212 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



ectodermal. In most other respects the development of the organs 

 is similar in the primary and secondary ascidiozooids. 



The growth and formation of the 

 colony has been deciphered by Seel- 

 iger and Neumann for Pyrosoma 

 aherniosum, and to some extent for 

 other species, as P. verticillatum. 

 Each primary ascidiozooid gives rise 

 first to a single bud, and these buds 

 move around to the left and occupy 

 positions dorsal to the interspaces 

 between the parent zooids (No. 1, 

 fig. 8). The next set of buds pro- 

 duced by the primary ascidiozooids 

 again move dorsally, this time occu- 

 pying positions directly above the 

 parent zooids, and in the same whorl 

 with the first formed four (No. 2, fig. 9). Thus far two rings, or tiers, 

 of zooids are established; an older one of four primary ascidiozooids, 



Fig. 8.— A diagram of an end view 



OF A COLONY OF PYROSOMA AHER- 

 NIOSUM, "WITH THE ZOSlDS PROJECTED 

 ONTO ONE PLANE. 



FIG 9.— A DIAGRAM SIMILAR TO FIGDRE 6, BUT ILLUSTRATING AN OLDER 

 COLONY. 



and a younger one of eight secondary ascidiozooids. Anothor set of 

 four buds is again produced by the primary ascidiozooids, these occu- 

 pying positions above the interspaces between the secondary zooids 



