13] 



inhalent aperture does not pass backward but passes 

 directly forward to the pre-branchial chamber, and only 

 bathes the anterior part of the sixth gill and all the gills 

 in front of this. The posterior gills are supplied by water 

 entering the posterior inlialent aperture. He denies that 

 there is a backward current caused by the flabella. 



My own observations on these points are as follows : 

 — There are two inhalent apertures — the anterior and 

 posterior. Between these two apertures the inner border 

 of the sub-branchial region is closely applied to the side 

 of the thorax, and there appears to be absolutely no inflow 

 of water along this border. Of the two inhalent 

 apertures, the anterior is decidedly the most important. 

 The current of water flow r ing in through the posterior 

 aperture is very small. I think it extremely probable 

 that some of the water drawn in at the anterior inhalent 

 aperture passes backward, but I do not accept the 

 explanation of Claus — that the backward current is caused 

 by the flabella of the maxillipedes. The presence of the 

 well-defined branchial ridge (see above) in the anterior 

 part of the branchial cavity has suggested another 

 explanation. The ridge is situated on the floor of the 

 branchial chamber immediately in front of the anterior 

 inhalent aperture. It arises near the inner side of the 

 chamber, and passes in front of the aperture as a 

 transverse wall. ( hi the outer side of the aperture it turns 

 backward and outward, and after extending half way 

 down the branchial chamber, it gradually dies away. 

 At its anterior end the branchial chamber is exceedingly 

 shallow, so that the ridge almost extends to the roof of 

 the chamber, leaving only a narrow slit which communi- 

 cates with the pre-branchial cavity. As the water flows 

 in through the anterior inhalent aperture, it will be drawn 

 forward by the vigorous action of the scaphognathite. 



