PHYLOGENY OF THE PELECYFODA. 



297 



■\vliich brood the young, as Uiiio and Pisidinm, tlie velum is highly reduced or wanting. 

 From Ilorst, we may gathei- the conclusion that the metembryo (gastrula) stage in the 

 European oyster, fig. 17, is not a pure gastrula, but has already acquired by concentra- 

 tion of development, some of the characters which would normally appear later. 



Professor Brooks says that the primitive blastopore closes over, becomes the dorsal as- 

 ])ect of the embryo and gives rise to the shell; the permanent mouth and anus forming 

 on the op])osite side of the embryo and independent of the blastopore. This differs so 

 essentially from the observations of Dr. Horst in the European oyster and from those of 

 othei" investigators except Rabl, in the development of various groups of mollusca, that 

 we prefer to follow Dr. Ilorst's observations. Professor Brooks himself says (9), p. 70, 

 that the whole evidence furnished by comparative anatomy and embryology tends to 

 show that the mouth, anus and shell-gland can be homologized perfectly in all the classes 

 of true molluscs and that they are not only homologous with each other, but must be 

 perfectly homologous also with similar structures in the ancestral form of whi( h the 

 molluscs are modifications. It is evident that these several oi'gans would not be ^' per- 

 fectly homologous " if in one case the shell-gland arose from the closed blastopore and in 

 another from the oj^posite pole of the body, and the same would be time of the derivation 

 of the mouth and anus. 



Dr. Horst in considering the development of the pi-econchylian gland' sa3\s that when 

 the embryo is viewed from the side the glandular depression is seen as in our fig. IS, sk,iin(\. 

 in fig. 17 in optical section. During the further development, our figs. 19 and 21, a small 



Bk- 



Fig. 1». 



Fig. 20. 



Fig. 22. 



DEVEI.Ol'MENT OF OSTREA EDULIS (AFTER HORST). 



Fig. 10. Embryo same stage as flg. 18, optical section ; Fig. 21. The same embryo, optical section. 



me, mesoderm ; d, primary gastrula invagination ; s!c, shell- Fig. 22. Embryo a clay older, with a wreath of vibra- 



gland. tile cilia, a stomachic cavity and the begiuuiug of a shell, s. 



Fig. 20. Embryo a day older, front view, \Yltli tlie prim- 

 itive month opening. 



invagination is formed bordered by walls of high cylindrical cells with a narrow opening. 

 Considering fig. 19, Horst says that the gastrula cavity has deepened, and behind the 

 mouth are cells, probably mesodermic. The blastopore continues to be very distinct, and 

 as far as he could ascertain, it did not disappear but remained and was transformed in- 

 to the mouth. 



' The prcconchylian gland was discovered by Lanke.ster 

 (44) in 1871, and has been figured by him (44) in Loligo, 

 Aplysia, Pleurobranchidinm, Linina'ns and Pisidium. It 

 has also been observetl and tignred by Fol in a Ileteropod 

 (see TJalfonr); by Rabl in Planorbis (see Clans); by 

 Butschli, in Palndina (see Clans) ; by Bobretsky, in 



Nassa (see Balfour); by Patten, in Patella; by Brooks 

 in a pulraonate Gasteropod ; by Hat.schek, in Teredo; by 

 Horst in Ostrea and in other genera by various inves- 

 tigators. Tins array of genera contlrms the opinion of 

 Lankester that it is typical of all developing molluscs. 



