GILL DEVELOPMENT IN MYT1LUS. 65 



longest, the most developed, and, apparently, the oldest. Lacaze- 

 Duthiers figures an almost exactly similar stage ('56, Fig. 6), as 

 well as one a little older ('56, Fig. 7) in which the ninth of 

 fifteen filaments is the longest. This suggests strongly a devel- 

 opment of the gill in both directions in the early stages. 



On the other hand, a comparison of these early stages with 

 others much older shows that the position of the anterior end 

 of the outer gill is somewhat definitely located relatively to the 

 inner gill. There seems to be some individual variation ; but the 

 extension of the inner gill in front of the outer is measured in all 

 cases observed by about ten filaments. Owing to the non-par- 

 allelism of the filaments of the two gills, it is very difficult to in- 

 terpret figures that have not been drawn with reference to this 

 particular point ; but a study of Lacaze-Duthiers's plate seems 

 to show ten such filaments in an earlier stage ('56, Fig. 6) and 

 eleven in a later ('56, Fig. 7). As the inner gill is growing only 

 at the posterior end, and as the filaments of the two gills are 

 arranged somewhat definitely in corresponding pairs, this indicates 

 almost certainly that the addition of filaments at the anterior end 

 of the outer gill is confined to very early stages, if present at all. 

 As regards these early stages my observations are very incon- 

 clusive. 



In this connection it is also interesting to note that in a com- 

 paratively early stage, when the animal is only about 1.60 mm. 

 in length and the outer gill is composed of only about sixteen 

 filaments, the anterior filament of this gill is already strongly 

 differentiated from its neighbors in both size and shape. It is 

 decidedly thicker and longer, and is furnished with a very pecu- 

 liarly enlarged and twisted termination. In both total prepara- 

 tions and sections it is very distinct from the other filaments and 

 very easily identified. From this time on the development of the 

 outer gill is clearly from front to back only, and the two gills 

 develop pari passu. 



Mode of Formation of Later Filaments in Mytilus. 



The early filaments of the Mytilus gill follow the mode of de- 

 velopment so beautifully worked out by Lacaze-Duthiers, and 

 outlined briefly above ; the later filaments follow a very different 



