The Development of Ischnochiton. 631 



the diagram (Fig. E) is probably not far astray. Certain it is that 

 the greater portion of the foot is derived from the first somatoblast 

 and the two bordering groups of third quartette cells which have 

 formed the ventral side. The doubt arises as to the part the anterior 

 third quartette products play in its formation. But since its anterior 

 border is comparatively wide and in contact for some distance with 

 the velum it appears probable that its antero-lateral borders arise in 

 the territory of the anterior third quartettes. 



Living embryos, especially during the free-swimming stage, show 

 a depression near the anterior border of the foot, and in preserved 

 specimens this may appear until the shell is nearly formed. A pro- 

 jection thus exists anterior to the depression and posterior to the 

 mouth such as is shown in Fig. 58. Sections and favorable surface 

 views show that along almost the entire edge of the projection the 

 opening of the "foot-gland" (Kowalevski) is situated (Fig. 58). This 

 is scarcely to be spoken of as a true duct but simply a series of 

 more or less clearly defined intercellular channels which do not ap- 

 pear sharply defined except in sections. 



Subjected as the young are to the violence of the waves, I am 

 of the opinion that Kowalevsiq is correct in speaking of this pro- 

 blematical organ as a foot gland, and that its secretion enables the 

 organism to retain its foothold. Young Chiton from ten to twenty 

 days of age are with much difficulty removed with a camel's hair 

 brush from the rocks on which they rest, and with later stages it is 

 necessary to immerse them in a killing fluid before they can be dis- 

 lodged with such an instrument. 



4. The Shell. 

 The shell arises on the dorsal side of the embryo and at its first 

 appearance is confined to the trunk region from whence it gradually 

 extends onto the head vesicle. It arises from the first somatoblast 

 and probably its lateral borders are formed from the neighboring third 

 quartette cells, these latter in the posterior quadrants exhibiting from 

 the commencement of their development a very great regularity in 

 their position. It will be recalled that a similar regularity also ap- 

 pears in the posterior arm of the cross whereby the cells become 

 arranged in rows somewhat after the fashion of the posterior third 

 quartette cells (Figs. 45—47). It is interesting to note that this 

 symmetrical arrangement of the blastomeres occurs in the exact locality 

 occupied by the future shell. This becomes more apparent after the 



Zool. Jahrb. XII. Abth. f. Morph. 4]^ 



