2i 4 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



centre of the body serves for a while as a broodpouch, 

 the embryos later passing out from an opening at the 



free end of the body 

 of the parent. The 

 eggs undergo a toler- 

 ably regular develop- 

 ment, producing a 

 velum, shell, and oper- 

 culum, the later stages 

 being found free in the 

 body -cavity of the 

 host." 



The preceding ex- 

 amples illustrate the 

 degenerating or cata- 

 genetic effect of a 

 parasitic life. We will 

 now observe the cor- 

 responding effect of a 

 sedentary life, which 

 may be called earth- 

 parasitism. As an ex- 

 ample of this I select 

 the well-known case 

 of the lowest of the 

 Vertebrata, the Tuni- 

 cata. 



The embryo ascid- 

 ian has the form of 

 a tadpole-like larva 

 which swims actively 

 through the sea by vibrating its long tail. After a 

 short free-swimming existence the fully developed, 

 tailed larva fixes itself by its anterior adhering papillae 



- 53- A Synapta digitata with para- 

 sitic Entoconcha ; B, a portion of Synapta, 

 with Entoconcha (F) enlarged a, point of 

 attachment; b, blood vessels; /, female por- 

 tion ; /', intestine ; in, male portion ; me, me- 

 sentery. From Kingsley. 



