GILL DEVELOPMENT IN MYTILUS. 7 1 



further advance in this precocity leads to the condition found in 

 a still later filament (Fig. 4), in which the flexure may be con- 

 sidered as having already occurred at the time when the filament 

 is first budded forth. 



Thus it is possible to conceive the later method of develop- 

 ment as derived from the earlier ; but it is equally possible, so 

 far as mere geometrical relations are concerned, to reverse the 

 series and consider the later mode as really primitive, the earlier 

 mode as a specialization. It may be that the feeding require- 

 ments of the young animal are such as to require an adaptive 

 modification in the direction of a speedy lengthening of the gill 

 filaments ; but it is likewise possible that the later mode of devel- 

 opment is an adaptation to some factor or combination of factors 

 in the life of the older animals. The startling similarity of the 

 anlage of the later filaments to the gill in young specimens of the 

 very primitive Nucula, as figured by Drew ('01, Fig. 46) make it 

 very tempting to interpret the later type as primitive ; but the 

 structures are in such undeveloped condition that such compari- 

 sons are hazardous. The safe course is a suspension of judgment. 



Filament Development in Other Lamellibranchs. 

 In connection with the above observations on Mytilus, the gill 

 development in various other forms has been hastily studied for 

 comparison. Mya is the only genus in which very young speci- 

 mens have been observed. Here the early development seems 

 to agree essentially with Lacaze-Duthier's description of Mytilus. 

 Later stages have been studied in Mya, Anomia, Modiola and 

 Area. In all these genera the development of the later filaments 

 follows the scheme here set forth for Mytilus. It is highly prob- 

 able that this modification of development is characteristic of the 

 later filaments in the Lamellibranchiata in general. 



Development of Interlamellar Connections in Mytilus. 

 The contrast of the interlamellar connections in the closely 

 related genera Mytilus and Modiola has been emphasized in a 

 former paper ('98). In Mytilus these connections (Fig. 7, B, b) 

 are irregularly cylindrical, and each contains a large blood cavity 

 communicating with the cavities in the two limbs of the filament. 



