The Development of Ischnochiton. 647 



of the truDk region of the trochophore , it will 1 think explain some 

 of its peculiarities. 



If we take such a type as that represented by the Chiton trocho- 

 phore and attempt to modify it into a segmented Annelid larva several 

 features present themselves which lead as to believe that metamerism 

 in its development pursued much the same course as that presented 

 in the following lines. 



The dorsal portion of the first somatoblast in Ischnochiton is in- 

 cluded in the shell and hence possesses a segmented character; the 

 ventral occupies a position in the foot and is ciliated and non-segmented. 

 And these two portions of the somatoblast grow in different ways; 

 the dorsal part increases little in length but considerably in width; 

 while the ventral area grows little in width but much in length. Let 

 it be supposed that during the phylogenetic development the segmented 

 portion increased in width and gradually extended more and more 

 over the dorsal surface. Were it to continue to a sufficient extent 

 its lateral borders would ultimately come in contact with the ventral 

 strip and the first somatoblast and the remaining cell groups might 

 have much the relations shown in diagram E, b. The first somato- 

 blast would appear as a great group of cells but they would consist 

 of two essentially unlike portions, a segmented and non-segmented, 

 the latter located along the median ventral line, the former composing 

 the larger portion of the trunk. 



If the ontogenetic development were undisturbed by secondary 

 factors the first somatoblast would probably exhibit these two methods 

 of growth — one of width in the dorsal tract, and a forward growth 

 of the cells behind the mouth whereby the ventral portion would be 

 formed and the dorsal region would come in contact with the ventral 

 in the manner described above for the phylogenetic development. But 

 in thus increasing so greatly the area occupied by the first somato- 

 blast secondary factors arise; precocious segregation endows it with 

 an increased size when it first appears and a rapidity of development 

 so great that before the mouth commences to shift the lateral borders 

 of the somatic plate nearly meet on the median ventral line. As the 

 mouth migrates the edges of the somatic plate concresce behind it. 

 There is no backward growth to form a ventral strip such as occurs 

 in Ischnochiton. This latter portion is now produced by the fusion 

 of the borders of the somatic plate in the process of concrescence 

 which is to be looked upon as a secondary phenomenon due to the 

 excessively rapid development of the somatoblast. But although 



Zool. Jahrb. XU. Abth. f. Morph. 42 



