IO4 



BIOLOGICAL LECTURES. 



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 

 AmpJdtrite 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 qd (Fig. 

 n, Amphitrite of fo'rty-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 Amphitrite, a fact 

 which is undoubtedly correlated with the unusual development 

 of the cells comprising 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 (prob., Fig. 12, Podarke of forty-two hours). 



cir 



FIG. ii. A mphitrite (after 

 Mead), forty-four hours. 



