48 COELENTERATA— PORIFERA phylum ii 



The water enters through the dermal pores, and passes through the 

 incurrent canals into ciliated chambers, which are lined with epithelial cells. 

 From these it is conveyed through all parts of the body by means of the 

 frequently branching excurrent canals, which open into a sac-like, tube-like 

 or funnel-shaped cloaca. The exhalent opening of the latter is termed the 

 osculum. Extremely thin-walled sponges have no cloaca, osculum or branch- 

 ing canal-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 shalloAV, or reduced to a mere sac-like 

 prolongation of the osculum. 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, 



Eeproduction 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. Re- 

 production by means of fission forming new colonies is of rare occurrence. 



The great majority of sponges secrete a skeleton composed either of horny 

 fibres or of siliceous or calcareous spicules, or they incorporate foreign bodies 

 into their framework. Only a few Recent forms {Myxospongiae) are without 

 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, etc. 



Siliceous spicules are sometimes encased in horny fibres, sometimes occur 

 detached in the cellular tissues, or are interlaced and consolidated with one 

 another in various ways to form a supporting framework. In each genus the 

 skeleton is composed of but a single type, or at the most of but a few regularly 

 repeated varieties of siliceous 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 siliceous 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 siliceous skeletal elements (Fig. 46) may 

 be grouped into a few fundamental types, as follows : — 



(a) Uniaxial spicules or monaxons (Fig. 46 ^"^") and (^^~^^). Straight or 

 bent, smooth, prickly or knotty, bevelled, sharpened or truncated needles, 

 rods, hooks, clasps, pins and anchors (ampJddiscs). They invariably contain 

 an axial canal, which may be either entirely sealed up, or open at one or at 

 both ends. 



