THE DEVELOPMENT OF DONDERSIA. 15 



When the larva changes into the adult, the external form is modi- 

 fied not only by the continuous lengthening of the posterior region 

 of the body but also by the rotation of the pre-oral region towards 

 the ventral surface. A comparison of Figs. 5 and 9 shows clearly 

 the beginning of this process. 



A further approximation towards the shape of the adult is brought 

 about by the development of a fold growing out on either side of 

 the body at its dorso-lateral angle, thus giving origin to the mantle. 

 At the same time, the ventral surface becomes specialised by the 

 growth of its cells, which multiply and give rise to the flattened foot. 

 The area between the mantle and the margin of the foot becomes 

 the slightly invaginated mantle-cavity (Fig. 4 C). 



'Nothing, to our knowledge, is known of the origin of the gills, but, 

 considering the simplicity of their form, it is very probable that they 

 arise as papilla-like prominences on the body-surface. In the adult 

 they form on each side a row of consecutive bipectinate ctenidia. 



Among the internal changes we note the formation of an ecto- 

 dermal invagination (Kowalevsky) at the posterior end of the body ; 

 this is the proctodaeum which gives rise to the anus and the base 

 of the intestine (Fig. 9, a). In the stomodaeum, the radular sac has 

 increased considerably in size, and the radula itself has already 

 appeared within it. In front of the latter another ventral outgrowth 

 of the wall of the stomodaeum has formed ; this widens later, and at 

 its base a thickened cell-mass can be recognised. We here appear to 

 have the so-called sub-radular organ of the adult, which has been 

 accurately described by Haller (No. 2). A further differentiation 

 of the alimentary canal is caused by a ventral swelling of the enteron, 

 which is perhaps the first indication of the liver (Fig. 9). 



The further development of the larvae of Chiton has not as yet 

 become known, but it is evident that they already resemble the 

 adult in various ways, apart from the incompleteness of their 

 internal organisation. 



II. The Development of Dondersia. 



The development of Dondersia banyulensis * has been investigated 

 by Pruvot. This form is allied to Proneomenia (Fig. 147, Chap. 

 xxxiii., Hubrecht, No. 5), possessing like the latter, a vermiform 

 body (40 mm. long and 1 mm. broad) capable of coiling up spirally. 



* [Simroth (Zeitschr. f. wiss. Zool., Bd. lvi., 1893) has created the genus 

 Myzomenia for this species. — Ed.] 



