20 



THE VOYAGE OF H.M.S. CHALLENGER. 



HexactincUida from tlie fundamental typo, I will begin with those forms which, like 

 Bathydonts fimhriatus, are only slightly removed from the original saccular form, and 

 which by a simple elongation have assumed a tube-like outline. In a section through 

 the lamelliform smooth wall of Bathydorus Jimbriatus, such as is somewhat diagram- 

 matically represented in PI. LVIII. fig. 2, the only essential modification of the funda- 

 mental ty|ie is a peculiar folding of the chamber layer. This layer exhibits a series of 

 closely disposed broad protrusions of approximately similar form and ec^ual size, which 



Fig. 1. — Section of the wall of Walteriafleininingii, n. geu. et sp. 



raise the smooth external skin in such a way that diverticula, traversed and divided by 

 trabeculse, extend inwards from the subdermal lacunse of the outer trabecular space. 

 The lumina of the protrusions which open by a wide round aperture into the inner 

 trabecular space are at first destitute of a trabecular framework. 



In all the numerous Lyssacina, which, like Acanthascus (Pis. LVI.-LVIL), Rossella 

 (PI. LV.), &c., resemble a thick- walled beaker or cup, the folding of the chamber layer is 

 continued by the formation of successive protrusions, so that branched eflferent passages 

 of roundish section are formed, between which corresponding complex incurrent passages 

 penetrate inwards from the outer trabecular spaces. This development of a more or less 

 richly branched system of afferent and efferent passages, which are, however, completely 

 separated by the chamber layer, remains essentially unchanged, even with such 



