8 Dr. W. Saleusky on Udckers Gastrsea Theory. 



Coelenterata and Vermes) tlie buccal aperture originates a-s an 

 invagination of the upper germ-laniella, and, indeed, first of all 

 in tlie form of a little tube closed i)osteriorly, which only opens 

 into the subsequently formed intestinal cavity at a very late 

 period (in nianv, if not in all cases, after the formation of the 

 anus). I mention this only because this buccal invagina- 

 tion may in some cases be confounded with the invagination 

 of the superior germ-lamella oi Aiiijihio.ciis, the Ascidia, and 

 other animals, which leads to the formation of the stomachal 

 cavity. The two processes, however, arc essentially different. 

 The invagination in Amjjhioxus 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 

 buccal aperture, on the contrary, the two germ-lamella? have 

 already long attained to their form and position, and by this 

 latter invagination only the anterior intestine (oesophagus, 

 gizzard, &c.) is formed. Undoubtedly Kiickel has this cir- 

 cumstance in his mind when he says that the buccal apertures 

 of the Yertebrata, 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 Kay 

 Lankester * when, in speaking of the developmental history 

 of tlie Xudibranchiata, he says, " and its occurrence {t. e. the 

 invagination or in-pushing of cells at one pole, just as Kowa- 

 levsky has drawn it in Amjjhioxus and PhaUusia) in a similar 

 stat;e in certain marine Lamellibranchs is clear from Loven's 

 admirable figures, though he has mistaken its significance." 

 According to the statements of Loven and the figures 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 (PI. 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 t, Mecznikofi' \, and Kowa- 

 levsky §, that the Gastrula-nUxgQ is very prevalent in the de- 

 velopmental history of these animals. 



" In the stock of the Arthropoda the Gfu^trula is indeed no- 

 Avhere any longer preserved in its original pure form ; but it is 



• Annals i<t Mag. Nat. Hist., Febiunrv l.>-7.1. 



+ Contrih. t<» the Nat. Ilii^l. of tho Tuited .Slates, vol. v. 



1 .MtMu. lie r.Vcad. df St. reteieb. tome xiii. 



^ I hid. tome xi. 



