88 Miscellaneous. 
name, but also figure, the primitive ancestor from which all types 
of the animal kingdom have been developed! This unknown an- 
cestor, he says, must have been built much like his Gastrula (only 
another name for what has long been known to all students of In- 
vertebrates as the Planula of Dalyell). Hackel would lead us to 
believe that this Gastrula is a newly discovered embryonic stage ; all 
he has done in reference to it is to recall the existence of Planule 
among Sponges, which had previously been discovered by N. 
Miklucho-Maclay*. Since the publication of Hiickel’s article, his 
special interpretation of fanciful affinities and homologies existing 
only in forms conjured up by Hiickel’s vivid imagination, have been 
sufficiently criticised by Metschnikoffr ; so that until we know some- 
thing more of the development of Sponges we may leave the discus- 
sion of their affinities with Coelenterates out of the question, in spite 
of the ingenious arguments advanced to support Leuckart’s views 
on the subject. 
The existence of Planule, the walls of which consist of an eeto- 
derm and entoderm, has been distinctly proved for Acalephs, Echi- 
noderms, Polyps, Worms, Arthropods, Tunicates, Mollusks?, and 
finally for Amphiowus; the papers of Johannes Miller, Krohn, 
Agassiz, Kowalevsky, Sars, Allman, Claparéde, Kupfer, Metschni- 
koff, and others are too well known to need citation in this connexion. 
So far we are in perfect accordance with Hiickel and cordially agree 
with him in his estimate of the systematic value of this early embry- 
onic stage, whether we call it Planula or adopt his latter name of 
Gastrula. But let us follow his subsequent steps and separate what 
is known from what is stated as known by Hickel. It is known that 
the Planula consists of an entoderm and of an ectoderm. It is 
known that the primitive digestive cavity is, in the case of Echino- 
derms, of Ctenophora, and of some Discophora, formed by the turn- 
ing-in of the ectoderm, so that the wall of this primitive cavity is, 
in their case at least, invariably formed by the ectoderm. It is 
known, on the other hand, that in Actiniw, in Worms, in Hydroids$ 
this primitive digestive cavity is hollowed out of the inner yolk mass 
of the embryo, and has its walls formed by the entoderm. We must 
lay great stress on this point, which is alluded to by Hiickel as of no 
consequence|| ; for this seems to us to destroy the very base of his 
argument. If the Gastrula can in one case, and in such closely 
allied classes as Actiniz and Hydroids on one side, and Echino- 
derms and Ctenophora on the other, be built so differently that in 
the first case the walls of the primitive cavity are formed by the 
entoderm, and in the other of the ectoderm, what becomes of all 
* N. Miklucho-Maclay, Jen. Zeitschrift, iv. 1868. 
+ E. Metschnikoff, “ Zur Entwickelungsgeschichte d. Kalkschwamme,” 
Zeits. f. wiss. Zool. xxiv. 1874. 
{ E. R. Lankester, “‘ On the Primitive Cell-layers of the Embryo,” 
Ann. Mag. N. H. May 1873. 
§ H. Fol, ‘“ Die erste Entwickelung d. Geryonideneies,”’ Jen. Zeitsch. 
vii. p. 471. 
| Mickel and Lankester both seem to think that because the result is 
a similar form it must be homologous. 
