162 E. RAY LANKESTER,. 
invaginate sac—the “ pedicle of invagination ”’—becomes in 
all cases the rectum. The mouth makes its appearance 
subsequently and eats its way into the pre-formed alimen- 
tary sac from the opposite pole. 
This being most positively the case in the Mollusca, I am 
led to raise the question as to whether the orifice of invagi- 
nation really persists in avy animal. Whilst I should not 
feel so much hesitation about a statement to the effect that 
it persists as anus, | would venture to ask for great certainty on 
the part of an observer who believes he has traced the mouth 
of any animal to this orifice. In the case of the Echinoderm- 
larve it is generally accepted that the anus is simply the 
persistent orifice of invagination. I am not aware that 
Agassiz—who most clearly points out the relation of the 
parts in question-—had before his mind the suggestion that 
the orifice of invagination may close and a new opening sub- 
sequently appear at the same spot. I cannot avoid the con- 
clusion that Kowalewsky’s observation, also, as to the 
persistence of the planula’s orifice of invagination—in the 
case of the Earth-worm—as “mouth,” or rather as con- 
tinuous with mouth, requires confirmation by an observer 
who shall distinctly keep the question as to identity of mouth 
andorifice of invagination before his mind. I makethese remarks 
inrelation to thestatements of two most eminentand competent 
observers, rather as suggesting the need of renewed examina- 
tion of this particular point than as believing that there is not 
a large probability in favour of their accuracy in observation. 
If it should prove to be true that the orifice of invagination 
does not persist in the cases just alluded to, it appears to 
me that the planula (as I have defined it in my paper, 
‘Ann. Nat, Hist.,’ May, 1873, p. 327), consisting of a hollow 
sac with two layers of cells forming its wall—and not the 
Gastrula—if by that term we are to understand “a planula 
plus a mouth” as Professor Haeckel does, is the important 
developmental form which can be recognised in all the 
Metazoa. The following words, which I have applied in my 
essay, already cited, to the Vermes, Echinodermata, Arthro- 
poda, Mollusca, and Vertebrata, would then apply equally to 
all Metazoa. ‘‘ The primitive orifice of invagination (mouth 
of the planula) does not persist either as mouth or, as has 
been erroneously supposed, as anus, but becomes entirely 
closed up, and a new mouth and an anus eat their way into 
the gastric cavity from the exterior’’ (p. 330, loc. cit.). There 
would not in this case be any animals which could be classed 
as retaining a primitive mouth. In all alike the mouth 
would be a late opening, which breaks into the already formed 
