CANAL-STRUCTURES. 53 
layer reached the ciliated chambers, and then passed to the exterior through them. 
This canal-system is typically seen in the genera Craticularia, Guettardia, 
Coscinopora, and Stawronema. 
(4) In which an inter-canal system is developed. In addition to the tubular 
blind canals in the proper wall of the Sponge, a secondary system of lacunar spaces 
and anastomosing channels is produced by the infolding and convolutions of the 
walls. As a well-known example may be mentioned the genus Ventriculites. In 
Sponges of this genus the spicular membrane of the wall is disposed in a series of 
closely-arranged vertical folds, and the outer surface exhibits vertical ridges and 
furrows in which are rows of elongated apertures. These apertures are connected 
with the interspaces between the folds of the wall, as may be seen in a transverse 
section through the Sponge, and therefore belong to the inter-canal system. The 
interior surface of the Sponge, next the cloacal cavity, or on the upper surface 
where no cloaca is present, is usually lined with a dermal membrane with regularly 
arranged circular apertures, which likewise connect inwardly with the interspaces 
between the folds of the wall. The genus Cephalites has a disposition of the 
inter-canal system similar to that in Ventriculites. 
In Camerospongia and Oystispongia the laminated walls of the Sponge are still 
further folded, and contained as in a sac by an outer spicular membrane, in which 
there may be a funnel-shaped cloaca, or merely several wide apertures leading 
directly into the interior of the sac. In the genus Plocoscyphia the folds of the 
wall are very complex, and they generally anastomose so as to form an intricate 
system of wide tubes and lacunar spaces which belong to the inter-canal system. 
The walls themselves usually possess a distinctive perforated dermal layer, and they 
are penetrated on both sides by blind canals, like the walls of those Sponges in 
which no folds occur. In the genus Cwloptychiwm the folded walls of the Sponge 
are contained as in a case by arigid spicular, perforated dermal layer, which freely 
admits the flow of water into the internal cavity; and on the ridges of the under 
surface of the Sponge, formed by the folding of the wall, there are rows of oval or 
elongated apertures, which may be compared to vents. 
Canal-system of Fessil Calcisponges.—In a single genus, Protosycon, there appears 
to be a similar arrangement of the canals as in Haeckel’s family Sycones. In this 
form the walls are penetrated by horizontal tubes or radiate canals which open into 
the tubular cloaca. In some calcisponges no distinctive canal-system is shown in 
the disposition of the spicular fibres of the skeleton, and the canals in the living 
animal must have followed the interspaces between the fibres. Thus in some 
species of Peronella, in which the Sponge is cylindrical, with an axial cloacal tube, 
no traces of canals appear in the skeleton, and the canals, which most probably in 
the living Sponge opened into the cloaca, entered through the ordinary interspaces 
of the skeletal fibres. Also in Pharetrospongia and Pachytilodia, in which the walls 
