256 



SUBKIKGDOM MOLLUSCA. 



another for the exit of water. They have a stomach, pulsating 

 heart, branchial appendages, and highly developed secretory 



organs. Some are simple, 

 others attached to a common 

 mantle, but each individual 

 a maintains a separate exist- 

 ence. 



Clavellinidse (little 

 knobs). These Ascidians are 

 united by a common stem.* 

 The elongated heart con- 

 tracts from behind forward, 



SO as to propel the blood in 



one direction, and after a 

 few moments, the pulsations 

 becoming fainter and fainter, 

 the heart gives the opposite impulse, 

 and the blood travels the other way, 

 through the same vessels. 



The Salpidae (stocked) are so 

 transparent that the structure of 

 their internal organs may be exam- 

 ined through their coatings. They 

 present an example of alternate 

 generation. An individual Salp re- 

 sembles " its grandparent, its grand- 

 children, and its own brethren," but 

 differs equally from its parent and 

 its own children. A single 



Clavettlna lepidiformis, Social Tunicates 

 or Ascidians. a. Incurrent orifice ; 

 b. Excurrent do. ; c. Stomach ; d. Intes- 

 tinal canal ; e. Common stem. 



* The history of a cluster is as follows i A sin- 

 gle individual just from the egg possesses active 

 powers of locomotion. After wandering a time 

 it attaches itself, and sends forth creeping stems, 

 from which, at brief intervals, there bud and grow 

 to maturity other individuals. From each of these Salpa cabotti. a. Posterior orifice ; 

 proceed young, to become in like manner the b. Anterior do. ; c. Processes by 

 parents of other colonies. This free motion of which the individuals of the 

 the immature Ascidians provides for the general chain were united ; h. Heart, -a. 

 diffusion of these animals, and prevents their Nervous ganglion ; o. Nucleus ; 

 being crowded in particular spots. r. Oitt. 



