668 K. KIRKPATRICK. 



narrow lumen (or excurrent canal). In the course of growth 

 the clusters expand at the same time that their lumen 

 increases, and they encroach on the incurrent spaces, which 

 may now consist of narrow channels or canals completely sur- 

 rounded by flagellated chambers. 



A tube of flagellated chambers may be compared to a 

 hollow central column (say with a "pore" at the summit 

 opening on the roof) supporting a groyned vault, the concavo 

 surfaces of which face the interior of the building: which 

 leads to a door (oscule). 



The excurrent spaces or channels in the upper part of the 

 sponge often form wide pouch-like out-foldings, but those in 

 the deeper parts which are crowded into the uppermost open 

 crypts of the hard skeleton are in the form of narrow 

 canals. 



In a section the in- and excurrent canals are easily distin- 

 guished by the direction of the apopyles and the collar-cells. 

 A terminal excurrent canal or space opens into the floor of one 

 of the large ectosomal spaces, and the latter open to the 

 exterior by an oscule. 



The flagellated chambers possess a remarkable structure 

 uot known to occur elsewhere. They are semi-oval or hemi- 

 spherical, and about 33 jw in diameter. They are usually 

 closely approximated to each other, being joined — or sepa- 

 rated — only by a few connective-tissue cells. 



Stretched across the excurrent aspect of each chamber is a 

 membrane with a thin-edged circular apopyle in the centre. 

 Nearly surrounding each apopyle is a single contractile cell 

 which acts as a sphincter muscle. The apopyles vary in 

 diameter from a point up to nearly the width of the flagellated 

 chamber. 



The prosopyles consist simply of spaces between the fused 

 rays of the stellate bases of the collar-cells (PI. 33, flg. 6) 

 — a character recalling the niembrana reticularis of Hexacti- 

 nellida. This stellate arrangement of the cell-plasma is 

 uot easy to see, and I am thereby led to believe that this 

 mode of union may possibly occur among other tetraxonid 



