REPORT ON THE TETRACTINELLIDA. 
143 
for I find that it marks the ends of those excurrent canals which terminate by simple ostia 
in an exceptional specimen to be further referred to later. The mesoderm of the choano- 
some is a sarcenchyma. The flagellated chambers (PI. XIII. fig. 9) are very clearly 
and sharply defined in all their characters. The choanocytes present a rounded base with 
the usual nucleus and nucleolus, a short collum deeply stainable extends from it, and is 
continued into an unstained structure seen as two sharply marked lines in section ; this 
is apparently the collar ; the collar enters into the fenestrated membrane ; the whole 
length of the cell is about 0*0118 mm. The prosopyle is single and large, about 0*0118 
to 0*016 mm. in diameter; the apopyle appears usually to be smaller, from 0*008 to 
0*012 mm. across; a narrow aphodus proceeds from it, on an average about 0*019 mm. 
long, but varying according to the distance to be traversed from the canal into which it 
flows and the chamber to which it belongs. The chamber always lies close to the 
ultimate wide branch of one of the incurrent canals, the prosopyle opening immediately 
into it or by means of a very short prosodus. The marked contrast between the large 
ultimate in current canals, with the layer of flagellated chambers immediately sur- 
rounding them, and the small branches of the excurrent system, with their repeated 
ramifications ending in a special aphodal canal for each flagellated chamber, is well 
shown in PI. XIII. fig. 8. 
The peripheral ends of the excurrent canals are no larger than those of the incurrent 
system, and only in a few specimens is an excurrent canal of large size to be met with 
at all, and this does not communicate directly with the exterior, but by means of smaller 
branches which run radially to the ectosome, to open by the usual cribriform pore-areas. 
These areas, which present the same characters whether serving for admission of water to 
the incurrent system or its ejection from the excurrent, occur between the deuterocladi 
of the dichotrisenes ; these, extending horizontally in the ectosome beneath the epithelium, 
form by their symmetrical disposition a very regular framework ; within the areas of this 
framework (PL XV. fig. 20) the ectosomal roof is perforated by pores, from 0*004 to 
0*015 mm. in diameter ; the margin about each pore is very thin, consisting of an 
inner and outer layer of epithelium, with an excessively thin intervening layer of 
collenchyma containing a few fusiform cells ; between the pores these increase in number 
as the collenchyma in thickness, forming a secondary framework of tissue within the 
primary spicular framework ; the thickening of the framework does not affect the level 
surface of the exterior epithelium, but bulges out on the under surface of the roof, so that 
in transverse section each pore forms an opening at the summit of a low dome, the walls 
of which are formed by the secondary framework. In one specimen the areas at one end 
of the sponge are each occupied by a single aperture or ostium which replaces the 
fenestration just described. These ostia are the openings of excurrent canals. 
In Anihastra pulchra and the present species we are presented with a series of 
stages in the modification of the excurrent canal system which serves to explain the 
