REPRODUCTION IN ASCIDIANS. 435 



its independent organization and its own mouth-opening. All 

 the individuals together have, however, but a single common 

 gill-pore, which is situated at the central point of the star- 

 shaped group. These star-shaped compound ascidian groups 

 {Botryllus, Polycliniiin, etc.) throw much light on the 

 Phylogeny of one of the most remarkable races of animals, 

 the Star-animals {Echinoderma). The parent-forms of 

 these are the Star-fish, or Asterids, which are, like the 

 compound Ascidians, star-shaped societies formed of Worms 

 connected by a common central intestinal opening."^ 



If we now once more glance back at the entire organiza- 

 tion of the simple Ascidians, Sea-squirts {Phallusia, Cyn- 

 thia, etc.), and compare it with that of the Amphioxus, we 

 find that the two present few points of resemblance. The 

 developed Ascidian is indeed like the Amphioxus in some 

 important points of internal structure, especially in the 

 peculiar construction of the gill-sac and intestine. But 

 it seems so far removed in most other particulars of its 

 organization, and is so dissimilar in outward appearance, 

 that the very near relationship of the two organisms is only 

 revealed by study of their germ-histories. We will now 

 consider and compare the individual development of the 

 two animals, and shall in this way find, to our great sur- 

 prise, that the same embryonic animal form develops from 

 the egg of the Amphioxus as from the egg of the Ascidian. 



k 



