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 osculum. Forms with a large and deep cloaca are regarded 

 as single individuals, those with numerous cloacae and oscula as colonies. I Jut 

 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 ami colonies. 



In the first process the fertilised 



ova comprete^aTtolerably 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 attachecfTicftire 

 parent individual, andthus 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, etc. 



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 Monaxons (Fig. 5 1 1 " 10 and 14 - 16 ). Straight or 

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

 hooks, clasps, pins, and anchors (amphidisrs). 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. 5 1 17 ). 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 

 abortion of one of the rays. One of the rays may become elongated or other- 

 wise modified so as to form anchors (triaens) with three simple or furcate hooks 

 (Fig. 5 1 18 ~ 23 ). Three of the rays may be numerously divided or foliately 

 expanded so as to produce forms resembling thumb-tacks (trichotriaens, phyllo- 



