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 Didemnidcc the body is more complex, of two parts, called 

 the "thorax" and "abdomen." In Amarcecinm, 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 DistomidcB and the Polycli- 

 nida:. 



In the suborder Lucim, includ- 

 ing the family PyrosomidcE, 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 ^^^ 284. -Botryllns magnus Ritter. 

 seas some colonies reach a length A compound Ascidian. Shumagin 



„ , . , J- . T. • . , Islands, Alaska. (After Ritter.) 



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 

 Salpidw. 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 Salpida; two kinds of individuals occur, the 



