No. 3.] PODARKE OBSCURA VERRILL. 449 



Mesoblast and Larval Mesoblast. 



As already stated in the descriptive portion of tiiis paper, 

 a double origin of mesoblast seems to occur in a number of 

 annelids and mollusks, the phenomenon apparently being more 

 widespread in the latter than in the former group. To the 

 one tissue Lillie gave the name "larval mesoblast," because of 

 its purely larval function. The cells composing it early lose 

 their connection with one another, and wander through the 

 segmentation cavity, where they become elongated and form 

 the larval muscles. The term "mesenchyme" has been so 

 generally applied to cells having this structure and position 

 that Lillie uses the name here and supposes that these cells 

 are phylogenetically related to the mesenchyne cells of ances- 

 tral forms which originally arose from ectoblast, but in this 

 case are massed into a single cell. 



The second source of mesoblast in Unio is at the posterior 

 end of the body, arising from the cell 4d, and differs from the 

 first in that its cells are grouped into definite germ bands, 

 and give rise to the mesoderm of the adult body. 



As we have already seen, while the definitive mesoblast of 

 annelids and mollusks usually arises as in Unio from the cell 

 4d, an additional larval mesoblast having a different origin but 

 similar function in each case has been described for Physa, 

 Planorbis, and Crepidula, among the mollusks, and for Aricia 

 and Podarke among the annelids. Conklin (No. 5, «, p. 15 i) has 

 stated his belief that the larval or primary mesoblast is phylo- 

 genetically the older, and indicates a primitive radial origin for 

 this layer, while it has been replaced in the ontogeny by the 

 secondary or bilateral mesoblast, arising from 4d. 



Professor Wilson, as a result of renewed investigation on 

 platodes ^ (see p. 462), has eliminated some apparent inconsist- 

 encies between these forms and the annelids, lamellibranchs, 

 and gasteropods, and has shown that Lang was probably correct 

 in his derivation of the mesoderm in polyclades from the second 

 and third quartettes of micromeres. In an exceedingly inter- 

 esting and suggestive article Professor Wilson has proposed the 



1 See also Appendix to Literature. 



