408 



UROCHORDA OR TUNIC ATA. 



On the other hand, the development exhibits marked alternation of 

 generations, both solitary and colonial forms being included in one 

 life history. 



In Doliolum the fertilised egg gives rise to a tailed larva, which 

 develops into an asexual "nurse, possessing the power of budding (cf. 

 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 divide up to form three series of buds — two lateral 



Fig. 176c. — Anatomy ot Afpendicularia. — After 

 Herdman. 



s.o., Sense organ ; br., branchial aperture ; at., dorsal tubercle ; ot., 

 otocyst ; n.g., nerve ganglion;^., peripharyngeal band; n., 

 nerve cord; 02., oesophagus; St., stomach; ov., ovary; tes., 

 testes; z., intestine; h., heart; u., urochord, cut at «' ; n.g 1 , 

 n.g", nerve ganglia of tail ; 7K., muscle band of tail ; app., tail 

 cut through ; a., anus ; at., one of the atrial apertures. 



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

 the sexual generation, but a few only 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 

 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 Ccelentera. 



