116 ZOOLOGY. 



surface of the sea. Some of these, as the Pyrosoma (Gr. 



pur, fire; soma, a mouth), are phosphorescent, and light 



up the sea in tropical regions 

 with a pale greenish light. 



165. Skeleton, or Cover- 

 ing. In the simple Asci- 

 dians the covering consists 

 of two coats, an outer and 

 an inner. The outer coat is 



rough and leathery in tex- 

 49.-SOCIAL ASCIDIAXS. tur ^ j t is ca]led tlie tegt 



or external tunic. It is largely composed of cellulose, a 

 chemical substance closely allied to the woody fibre of 

 plants. This substance is rarely found in animal tissues. 

 The inner integument, or mantle, is muscular. It is 

 by means of its contractability that the animal is able 

 to squirt out water through one of its apertures. 



166. Digestion. The two orifices are usually guarded 

 by short tentacles, which are not furnished with cilia. 

 The upper or oral opening leads into a chamber which 

 occupies a large portion of the interior of the animal. 

 This is usually called the respiratory or branchial sac. 

 Its walls are perforated like a sieve, the apertures being 

 lined with cilia. At the lower extremity of this sac the 

 entrance to the gullet is situated. The gullet leads to a 

 stomach and intestine, which is bent round, so as to reach 

 a cavity connected with the other aperture, called the 

 cloaca or atrial (Lat. atrium, a hall) chamber. A cur- 

 rent of water constantly passes in at the oral aperturo 

 into the respiratory chamber, and through its perforated 

 walls to the atrial chamber. This current is produced by 

 the movements of the cilia which line the apertures of 

 the respiratory sac, the cilia working so as to produce a 

 current in that direction. From the atrial chamber the 

 water is discharged through the atrial aperture to the 

 surrounding medium, having in its passage not only 

 brought food to the stomach, but also carried off the ex- 

 cretions supplied by the intestine. 



