434 



THE EVOLUTION OF MAN. 



Of the other important organs, we have yet to mention 

 those of reproduction, which lie at the posterior extremity 

 of the body-cavity. All the Sea-squirts are hermaphrodites. 

 Each individual has a male and a female gland, and is thus 

 capable of self-fertilization. The mature eggs (Fig. 154, o) 

 fall directly from the ovary (o) into the gill-cavity. The 

 male sperm, on the contrary, is carried from the testes (t) 

 into the same cavity by a special seed-duct (yd). Here 

 impregnation takes place, and here in many Sea-squirts 

 developed embryos are found (Plate XL Fig. 14, z). These, 

 with the water that has been inhaled, are then thrown out 

 at the gill-pore (q) ; they are thus " born alive." 



Many Sea-squirts, especially of the smaller species, 



'aolooS^v, Fig. 154. — Structure of an Ascidian (observed from 



rSSs^'*- tbe left side ' as in Fig - 153 > and Fig - 14 ' Tabie xl ) •• 



«nmSsR 1 s ^' giU" sac > v > stomach ; i, large intestine ; c, heart ; 



^iJPjTu i t, testes ; vd, seed-duct ; o, ovary ; o', matured eggs in 

 the gill-cavity. The two little arrows indicate the en- 

 trance and exit of the water through the two openings 

 of the tunic. (After Milne Edwards.) 



multiply, not by sexual reproduction, but 

 asexually by the formation of buds. Great 

 numbers of the individuals thus produced 

 from buds remain permanently attached to 

 each other, thus forming large masses, or 

 comes like the well-known coral societies. 

 Among these social or compound Ascidians, 

 those species are peculiarly interesting in 

 which the mass seems to be beautifully 

 combined of many star-shaped groups. Each 

 star-shaped group consists of a larger or 

 smaller number of individuals, of which every one possesses 



