CANAL-STRUCTURES. 51 



and without cloaca. Typical examples belong to Verruculina, Chenendopora, and 

 Kalpinella. 



(3) In this system a well-marked cylindrical or funnel-shaped cloaca is present. 

 The excurrent canals can be traced from near the outer surface to the cloacal 

 cavity, in which the vents are disposed either in rows or irregularly ; the canals are 

 either simple or branched, and nearly horizontal in direction. The incurrent 

 canals begin near the cloacal surface and radiate outwards, opening at the surface 

 of the sponge-wall. The Sponges are usually cylindrical or clavate. This system 

 is present in the genera Cylindrophyma and Phymatella. 



(4) This modification closely approximates to the preceding. It occurs in 

 sub-spherical or cylindrical Sponges with deep and narrow cloacal tubes. The 

 incurrent canals are numerous, fine, and unbranched ; they extend either horizon- 

 tally or obliquely from the outer surface to the interior of the wall. In some 

 instances the excurrent canals open into the cloacal tube as in the preceding 

 modification, and they also appear as open furrows radiating down the sponge- 

 wall from the margins of the cloacal aperture. This modification occurs in 

 Scytalia and Pachinion. 



(5) In this the incurrent system is represented by numerous delicate canals 

 extending from the outer surface of the Sponge in an arched direction towards the 

 centre, whilst the excurrent system consists of relative large canals which extend 

 from the basal portion of the Sponge in a generally vertical direction parallel to its 

 contour, and open into the cloaca. These excurrent canals are frequently shown 

 as open furrows on the outer surface of the Sponge, extending from the margins 

 of the cloaca to the lower portion of the body. In the living condition they 

 were covered over by the soft dermal tissues, as well as by the skeletal dermal 

 layer of spicules, which is now rarely preserved in situ. These superficial ex- 

 current canals, now represented as open furrows, would, in the further growth of 

 the Sponge, become enclosed by the skeletal mesh, and then resemble the present 

 internal canals, which have all, in their turn, been formed just beneath the dermal 

 surface of the Sponge. 



This canal-system is typically developed in Siphonia (fig. 1), Melonella, 

 Aulocopium, and Astylospongia ; and it is also present in those lithistid Sponges of 

 conical, cylindrical, or branching form, in which no cloaca is developed, and the 

 vertical excurrent canals extend the entire length of the Sponge, and open at its 

 summit, either grouped in bundles or apart from each other. In the branching 

 Sponges the vertical canals extend the entire length of the branches, and open at 

 their distal ends. In these Sponges the finer incurrent canals are either horizontal 

 or oblique in their course, similar to those in Sponges where a cloaca is present. 

 Examples of this type belong to Jereica, Stichophyma, Doryderma, and Jerea. 



(6) In this last division the massive wall of the Sponge is divided up into 



