194 THE SCIENCE OF LIFE. 



sion, circulation, and respiration. Many species are fur- 

 nished with organs of a remarkable and peculiar kind, 

 called Avicularia, (avicula, a little bird,) or " bird's 

 heads," which during life, and even after the death of 

 the animal, keep up a continual motion, see-sawing, and 

 snapping, and opening their jaws in the most singular 

 manner. Their use is unknown, but Mr. Gosse conject- 

 ures that they may seize and hold minute animals until 

 decomposition attracts a crowd of Infusoria, which may 

 serve the Polyzoan for food. Some species of Polyzoa 

 are found in fresh water. 



3. TUNICATA, named from the Latin tunica, a cloak, 

 is a class of Molluscoida which are enveloped in a tough, 

 leathery sac, or " test." This sac is double-walled, but 

 not capable of protrusion. The mouth of the animal 

 opens into the bottom of a respiratory sac whose walls 

 are lined by a net-work of blood-vessels. The tubular 

 heart exhibits the curious phenomenon of reversing its 

 action at brief intervals, so that the blood oscillates 

 backward and forward in the same vessels. The wall of 

 the tunic contains cellulose, which is generally a vege- 

 table product. 



These bottle-shaped creatures are found in the ocean, 

 " solitary," attached to rocks or sea-weed, and often 

 glued together in bunches. Sometimes they are in " so- 

 cial " groups, as in Fig. 94, or " compound," as Fig. 95. 



The Salpce are free swimming, transparent Ascidians, 

 (askos, a bag ; eidos, like,) or Tunicates, often found ad- 

 hering to each other in long chains, which give birth to 

 solitary individuals of different form by alternation of 

 generations. 



