SPONGOZOA. 
533 
two concentric layers, the ectoderm, with the spicula, and the endoderm ; 
the gastro-vascnlar cavity is formed somewhat later, but the “mouth” 
was still wanting in the most advanced stage observed. The syncytial 
skeletigerous outer layer of the sponge is not homologous with the ecto- 
derm of the Codenterata, but with the mesoderm ; the true ectoderm exists, 
e. g., in larvsB of siliceous sponges, but disappears early. The l3ody- 
cavity of diJfferent zoological types may, from a morphological point of 
view, be formed in very different manners ; the whole argument and 
classification, which is built up from the homology of the “ Gastrula ” 
with the larvae of other animals, is therefore devoid of any solid base.* 
* This is tho most proper place to notice E. Hilckel’s “Die Qastrroa- 
Theorio. Dio phylogonotischo Classification dos Thiorreichs und die Homologio 
der Keimblatter,” Jen, Z. Nat. viii. pp. 1-55, pi. i. (translated by E.P. Wright in 
Q. J. Micr. S. xiv. pp. 142-165 & 233-247, pi. vii., abstracted by G. 0. J. Schneider 
in Arch. Z. exper. iii. pp. 239-256 ; criticized by C. Claus in a special pamphlet, 
“ Die Typenlehre und E. Haeckel’s sogenannten Cras'r^pa-Theorie,” Wien, 1874, 
and by W. Salonsky, “ Bomorkungen iibor Haeckel’s GasfrrtJa-Thcorie,” Arch. f. 
Nat. xl. pp. 136-174. Cf. also C. Semper, “ Kritische Gange, iii.. Die Keimblatter- 
theorie und die Genealogie der Thiere,” Arb. Inst. Wiirzb. i. p. 22). It is a further 
development of the ideas sketched in the “Monographie der Kalkschwamme ” 
und “Ueber die Morphologie der Infusorien " [Zool. Rec. x. p. 523], and proposes, 
like the theory of E. Ray Lankester (ibid.), established about the same time, to 
replace the type-theory of Cuvier and Von Baer, or the doctrine of distinct archi- 
tectural features for the chief divisions of the animal kingdom, by a genealogical 
exposition of the relations between its principal branches. Excluding the 
Monera” from the animal kingdom, and relegating them to the Pro^^s^a, his 
lowest division is that of the Protozoa {Infusoria, Gregarina, Rhizopoda) ; being 
typically unicellular, they do not, of course, afford any differentiation into 
Ectoderm and Endoderm, though these may be suggested by I “ Exoplasm ” and 
“Endoplasm.” All other (higher) animals are Metazoa or Oastrozoa: the pluri- 
cellular embryo or “ Gastrula ” is early differentiated into, at least, two cell-layers ; 
the Endoderm or inner (vegetative) germ-leaf, and the Ectoderm or outer (animal) 
germ-leaf ; in all the higher types, a third (or Mesoderma) is developed between the 
two primary layers. All Metazoa are typically (where the mouth, &c., is not lost 
through a retrograde development, as in some parasitical worms) provided with a 
mouth and stomach or intestine. The Zoophyta (Coelenterata, Spongiee, Acalephre, 
Anthozoa) and lower worms {Acoelomi, Plathelmintha) have only this intostino- 
gastral cavity, but no true body-cavity (coeloma), which is in the higher branches 
of the animal kingdom formed by the separation of the secondary layers of the 
mesoderm. The Zoophyta and Acoelomi therefore are truly bloodless animals 
{Anremaria) ; a blood-fluid or “ haemolymph” cannot exist without or before the 
“ coeloma,” which is fllled with transuding nutritive fluid from the gastral 
cavity, develops the nutritive organs, etc. The difference between the radiate 
Zoophyta and the bilateral worms is deduced — and this is apparently one of the 
most vulnerable points of the theory— from the oldest descendants of the common 
ancestor (Gastreea) becoming either fixed (Protascus) or creeping (Prothelmis) . 
The higher worms (Corlomati), the first blood-animals or HcemataHa, were dovol- 
lopod from tho Plathelmintha ; from them again, tho four higher “ phylro,” 
Mollusca, Echinodermata, Arthropoda, and Vertehrata originated independently 
of each other. Tho Gastrula stage is still preserved in tho first embryonal stage 
of Amphioxus ; in all other Vertehrata it is passed over, owing to the, as Claus 
terms it, dangerous principle of the abridged evolution. C/. also the classification 
of the animal kingdom, proposed by T. Huxley, Nature, xi. p. 101 [read before the 
Linncan Society, Dec. 4, 1874]. 
