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 Craticidaria, Guettardia, 

 Coscinopora, and Stauronema. 



(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 Cystispongia 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 Coeloptychium the folded walls of the Sponge 

 are contained as in a case by a rigid 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 Fossil > 'alcisponges. — In a single genus, Protosycon, there appears 

 to be a similar arrangement of the canals as in Haeckel's family Sy cones. 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 



