292 



LIFE BY THE SEASHORE. 



greater part of it is always filled up by the large branchial 

 sac (&?), which usually runs from the mouth or upper 

 opening to the other extremity of the body. It is a 

 beautiful structure made up of bars crossing one another 

 at right angles, the rectangles so formed being filled up by 

 smaller bars with slits between them. The whole sac is 

 thus a sieve, but a sieve of beautiful and elaborate struc- 

 ture. To the exterior this sieve opens by the mouth (w), 

 and in life a continuous stream of water passes into it, then 

 through the slits into the space between branchial sac and 

 body-wall, and out by the lower (atrial, at) opening which 



communicates with this space. 

 As the current passes through 

 the slits of the branchial sac 

 it washes the blood contained 

 in the bars, which are really 

 blood-vessels. The current is 

 thus primarily respiratory, but 

 it brings with it also the 

 minute particles on which the 

 sea-squirt feeds. These par- 

 ticles would be swept out 



FIG. 83. Corella parallelogramma, a with the Water of respiration 

 simple sea-squirt, so transparent * fU PT , p W{ , Q ^.-.f cnTno cr >nm'nl 

 that the internal organs can be tnere Was not SOine Special 



made out without dissection. For mechanism to retain them. 



The mechanism is of somewhat 



complex nature, and consists of two parts. To understand 

 their position we must first determine the orientation of the 

 body. We have noticed already the little nerve mass lying 

 between the apertures; now development shows that this 

 lies on the dorsal surface, and therefore that the short 

 region between the apertures corresponds to the back of a 

 fish, while the opposite edge is the equivalent of the ventral 

 or under surface of the fish. Along the ventral surface of 

 the branchial sac, then, lies a groove, the endostyle ; while 

 dorsally there is in Ciona a series of processes called 

 languets. The grooved endostyle secretes sticky mucus, 

 in which the food particles are entangled, and they are 

 then swept backwards apparently by the aid of the languets 

 into a slit-like opening at the posterior end of the branchial 

 sac. This opens into the stomach (st\ the stomach into a 



