REPOKT ON THE TETEACTINELLIDA. 
169 
that the lacunae of one canal are in communication with those of that adjoining. 
Numerous fusiform cells are present in the collenchyma as well as the usual branched 
collencytes, and large round or oval vesicular cells occur as well. These cells (PI. XVIII. 
■figs. 20, 21), which vary in abundance, sometimes lying so close together that they touch, 
are about 0’04 mm. in diameter, with a very definite outer wall, appearing in optical section 
as a thin double-contoured line ; circular when seen face on ; and, since they are much 
depressed, oval when seen in transverse section. Their thickness as shown in such sections 
is about 0’015 mm. Excentrically situated is a small circular nucleus, 0‘005 mm. in 
diameter, containing a small spherical nucleolus, and surrounded with a small irregular 
heap of granular protoplasm, from which irregular, branching, and often anastomosing 
granular threads extend to the outer wall, within and over which they expand into a 
continuous, granular, protoplasmic layer. These cells lie close to the epithelium of the 
lamella within which they occur, in a single layer facing the surface ; occasionally the 
layer may become double. 
The fusiform cells, often over 0‘296 mm. long, are variously arranged; some run 
radially through collenchyma from the spicular wall to the inner wall of the canals, 
entering a layer of concentrically arranged fibres with which the inner wall is surrounded. 
At varying intervals (PI. XYIII. fig. 17), on an average about 2 mm. apart, the lumen of 
the canal is crossed by a membranous diaphragm (homologous with the usual velum of the 
water canals), centrally perforated ; around this perforation dark gray, highly granular, 
fusiform cells are concentrically arranged, forming a powerful sphincter. 
The spicular fibres are encircled by sheaths of concentric fibres, and similar fibres 
wrap round individual spicules ; the spicules also serve as points of attachment for the 
fibres which radiate through the collenchyma ; when these fibres pass on to the sjDicules 
they appear to expand into a finely fibrillated film ; between the filmy ends of adjacent 
fibres, large oval cells with granular protoplasmic contents are developed close to the 
sides of the spicules (PL XVIII. fig. 24); the meaning of this arrangement is not obvious, 
as the cells do not appear to be scleroblasts. Between the spicular columns, running 
parallel with them longitudinally, are strong fibres about 0’04 mm. thick, composed of 
fusiform cells, which on reaching the distal disc of the cloacal tube enter it divergently, 
and radiate towards its margin and surface, terminating partly by enveloping the spicules 
of the disc, partly by entering a fibrous layer which coats its distal face. 
The outer wall (PI. XVIII. fig. 20) of the cloacal tube consists of a collenchymatous 
layer faced with epithelium on both sides, and continued on the inner face into the 
strands and lamellae of collenchyma before mentioned. Parallel longitudinal fibres of 
fusiform cells traverse it, bulging it on the inner side ; they are about 0*028 mm. wide, 
and on an average lie about 0*08 mm. apart. The outer wall is thus ruled out, as it were, 
into a number of longitudinal strips separated by the fibrous tracts ; beneath these strips 
lie the lacunae of the canals. Transverse thickenings of collenchyma, containing trans- 
(ZOOL. CHALL. EXP. — PART LXIII. — 1887.) Err 22 
