The Tunicates, or Ascidians 



477 



masses incrusting on seaweeds of various kinds, and on other 

 Ascidians, are frequently thrown ashore in great quantities 

 during heavy storms. 



In Didemnida the body is more complex, of two parts, called 

 the "thorax" and "abdomen." In Amarcecium, the "sea 

 pork " of the fishermen, the body 

 is in three parts and the indi- 

 viduals are very long. These some- 

 times form great masses a foot or 

 more long, "colored like boiled 

 salt pork, but more translucent." 

 Other families of this type are 

 the Distomidg and the Polycli- 



Ritter. 



A compound Ascidian. Shumagin 

 Islands, Alaska. (After Ritter.) 



In the suborder Luciaz, includ- 

 ing the family Pyrosomidaz, the 

 colonies are thimble-shaped and 

 hollow, the incurrent openings 

 being on the outer surface of the 

 thimble, the outgoing stream open- 

 ing within. Pyrosoma is highly 

 phosphorescent. In the tropical Fi(} 28 ^_ Botryttu , 

 seas some colonies reach a length 

 of two or three feet. It is said 

 that a description of a colony was once written by a naturalist 

 on a page illumined by the colony's own light. "Each of the 

 individuals has a number of cells near the mouth the function 

 of which is to produce the light." 



Thaliacea. In the order Thaliacea the Tunicates have the two 

 orifices at opposite ends of the body. All are free-swimming 

 and perfectly transparent. The principal family is that of 

 Salpida. The gill-cavity in Salpa is much altered, the gills 

 projecting into it dividing it into two chambers. 



In these forms we have the phenomena of alternation of 

 generations. A sexual female produces eggs, and from each 

 hatches a tadpole larva which is without sex. This gives rise 

 to buds, some at least of the individuals arising which in 

 turn produce eggs. 



In the family Salpidce two kinds of individuals occur, the 



