PHYSIOLOGY OF THE INVERTEBRATA. 453 



a free-swimming tailed larva is developed. Just before 

 reaching maturity, the larva attaches itself by means of 

 papillae, and undergoes a series of changes, of which the 

 following are the most important : The tail aborts, the 

 muscles and notochordal sheath degenerate, and the noto- 

 -chordal axis contracts. The nervous system and organs of 

 sense degenerate, and the cavity in the nerve-cord and 

 cerebral ganglion disappear. The pharynx increases largely, 

 and the branchial slits become visible. After this the adult 

 state is reached.* 



At this point we give a diagram (Fig. 8i) which indicates 

 roughly the evolution of the Invertebrata. 



Concluding Eemakks. 



This chapter has its moral— that like always begets like. 

 It does not matter by what mode of reproduction it has 

 been produced, the offspring is always of the same kind 

 as the parent. It appears that transformation or pleo- 

 morphism does not exist in the animal kingdom. The life- 

 cycles of the lowest as well as the highest animals repeat 

 themselves according to a well-known law. "To one who 

 has fully comprehended the meaning and the operation of 

 the Darwinian law, it will be at once apparent that there 

 must be error somewhere in the matter of pleomorphism." 

 How can Bacillus subtilis be transformed into Bacillus 

 anthracis;] or an amoeba into a gregarina by the experi- 

 mentalist? "If the law of actual variation," says Dr. 

 Dallinger, "with all that is involved in the survival of 

 the fittest, could he so readily hrought into complete opera- 

 tion, and yield so pronounced a result, where would be the 



* In some of the Tkmicata, the development of the embryo takes place 

 within the parent, and there is a placenta. See also Giard's papers in 

 Revue Scientifique, 1874 ; Comptes Bendus, 1874-5 ! Archives de Zoologie 

 Expirimentah, 1872. 



+ See Dr. GriflBths' book : Beaearches on Micro-Organisms, p. 41 

 (Baillifere & Co.). 



