FISHES AND SEA-SQUIRTS. 293 



coiled intestine (in), this into a rectum (r), which runs 

 forward to end within the lip of the atrial opening. 



This description may sound a little complicated, but with 

 the help of the diagram there should he no difficulty in 

 following it. Let us summarise the salient points. A sea- 

 squirt may be compared to what chemists call a two-necked 

 flask, or, as the more familiar object, to a narrow-mouthed 

 coffee-pot. Within the mouth of the coffee-pot let us 

 suspend a muslin bag, which may represent the branchial 

 sac. At the bottom of the muslin bag let us make a slit, 

 and fasten to the outer side of the slit a U-shaped tube, 

 so that one of the arms of the U reaches up to the spout, 

 and its base to the bottom of the jug. "We have then a 

 pretty close model of* a sea-squirt, but to complete the 

 resemblance we must suppose that the muslin is covered 

 with fine hairs, which continually bale the water through 

 its holes. Now pour in water containing coffee-grounds, 

 and we find that owing to the hairs (cilia) on the walls of 

 the bag, a current is created which drives the water through 

 the bag, and ultimately out by the spout. But owing to the 

 arrangement of groove and processes already mentioned the 

 coffee-grounds are, on the other hand, swept into the slit 

 and so into the U tube. 



The muslin bag is the branchial sac, the mouth of the 

 coffee-pot corresponds to the mouth of the sea-squirt, the 

 U tube to the alimentary canal, the spout to the atrial 

 opening. The water, which is swept through the muslin 

 bag to ultimately gush out at the spout, is the water which 

 is used in respiration, for as it passes through the slits it 

 washes the blood contained on their walls, and so purifies 

 the blood. The coffee-grounds correspond to the food par- 

 ticles which are sifted out from the water, and pass from 

 branchial sac to stomach. Here they are digested, while 

 the indigestible residue passes into the rectum, and so to 

 the lip of the atrial opening. The matter is of course not 

 quite so simple as this analogy would suggest, especially in 

 that in most Tunicates the branchial sac is so large that it 

 has, as it were, squeezed the alimentary canal to one side, 

 and the relation of the parts becomes in consequence some- 

 what complicated. But the coffee-pot model indicates the 

 gist of the matter. 



