THE DEVELOPMENT OF THE ODONTOPHORA. 427 



body, which extends toward and eventua]ly opens into the 

 blind end of the archenteron or primitive ahmentary sac. A 

 ciliated velum is dev^eloped on the heemal side of the mouth, 

 and a " shell gland " appears in the centre of the area which 

 gives rise to the mantle. 



In Lymnmus,^ also, cleavage ends in the production of 

 blastomeres of equal size, whether with or without a transi- 

 tory stage of inequality, and the vesicular morula undergoes 

 invagination to give rise to the archenteron. The blastopore 

 is elongated, and it appears to be likely that its anterior and 

 posterior ends may coincide with, if they do not give rise to, 

 the mouth and anus respectively. 



In most Olontopliora^ the process of yelk-division goes 

 on unequally, and results in the production of large and small 

 blastomeres (macromeres and micromeres). The latter form 

 a layer which gradually extends over the macromeres and in- 

 closes them. Obviously, this comes to the same result as 

 invagination ; and the included macromeres and their progeny 

 either become converted into the archenteron with its ap- 

 pendages, and more or less of the mesoblast, or a portion of 

 them may serve as food-yelk. 



In the Fteropoda and Hiteropoda^ and in N'assa^ JSTatica^ 

 and lusus^^ the blastopore, or aperture circumscribed by the 

 edges of the micromeral layer as it grows round the macro- 

 meres, closes, but corresponds in position to the invagination 

 of the ectoderm which gives rise to the future mouth ; and 

 the anus is a new formation. 



In such land Ptdmonata as Lunar, the process of velk- 

 division gives rise to macromeres and micromeres, and the 

 latter inclose the former. What becomes of the blastopore 

 is not clear, though I am inclined to think that it corresponds 

 in position with the mouth. The latter is seen very early as 

 a funnel-shaped invagination of the epiblast bounded by lat- 

 eral lips. Behind it, the foot grows out and rapidly attains 

 a considerable size. Its posterior extremity becomes flattened 

 from above downward, and converted into an orbicular ap- 

 pendage, the opposite walls of which are connected by retic- 

 ulated muscle-cells. This appendage undergoes rhythmical 



iLankester, " Observations on the Development of the Pond-Snail" {Quar- 

 terly Journal of Microscopical Science, 1874), and C. Rabl, " Die Outogenie der 

 Siisswasser Pulraonaten " {Jen. Zeitschrift, 187'^). 



2 Fol, " fitudes sur le developpement des MoUusques." (" Arch, de Zoologie 

 experiraentale," 1875, 1876.) 



3 Bobretskv, " Studien liber die erabrj'onale Entwickelung der Gasteropo-. 

 den." (" Archiv f. Mikr. Anat.," 1876.) 



