52 
CAULOPHACUS SCHULZEI. 
connecting their ends, and 7-26 n thick near the middle. The central tyle 
measures 11-32 n in transverse diameter and is 2-9 n, on an average about 4.4 n, 
thicker than the adjacent parts of the spicule. In the central tyle a transverse 
cross is observed, composed of four rudimentary axial threads, about 1 n long. 
At the ends, these rhabds are 0-4 n thinner than near the centre. Their aniso- 
actinity is not great, the difference in the thickness of the two ends usually not 
exceeding 2-3 n. The terminal spiny regions are 20-45 n long. The spines in 
them stand close together, attain a length of about 1 /x, and arise vertically. 
They are either straight or slightly curved backwards, towards the middle of 
the rhabd, at the end. 
In comparing the measurements of the rhabds of the body of the small 
specimen (with a disc about 30 mm. in diameter) with those of the rhabds of 
the large specimen D (with a disc about 60 mm. in diameter), I found no per- 
ceptible difference in their length, but a well-pronounced difference in their 
thickness, the body-rhabds of the 30 mm. -specimen B being 10-21 u thick and 
having central tyles 15-26 m in diameter ; those of the 60 mm. -specimen D being 
10-26 n thick and having central tyles 16-32 n in diameter. 
Wilson ( loc . cit., p. 45) states that in the specimens examined by him the 
rhabds were 1-4 mm. long, usually 1.5-2. 5 mm.; 8—12 thick, exceptionally 
24 n] and “ subterminally roughened with microtubercles.” To me the sub- 
terminal protuberances appear as sharp-pointed spines and I should not call 
them “microtubercles.” 
The hexactine megascleres of the choanosome (Plate 7 , figs. 20-31) measure 
1.2-3. 2 mm. in total diameter. The rays of the same hexactine are fairly equal 
in thickness but differ more or less, often very considerably, in length. In 
many hexactines the longest ray is two to three times as long as the shortest. 
The rays are 250 ^-1.4 mm. long, conic, blunt, 25-74 n thick at the base, and 
7-18 yu just below the end. They are more or less, often considerably, curved, 
rarely angularly bent (Plate 7 , fig. 28). The long rays are invariably smooth 
and attenuated toward the end. The rays reduced in length are either conic, 
pointed, and spiny (Plate 7 , fig. 21) or, more rarely, cylindrical, terminally 
thickened, and smooth (Plate 7 , fig. 29). 
In the shortened conic and spined rays there is a correlation between the 
number and size of the spines on the one hand, and the degree of longitudinal 
reduction of the ray on the other, the development of the spines being in pro- 
portion to the degree of reduction. Here, as in the similar case of Calycosilva 
cantharellus, this correlation between spine-development and reduction in length 
