THE FURTHER DEVELOPMENT OF THE LARVA AND METAMORPHOSIS. 7 



these forms have received more attention than the others, they will 

 serve for description, as these processes have a very similar course in 

 all Chitones. 



To the organs which we have already found in the larva, a new 

 one is added at a somewhat early stage : this is a sac-like gland, 

 opening behind the oral aperture (Fig. 2 B, fd.). According to 

 KOWALEVSKY this organ, which he regards as the pedal gland, is 

 unpaired and originates as a thickening and depression of the 

 ectoderm, but this origin has not been fully established. Structures 

 corresponding to the " pedal gland " will again be met with in the 

 Gastropoda. The nature of this organ in Ohiton is not yet quite 

 clear, and, as it appears to degenerate later, its efferent duct closing 

 first, KOWALEVSKY regards it as a larval organ. 



As the post-oral portion of the body elongates, the intestine also 

 increases in length (Fig. 5) ; but its posterior end does not as yet 

 communicate with the exterior. It has already been shown that, in 

 the Annelidan Trochophore, the anus is wanting, or else the intestine 

 is in a very backward stage of development. 



The mesoderm which, as has already been described, first arose as 

 two groups of cells continuous with the entoderm, and shifted later 

 into the cleavage-cavity near the blastopore, has become richer in 

 cells, and with the progressive growth of the larva has extended 

 between the ectoderm and the entoderm. Now, as when it first 

 appeared, it shows a bilateral arrangement, i.e., in young embryos 

 (aged twelve hours or slightly older, Fig. 3 /?), the mesoderm takes 

 the form of two compact cell-bands situated on the ventral side of 

 the embryo and lying internal to the ectoderm. We may claim 

 these as mesoderm- bands, since they later undergo division into a 

 splanchnic layer applied to the embryonic intestine and a somatic 

 layer lining the ectoderm (Fig. 4 A and B). The cavity lying 

 between them (coe) which is paired, is consequently to be regarded 

 as a true coelom. It develops earlier in the anterior than in the 

 posterior part of the body. Thus, when two distinct cavities almost 

 lined with epithelium are to be found in front, there is still a solid 

 mass of cells in the posterior region. The reader will here doubtless 

 recall conditions prevailing among the Annelida. It should also be 

 mentioned here that the regular arrangement of the mesoderm just 

 described is soon lost, as the mesoderm-cells grow into the primary 

 body-cavity in a manner similar to that observed in other Molluscs 

 (Figs. 5, 9). A large complex of mesoderm-cells, which at first 

 remains lying at the posterior end of the body (Fig. 5) yields later 



