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 hthistid 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 
