SPONGES 



45 



results (fig. 17 h). The pterocymba is subject to considerable modi- 

 fications : the prows may be similar (homoproral) or dissimilar 

 (hetcroproral) ; the pteres may be lamellar or ungual ; additional 

 lamellae (tropidial pteres) may be produced by a lateral outgrowth 

 of the keel (fig. 17 k) ; and by growing towards the equator the 

 opposed proral and pleural pteres may conjoin, producing a spicule 

 of two meridional bands (oocymba ; fig. 17 I). A curious group of 

 flesh spicules are the trichites. In this group silica, instead of being 

 deposited in concentric coatings around an axial fibre, forms within 

 the scleroblast a sheaf of immeasurably fine fibrillse or trichites, 

 which may be straight (fig. 17 m) or twisted. The trichite sheaf 

 may be regarded as a fibrillated spicule. Trichite sheaves form in 

 some sponges, as Dragmastrd, (25), a dense accumulation within 

 the cortex. In Hexactinellid sponges the rays of the aster are 

 limited to six, arranged as in a primitive sexradiate spicule, but 

 divided at the ends into an indefinite number of slender filaments, 

 which may or may not be tylotate, rosettes (fig. 17 1). 

 Spongin Spongin is a horny substance, most similar to silk in 

 Bcleres. chemical composition, from which it differs in being in- 

 soluble in an ammoniacal solution of copper sulphate 

 (cuproso-ammonium sulphate). In Darwinella aurea, F. 

 Miiller, it occurs in forms somewhat resembling tri-, 

 quadri-, and sex-radiate spicules. But usually the spongin 

 skeleton takes the form of fibres, consisting of a central 

 core of soft granular substance around which the spongin 

 is disposed in concentric layers, forming a hollow cylinder 

 (fig. 23 b). The relative diameters of the soft core and 

 of the spongin cylinder differ greatly in different sponges. 

 The fibres branch so as to form antler-like twigs or bushy 

 tree-like growths, or anastomose to form a continuous net- 

 work, as in the bath sponge (Eusponyia offidnalis). The 

 detailed characters of the network differ with the species, 

 and are useful in classification. In Janthella certain cells 

 (sponginblasts) become included between the successive 

 layers of the spongin cylinder, and their deep violet colour, 

 contrasting with the amber tint of the spongin, renders 

 them very conspicuous. 



Union of In some sponges the scleres are simply scattered through the 

 scleres mesoderm and do not give rise to a continuous skeleton, Cortirium, 

 into a Ch-ondrilla, Thrombus. In the Cakarea and many silicious sponges 

 skeleton, they are dispersed through the mesoderm, but so numerously that 

 by the overlapping of their rays a loosely felted skeleton is pro- 

 duced. In the calcareous sponges the spicules are frequently regu- 

 larly disposed ; and in the Sycons in particular a definite arrange- 



Fio. 1& Articulate and inarticulate tubar skeletons of calcisponges. a, articu- 

 late ; b, inarticulate skeleton. After Haeckel. 



ment, on two plans, the articulate and inarticulate, can be traced 

 in the skeleton of the radial tubes. On the latter plan the triradi- 

 ate or quadriradiate spicules, the apical rays of which are of con- 

 siderable length, are arranged in two sets, one having the basal 

 rays lying in the mesoderm of the paragastral wall and the other 

 with the corresponding rays in the dermal mesoderm. The apical 

 rays of each set lie in the mesoderm of the radial tubes parallel to 

 their length, but pointing in opposite directions (fig. 18 b). In the 

 articulate division numerous spicules, small in comparison with 

 the size of the radial tubes, form a series of rows round the tubes, 

 their basal rays lying parallel to the paragastric surface and the 

 apical pointing towards the ends of the radial tubes (fig. 18 a). 



In the Silicispongia sheaves of long oxeate spicules radiate from 

 the base of the sponge if of a plate-like form, or from the centre if 

 globular, and extend to the surface. If trianes are present their 

 arms usually extend within the mesoderm immediately below the 



dermal surface (fig. 19). Single spicules reach from centre to sur- 

 face only in small sponges. As the sponge increases in size the 

 spicules must either correspondingly lengthen, or fresh spicules 

 must be added, if a , / 



continuous skeleton is 

 to be formed. The 

 latter is the plan fol- 

 lowed in fact : the ad- 

 ditional spicules over- 

 lap the ends of those 

 first formed like the 

 fusiform cells in a 

 woody fibre. "With the 

 formation of a fibre, 

 often strengthened by 

 spongin or bound to- 

 gether with connective 

 tissue, there appears to 

 be a tendency for the 

 constituent spicules to 

 diminish in size, and 

 the length of each iu 



the most markedly Fio. 19. Mode of arrangement of spicules in a 

 fibrous sponges is in- 3g A^SouL SP ^' Dragmastm normini ' 

 significant when com- 

 pared with the length of the fibre. The spicular fibre thus 

 formed may be simple or echinated by spicules either similar to 

 those which form its mass or different. More usually they are 

 different, and generally styles, often spinose about their origin. 

 The spongin which sometimes cements together the spicules of a 

 fibre may progressively increase in quantity and the spicules di- 

 minish in number, till a horny fibre containing one or more rows 

 of small oxeas results. In an echinated fibre the axial spicules 

 may disappear and the eehinating spicules persist. Finally all 

 spicules may be suppressed and the norny fibre of the Ceratose 

 sponges results. The horny fibres may next acquire the habit of 

 embedding foreign bodies in their substance, though foreign en- 

 closures are not confined to the Ceratosa but occur in some Silici- 

 spongix as well. The included foreign bodies may increase in 

 quantity out of all proportion to the horny fibres ; and finally the 

 skeleton may consist of them alone, all spongin matter having 

 disappeared. 



In the Lithistid sponges a skeleton is produced by the articula- 

 tion of desmas into a network. The rays of the desmas (figs. 12 f, 

 13 s, 14 e) terminate in apophyses, which apply themselves to some 

 part of adjacent desmas, either to the centrum, shaft, arms, or 

 similar apophyses, and then, growing round them like a saddle on 

 a horse's back, clasp them firmly without anchylosis. Thus they 

 give rise to a rigid network, in conjunction with which fibres com- 

 posed of rhabdus spicules may exist. In the Hexactinellida both 

 spicular felts and fibres occur, and in one division (Dictyonina) a rigid 

 network is produced, not, however, by a mere clasping of apophyses, 

 but by a true fusion. The rays of adjacent spicules overlap and a 

 common investment of silica grows over them. 



Histology. 



The ectoderm usually consists of simple pavement Ecto- 

 epithelial cells (pinnatocytts), the margins of which can derm, 

 be readily rendered visible by treatment with silver nitrate, 

 best by Harmer's method. 1 The nucleus and nucleolus 

 are usually visible in preparations made from spirit speci- 

 mens, the nucleus being often readily recognizable by its 

 characteristic bulging beyond the general surface. In some 

 sponges (Thecapliord) the epithelium may be replaced 

 locally by columnar epithelium, and the cells of both pave- 

 ment and columnar epithelium may bear flagella (Aplysilla 

 violacea, Oscarella lobularis). The endoderm presents the Endo- 

 same characters as the ectoderm, except in the Ascons and derm, 

 the flagellated chambers of all other sponges, where it is 

 formed of collared flagellated cells or choanocytes, cells 

 with a nearly spherical body in which a nucleus and nucleo- 

 lus can be distinguished and one or more contractile vacu- 

 oles. The endoderm extends distally in a cylindrical neck 

 or colhim, which terminates in a long flagellum surrounded 

 by a delicate protoplasmic frill or collar (fig. 21 g). In 

 Tetractinellida, and probably in many other sponges cer- 

 tainly in some the collars of contiguous choanocytes 

 coalesce at their margins so as to produce a fenestrated 

 membrane, which forms a second inner lining to the flagel- 



1 S. F. Harmer, " On a Method for the Silver Staining of Marine 

 Objects," Mitth. Zoolog. Station zu Stopd, 1884, p. 445. 



