362 



GILMAN A. DREW. 



blood, they are probably not directly concerned in the circu- 

 lation of the blood. 



Between the chitinous trough and the suspensory mem- 

 brane there is a small bundle of muscle-fibres that are con- 

 tinued the whole length of the gill (fig. 53, Im.). This 

 bundle lies in the open part of the chitinous trough, and 

 probably corresponds to the large bundle that occupies a 

 similar position in the gill of Yoldia (Text-fig. U, Urn.). A 

 second longitudinal bundle of muscles is found in the gill of 

 Yoldia {ulm.), but this does not seem to be present in this 

 species. A few of the muscle-fibres in the suspensory mem- 

 brane seem to be continued into the plates. They are not 

 numerous, and they have not been carefully followed. The 

 gill of this species is so small that it is not favourable for the 

 determination of minute details. 



The gills probably act as respiratory organs, but their 

 small size, together with the blood-supply of other parts, 

 makes it seem probable that other organs, such as the mantle 

 and the palps, are also concerned in respiration. The opaque 

 character of the shells of adult animals makes it quite im- 

 possible to observe the normal movements of the gills. They 

 can be seen to move slightly, however, and it seems probable 

 that the suspensory membranes contract slightly at intervals. 

 Such movements would be useful in causing movements in 

 the contained blood, but they are not sufiicient to cause 

 strong currents of water. The shape of the gills is not such 

 as would make them efficient pumping organs. (Compare 

 fig. 53 and Text-fig. U.) Inasmuch as these animals live 

 entirely covered by mud, the production of strong currents 

 of water could not be beneficial. As the animal wanders 

 around in the mud the fgeces naturally drop out of the open 

 mantle chamber. 



It would be a matter of some interest if the exact relation- 

 ship of the gills of Nucula and Yoldia could be determined. 

 It would seem to be a comparatively easy task to account for 

 the changes in the shape and structure of the gill of Yoldia 

 if we were to start with a gill such as has been described for 



