88 Miscellaneous. 



name, but ulso figure, the primitive aucestor from which all types 

 of the animal kingdom have been developed I This unknown an- 

 cestor, he says, must have been built much like his Gastnila (only 

 another name for what has long been known to all students of In- 

 vertebrates as the Plannht of Dalyell). Hikkel would lead us to 

 beheve that this Gastnih is a newly discovered embryonic stage ; all 

 he has done in reference to it is to recall the existence of Planuhe 

 among Sponges, whioh had previously been discovered by N. 

 Miklucho-Maclay*. Since the publication of Hackel's article, his 

 special interpretation of fanciful affinities and homologies existing 

 only in forms conjured up by lliickel's vivid imagination, have been 

 sufficiently criticised by Metschnikoff f ; so that until we know some- 

 thing more of the development of Sponges we may leave the discus- 

 sion of their affinities with Ca-lenterates out of the (juestion, in spite 

 of the ingenious arguments advanced to support Leuckart's views 

 on the subject. 



The existence of Planuhe, the walls of which consist of an ecto- 

 derm and entoderm, has been distinctly proved for Acalephs, Echi- 

 noderms. Polyps, AVorms, Arthropods, Tunicates, Mollusks:J:, and 

 finally for Amphioj'us ; the papers of Johannes Miiller, Krohn, 

 AgaSvsiz, Kowalevsky, Sars, Allman, Claparede, Kupfer, Metschni- 

 koff, and others are too well known to need citation in this connexion. 

 So far we are in perfect accordance with Hixckel and cordially agree 

 with him in his estimate of the systematic value of this early embry- 

 onic stage, Avhether 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 Hiickel. It is known that 

 the Plamda consists of an entoderm and of an ectoderm. It is 

 known that the primitive digestive cavity is, in the case of Echino- 

 derms, of Ctcnophora, 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 ectodeiTn. It is 

 known, on the other hand, that in Actinise, in "Worms, in Hydroids§ 

 this primitive digestive cavity is hoUowed 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 Actiniae and Hydroids on one side, and Echino- 

 dcrms and Ctenophora on the other, be built so difi'erently 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 aU 



♦ N. Miklucho-Maclay, Jen. Zeitschrift, iv. \8C<8. 



t E. Metschnikoff, '• Zur Entwickeluugsgeschichte d. Kalkschwamme," 

 Zeits. f. wi.«s. Zool. .\xiv. 1874. 



X E. H. Lankester, " On the Primitive Cell-lavers of the Embrvo," 

 Ann. Mag. N. 11. May 1873. 



§ II. Pol, " Die erste Entwickelung d. Geryonideneies," Jen. Zeitsch. 

 vii. p. 471. 



II Hackel and Lankester both seem to think that berause tlip result i-o 

 a .similar form it must be homologous. 



