stops, and then its wave of contraction 

 starts again, but in the reverse direction, so that what 

 were veins bringing in the blood become arteries carrying 

 it away, whilst the former arteries act as veins. After 

 the same number of beats or contractions of the heart 

 there is again a brief pause, and the action is again 

 reversed. And so it goes on. There are an extraordinary 

 number and variety of kinds of Ascidians, some large 

 some minute, some sac-like and fixed, some glass-like in 

 transparency and floating in the sea. Others are con- 

 joined to form star-like flat encrusting growths of brilliant 

 colours on submarine rocks (Fig. 33). They all have this 

 peculiar reversible heart (which can be watched for hours 

 with the microscope in the small transparent kinds), and 

 no other animals are known with a heart acting in the 

 same way. 



The common sac-like sea-squirt which we are anato- 

 mising is a big kind called " Ascidia mentula" There 

 are a few bigger, as much as 4 in. in length, but most are 

 very much smaller, not larger than beans or peas. Our 

 sea-squirt has a digestive gland (of red colour), and also 

 masses of germ-cells (egg-cells and sperm-cells) between 

 the folds of its intestine. It has a fine set of muscular 

 bands and slips, which by their contraction can elongate 

 or shorten the sac, close or expand the mouth and the 

 branchial orifice, and squeeze the water from it. It has 



