STRUCTURE AND FUNCTION OF THE SPHAERIDIA. Ad 
selected for the reason that they control each other in possible 
points of error, that the reticulated tissue of Lovén is a system 
of canals running through the vitreous calcareous substance, 
and not another sort of finer carbonate. This canal system, as 
above stated, is but a modified form of the canalicular spaces 
of the spines. In the neck of the spheridia the canalicular 
spaces occupy a large part of the room enclosed by the outer 
crust, while in the head the meshes are modified into long 
branching (anastomosing) canals. The central canals, of which 
there may be few (2—5) or many (10), run perpendicularly 
through the head and neck. In the neck the structure 
resembles that of the spines of the species—there being an 
outer crust and an inner network or reticulation of calcareous 
bars; both of which possess characteristic forms. In the head 
the reticulations as such have almost disappeared, persisting 
only near the centre, their place is filled by long irregular 
canals, which, however, now and then indicate by the regularity 
of their arrangement their source (compare figs. 16,17). In 
fig. 16 five rows of the external openings of the canal system 
are shown ; in fig. 17 only one such row of canals, leading from 
the external mouths to the central canals, is shown. The 
external openings or mouths of the canal system (figs. 1, 4, 5, 
6, 7, 8, 10, 15, 16, 17) are usually funnel shape, quite fre- 
quently is the inner part of the canal obliterated by the growth 
of the lamelle, leaving only the funnel-shaped mouth to indi- 
cate its former existence (figs. 4, 7, 8, 10). The internal 
anastomoses of the canals of the head are very irregular. It is 
impossible in some cases to trace the connection between the 
canals of the neck and those of the head (in optical section), 
the critical point being at the junction of the two parts, z.e. at 
the commencement of the swelling of the globule. 
The contents of these canals is mostly nervous cells, though 
frequently there is found besides the nerve-tissue a chlorophyl- 
green fluid (e. g. Echinus melo). 
The soft tissues of the spheridia are the epithelial covering, 
the nervous filaments and nerve-cells of the canal system and 
the contractile and fibrous elements joining the globule to the 
