I04 



BIOLOGICAL LECTURES. 



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Extending the comparison into the later larval stages, we see 

 that this difference still persists, and is even more noticeable 



than in the earlier. In the larva of 

 AmpJiitj-ite of forty hours there is a 

 very marked development of meso- 

 dermal and trunk ectodermal tissue, 

 or, in other words, a very large 

 amount of material derived from 

 the descendants of 2d and ^J-d (Fig. 

 II, AmpJiitrite of forty-four hours, 

 from Mead). AmpJiitrite has four 

 well-marked body segments, with 

 strong setae on two of them (ecto- 

 dermal structures which have arisen 

 from 2d), and, according to Dr. 

 Mead, well-developed mesodermal 

 tissue lining the gut and body wall. 

 To be* noted, also, is the relatively 

 small development of the umbrella 

 portion (the portion in front of the 

 prototroch) and the extreme width 

 of the prototroch itself. The large 

 mucous glands and the " proble- 

 matic bodies" of Mead occupy a 

 large part of the surface of the um- 

 brella, the former extending back 

 into the subumbrella portion. 

 Turning now to Podarke, we find a very different condition 

 of affairs. The most noticeable feature is the complete lack of 

 any trace of segmentation. The subumbrella has elongated con- 

 siderably, but shows no trace whatever of a division into somites. 

 The umbrella is relatively much larger than in AmpJiitrite, a fact 

 which is undoubtedly correlated with the unusual development 

 of the cells compfising the cross in the former animal. I have 

 not succeeded in finding any mucous glands, but what are prob- 

 ably identical with Mead's "problematic bodies" and Wilson's 

 " frontal bodies" are found lying just in front of the small eye 

 spot on either side {pj-ob., Fig. 12, Podarke of forty-two hours). 



Fig. II. — Ainphitrite{3.i\^x 

 Mead), forty-four hours. 



