GENERAL NOTES ON TUNIC AT A. 



455 



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 

 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. 



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 

 development is in consequence much abbreviated, the tailed larva not 

 being represented. This embryo gives rise to a solitary "nurse" form, 



bv 



an 



PPb 



ph 



FlG. 246. Diagram of Salpa africana. 



o.a. t Oral aperture ; d.t., dorsal tubercle ; te. t tentacle; g., ganglion ; #z., 

 muscle bands; atr., atrium; b.v. t blood-vessel; an., anus; a. a., 

 exhalant aperture ; v.n., visceral nucleus; &, heart ; st., stolon ;</./., 

 dorsal lamina ; ., endostyle ; s.n.g., sub-neural gland ; ph., pharynx ; 

 p.p.b.\ peri-pharyngeal band. 



which by budding produces a chain of embryos. This chain is set free, 

 its members become sexual, and, either while still united or. after 

 separation, give rise to the eggs which develop into the nurse form. 



The remaining order of Tunicates includes minute simplified forms 

 like Appendicularia, also pelagic in habitat, but without any power of 

 budding, and never forming colonies. These forms have a distinct tail, 

 which is bent at an angle to the body, and is the main organ of locomo- 

 tion. The mouth is at the anterior end ; the anus, which is distinct 

 from the atrial openings, is at the root of the tail. These atrial openings 

 lie slightly behind the anus, and are merely small ectodermic invagina- 

 tions communicating with the two gill-slits of the pharynx. They 

 correspond to the similar imaginations in the Ascidian larva. The test 

 may form a large investing " house," but it does not contain cells, and 

 is periodically cast and renewed. The important points as regards 

 internal structure are the presence of the notochord throughout life, and 

 the structure of the nervous system. The latter consists of a lobed 



