8 Dr. W. Salensky on Hickel’s Gastraea Theory. 
Coelenterata and Vermes) the buccal aperture originates as an 
invagination of the upper germ-lamella, and, indeed, first of all 
in the form of a little tube closed posterior ly, which only opens 
into the subsequently formed intestinal cavity at a very late 
period (in many, if not in all cases, after the formation of the 
anus). | mention this only because this buccal invagina- 
tion may in some cases be confounded with the invagination 
of the superior germ-lamella of Amphioxus, the Ascidia, and 
other animals, which leads to the formation of the stomachal 
cavity. ‘The two processes, however, are essentially different. 
The invagination in Amphioxus and others is a process by which 
the two germ-lamella acquire a definite form and position and 
the intestinal cavity is formed; at the time of that invagination 
which leads to the formation of the anterior intestine and 
buceal aperture, on the contrary, the two germ-lamell have 
already long attained to their form and position, and by this 
latter invagination only the anterior intestine (cesophagus, 
gizzard, &e.) i is formed. Undoubtedly Hiickel has this cir- 
cumstance in his mind when he says that the buccal apertures 
of the Vertebrata, Arthropoda, and Echinodermata (to which 
the Mollusca may be added) are peculiar new formations and 
certainly not homologous with the primitive mouth. ‘The 
confusion appears to me, however, to have been made by Ray 
Lankester * when, in speaking of the developmental history 
of the Nudibranchiata, he says, “ and its occurrence (7. e. the 
invagination or in- pushing of cells at one pole, just as Kowa- 
lev sky has drawn it in Amphioxus and Phallusia) in a similar 
stage in certain marme Lamellibranchs is clear from Lovén’s 
admirable figures, though he has mistaken its significance.” 
According to the statements of Lovén and the fioures given 
with his researches we see at once that in the Lamellibran- 
chiata investigated by him the buccal aperture and anterior 
intestine are formed by invagination ; and therefore these in- 
vaginations cannot be compared to those which were de- 
scribed by Kowalevsky. To make the circumstances clear I 
give figures (Pl. V. figs. 1-3) of three stages in the develop- 
ment of the oyster which are characteristic in this respect. 
As regards the Echinoderms we must conclude, from the 
beautiful investigations of Agassiz +, Mecanikoff J, and Kowa- 
levsky §, that the Gastrula- stage is very prevalent in the de- 
velopmental history of these animals. 
“In the stock of the Arthropoda the Gastrulais mdeed no- 
where any longer preserved in its original pure form ; but it is 
* Annals & Mag. Nat. Hist., February 1873. 
+ Contrib. to the Nat. Hist. of the United States, vol. v. 
| Mém. de l’Acad. de St. Pétersh. tome xiii, 
§ Ibid. tome xi. 
