CLASS I SPONGIAE 43 
system, but the excurrent canals terminate directly in small openings situated 
on the upper surface of the body. The cloaca when present is often of 
considerable depth, although sometimes shallow, or reduced to a mere sac-like 
prolongation of the oseulum. Forms with a large and deep cloaca are regarded 
as single individuals, those with numerous cloacae and oscula as colonies. But 
since all the cloacae of a colony communicate by means of canals, while the 
oscula are never surrounded by a crown of tentacles, it is often difficult to 
distinguish between large excurrent canals and true cloaca, and hence also 
between individuals and colonies. 
Reproduction is either sexual or asexual. In the first process the fertilised 
ova complete a tolerably regular segmentation, develop into a gastrula, pass out 
through the osculum, and attach themselves to some foreign object. Asexual 
reproduction takes place by budding, the young buds remaining attached to the 
parent individual, and thus giving rise to colonies. Reproduction by means of 
fission is of rare occurrence. 
The great majority of sponges secrete a skeleton composed either of horny 
fibres or of silicious or calcareous spicules, or they incorporate foreign bodies 
into their framework. Only a few recent forms (Myxospongiae) are with- 
out a skeleton. In the horny sponges (Ceratospongiae) the skeleton consists 
of anastomosing and reticulated fibres of spongin, an organic nitrogen compound 
resembling silk. The fibres are either solid, or they contain an axial canal, 
which is sometimes cored with foreign bodies, such as sand-grains, fragments of 
sponge-spicules, Foraminifers, Radiolarians, ete. 
Silicious spicules are sometimes encased in horny fibres, sometimes occur 
detached in the cellular tissues, or are interwoven and consolidated with one 
another in various ways to form scaffoldings. In each genus the skeleton is 
composed of but a single form, or at the most of but a few regularly repeated 
varieties of silicious bodies, which are called the skeletal elements. In addition 
to these there occur more or less abundantly, especially on the outer surface 
and in the cloacal and canal walls, extremely delicate flesh-spicules, usually of 
small size and of great diversity of form. The flesh-spicules are as a rule 
destroyed during fossilisation. All the silicious skeletal elements are secreted 
by nucleated cells, and are composed of concentric layers of colloidal silica, 
deposited usually about a slender axial canal. In some spicules, notably those 
having spherical or stellate contours, the axial canal is wanting. It is very 
delicate in fresh spicules, but becomes enlarged by maceration, and in fossil 
specimens it is often coarsely calibrated. 
The multitudinous varieties of silicious skeletal elements (Fig. 51) are 
resolvable into a few fundamental types, as follows :— 
(a) Uniaxial spicules or JMonaxons (Fig. 511~' and 4-16). Straight or 
bent, smooth, prickly or knotty, bevelled, sharpened or truncated needles, rods, 
hooks, clasps, pins, and anchors (amp/hidiscs). They invariably contain an axial 
canal, which may be either entirely sealed up, or open at one or at both ends. 
(b) Tetraxial spicules or Tetraxons (Fig. 51). The normal form is 
characterised by four equal rays intersecting like the bisectrices of the plane 
angles of a regular tetrahedron. Triaxial forms result from the occasional 
abor tion of one : of the rays. One of the rays may become elongated or other- 
wise modified so as to form anchors (friaens) with three simple or furcate hooks 
(Fig. 5118~?5). Three of the rays may be numerously divided or foliately 
expanded so as to produce forms resembling thumb-tacks (¢richotriaens, phyllo- 
