512 



UROCHORDA 



Compound Ascidians). The ventral stolon of the nurse gives rise to a 

 number of primitive buds, which migrate over the body until they reach 

 a dorsal outgrowth, apparently well supplied with blood. Here they 

 fix themselves and di\'ide up to form three series of buds — two lateral 

 and one median. All these buds develop into individuals belonging to 

 the sexual generation, but only a few become truly sexual. The two 

 lateral series develop into nutritive forms, which supply the nurse with 

 food. The nurse itself loses its alimentary and respiratory organs, and 



PP 7^ 



7?b' 



Fig. 294. — Structure of Appendtcularla. 



Herdman. 



-After 



S.O., Sense organ; br., branchial aperture ; at. (upper side), dorsal 

 tubercle; ot., otocyst ; n.g., nerve ganglion; pp., peri- 

 pharyngeal band ; n., nerve-cord ; ce., oesophagus; St., stomach ; 

 ov., ovary ; tes., testes ; 1., intestine ; /;., heart ; tt., urochord, 

 cut at u'. ; n.g'., n.g"., nerve gangliapf taU ; m., muscle band ; 

 ' app., tail cut through ; a., anus ; at., one of the atrial aper- 

 tures ; end., endostyle. 



becomes a mere organ of locomotion. The median buds develop into 

 " foster mothers," which ultimately go free, bearing with them other 

 buds destined to develop into the solitary sexual forms. In these, first 

 ova and then spermatozoa are produced, which start the life-cycle afresh. 

 It is thus obvious that there is considerable division of labour in the 

 sexual form, accompanied by polymorphism ; the whole process presents 

 some curious analogies to the conditions seen in the Coelentera. 



In Salpa the single egg is fertilised within the body of the mother, 

 and becomes attached to the wall of the peribranchial chamber. Here 

 the developing egg is nourished by means of a " placenta," and the 



