STRUCTURE 475 



If we pin down the Ascidian on a dissecting block under 

 water, and cut up the middle of the right side we expose a large 

 cavity with a beautiful basket-work wall with innumerable 

 minute ciliated meshes. The cavity is the pharynx and the 

 meshes are the (secondary) gill-slits. (Plate XXXIV., E.) 

 Water is continually passing in by the mouth, through the 

 basket-work, and out by the exhalant aperture. The details 

 of this are not easy, and we shall simply mention that between 

 the body-wall and perforated pharynx-wall there is a cavity 

 called peribranchial or atrial which communicates with the 

 exhalant aperture. As the water passes through the slits in 

 the pharynx it parts with oxygen to the blood-vessels (spread 

 out between the slits) and gains carbon dioxide. The chief use 

 of the pharynx is thus respiratory. 



The food supply consists of microscopic plants and animals 

 which are swept in along with the water. They would be 

 swept out again through the slits were it not for an interesting 

 contrivance. They become entangled in a " spider's web " of 

 viscid mucus which collects at the front end of the pharynx or 

 branchial sac. This mucus is made by a gutter (called the 

 endostyle) which runs along the ventral margin of the pharynx 

 — a gutter which has its precise counterpart in a groove along 

 the floor of the pharynx in the Lancelet, and, according to 

 some authorities, in the thyroid gland of higher Vertebrates. 

 The entangled food particles form thread-like ropes which are 

 wafted along the dorsal wall of the pharynx where there is a 

 ciliated ridge or a row of ciliated tags. The ropes of food- 

 particles are led to the opening of the digestive portion of the 

 gut, thus passing to the left-hand side. 



We rightly regard the sea-squirt as a sluggish animal, but 

 we must not forget that here, just as in a sponge, there is great 

 internal activity, namely that of the microscopic lashes or cilia. 

 " All the stigmata (of which there may be several hundred 

 thousand) in the wall of the branchial sac are bounded by 

 cubical or columnar epithelial cells, which are ciliated. These 

 cilia, so long as the animal is alive, are in constant motion, so 

 as to drive the water onwards, and it is this constant ciliary ac- 

 tion in the walls of the branchial sac that gives rise to the all- 

 important current of water streaming through the body " 

 (Herdman). 



