MOLLUSCOIDA POLYZOA. 117 



167. Circulation. The heart is iii the form of a simple 

 muscular tube, open at both ends, and without valves. 

 By the peristaltic motions of this tube circulation is 

 effected. For a definite number of times the blood is 

 propelled in one direction ; then, the action of the tube 

 being reversed, it flows as many times in the opposite 

 direction. Thus the two ends of this tubular heart per- 

 form alternately the functions of an artery and a vein. 



168. Respiration. The walls of the respiratory cham- 

 ber are richly supplied with blood-vessels. The blood in 

 these vessels is aerated by the air contained in the water 

 which constantly flows through it. This current of water 

 not only brings food to the animal, but also brings oxygen 

 for the purification of its blood. Thus, in the ascidians, 

 as well as in the Mollusca proper, the functions of respira- 

 tion and alimentation are intimately associated. 



169. The nervous system consists of a single ganglion 

 placed between the oral and atrial apertures. There are 

 generally eye-like spots between the tentacles that sur- 

 round the mouth. There is also an auditory vesicle con- 

 taining an otolith. 



170. Development. The ascidians are all hermaphro- 

 dite, the sexes being united in the same individual. The 

 young in some groups resemble tadpoles, being free- 

 swimming, and provided with long tails. 



CLASS 3. Polyzoa. 



171. The Polyzoa (Gr. polus, many ; zoon, an animal) 

 is the lowest class of the Molluscoida. It includes 

 the " sea-mosses " and the Flustra or sea-mats. These 

 animals were formerly placed along with the Hydrozoa, 

 which they very much resemble in outward appearance ; 

 their internal structure, however, is widely different. 

 From their resemblance to plants, they are often called 

 Zoophytes (Gr. zoon, an animal ; phuton, a plant). Most 

 of them inhabit the sea, but some species live in fresh 

 water. They are compound animals, forming colonies 



