344 PASSAGE FROM THE PROTOZOA TO THE METAZOA. 



&c., though in most of these cases both kinds of digestion probably go on 

 simultaneously 1 . 



Another hypothetical mode of passage, which fits in with delamination, 

 has been put forward by Lankester, and is illustrated by fig. 205. He 

 supposes that at the blastosphere stage the fluid in the centre of the colony 

 acquired special digestive properties ; the inner ends of the cells having at 

 this stage somewhat different properties from the outer, and the food being 

 still incepted by the surface of the cells (fig. 205, 3). In a later stage of the 

 process the inner portions of the cells became separated off as the hypo- 

 blast ; while the food, though still ingested in the form of solid particles by 

 the superficial cells, was carried through the protoplasm into the central 

 digestive cavity. Later (fig. 205, 4), the point where the food entered became 

 localised, and eventually a mouth became formed at this point. 



The main objection which can be raised against Lankester's view is that 

 it presupposes a type of delamination which does not occur in nature except 

 in Geryonia. 



Metschnikoff has propounded a third view with reference to delamina- 

 tion. He starts as before with a ciliated blastosphere. He next supposes 

 the cells from the walls of this to become budded off into the central cavity, 

 as in Eucope (fig. 202), and to lose their cilia. These cells give rise to 

 an internal parenchyma, which carries on an intracellular digestion. At a 

 later stage a central digestive cavity is supposed to be formed. This view 

 of the passage from the protozoon to the metazoon state, though to my 

 mind improbable in itself, fits in very well with the ontogeny of the lower 

 Hydrozoa. 



Another view has been put forward by myself in the chapter on the 

 Porifera*, to the effect that the amphiblastula larva of Calcispongias may 

 be a transitional form between the Protozoa and the Metazoa, composed of 

 a hemisphere of nutritive amoeboid cells, and a hemisphere of ciliated cells. 

 The absence of such a larval form in the Ccelenterata and higher Metazoa 

 is opposed, however, to this larva being regarded as a transitional form, 

 except for the Porifera. 



It is obvious that so long as there is complete uncertainty as 

 to the value to be attached to the early developmental processes, 

 it is not possible to decide from these processes whether there is 

 only a single metazoon phylum or whether there may not be two 

 or more such phyla. At the same time there appear to be strong 



1 J. Parker, "On the Histology of Hydra fusca," Quart. Journ. Micr. Science, 

 vol. xx. 1880; and El. Metschnikoff, " Ueb. die intracellulare Verdauung bei Ccelen- 

 teraten," Zoologischer Anzeiger, No. 56, vol. in. 1880 and Lankester, " On the 

 intracellular digestion and endoderm of Limnocodium, " Quart. Journ. Micr. Science, 

 vol. xxi. 1881. 



! Vol. n. p. 149. 



