Page:Encyclopædia Britannica, Ninth Edition, v. 22.djvu/446

 422 SPONGES eurypylous Rliagon type, but the apliodal is not unknown. The Ceratosa contain all sponges with a horny skeleton, except those in which the horny fibres are cored or spinecl with silicious spicules secreted by the sponge ("proper" spicules) ; these are arbitrarily assigned to the Monaxona. There is convenience in this proceed- ing, for horny matter is widely disseminated throughout the Demo- spongiae, occurring even in the Lithistidci, and it frequently serves to cement the oxeate spicules of the Monoxonct into a fibre, without at the same time forming a preponderant part of the skeleton. It would be wellnigh impossible to say where the line should be drawn between a fibre composed of spicules cemented by spongin and one consisting of spongin with embedded spicules, while there is com- paratively no difficulty in distinguishing between fibres containing spicules and fibres devoid of them. That the distinction, however, is entirely artificial is shown by the fact that, after spicules have disappeared from the horny fibre, they may still persist in the mesoderm ; thus Von Lendenfeld announces the discovery of micro- scleres (cymba) in an Aplysillid sponge and of strongyles in a Cacospongia, both horny sponges. (A form intermediate between this Aplysillid and the Desmacidonidsz would appear to be Toxo- chalina, Ridley.) The Ceratosa frequently enclose sand, Fora- minifera, deciduous spicules of other sponges and of compound Ascidians, and other foreign bodies within the horny fibres of their skeleton ; they also sometimes attach this material, probably by a secretion of spongin, to their outer surface, and thus invest them- selves in a thick protective crust. In some Ceratosa no other skeleton than that provided by foreign enclosures is present. The canal system is syconate or eurypylous in the simpler forms and diplodal in the higher. The Monaxonida make their earliest ap- pearance in the Silurian rocks (Climacospongia, Hiude), and are now found in all seas at all depths. The only sponges inhabiting fresh water belong to this group. The TetractincUida adhere to the Monaxonida at more than one point, and one of these groups has probably been a fruitful parent to the other, but which is offspring and which parent is still a subject for discussion. The Choristida in its simplest forms presents a eurypylous Rhagon system, in the higher an aphodal system. It is in this group that the most highly complex cortex is met with ; in the Gcodinidse, for instance, it consists usually of at least five distinct layers. Thus, proceeding outwards, next to the choanosome is a layer of thickly felted clesniachyme, passing into collenchyme on its inner face ; then follows a thick stratum of sterrasters united together by desmacytes ; this is succeeded by a layer of cystenchyme or other tissue of variable thickness ; external to this is a single layer of small granular cells and associated dermal asters ; and finally, the surface is invested by a layer of pavement epithelium. The Lithistida, like the Ceratosa, are possibly of polyphylitic origin ; in one group (Tctracladina) the articulated scleres are evidently modified calthrops spicules (see fig. 14 c), and associated with them are free truenes, which support the dermis and resemble precisely the trisenes of the Choristida. In another group (Bhabdacrepida) the scleres are moulded on a Monaxonid base (see fig. 13 q-s) ; but, associated with them, tria-nes sometimes occur similar to those of the Tctracladina. Both these groups are in all probability derived from the Choristida, and a distinct passage can be traced from the Tetracladose to the Rhabdocrepid group. In the Rhabdocrcpida we find forms without trirenes ; these may possibly be degenerate forms. The third group of Lithistids is derived from the Rhabdo- crepida, the Anomocladine desma being derivable from the Rhabdo- crepid by a shortening of the main axis into a centrum. The thick centrum, from which the arms, variable in number, ori- ginate, is hollowed out by a cavity, which appears during life to have been occupied by a large nucleus, like that of a scleroblast, and it is quite conceivable that the scleroblast, which in the Tetracladine Lithistids lies in an angle between the arms, may have become enclosed in an overgrowth of silica, from which addi- tional arms were produced. The constancy with which spicules in other sponges maintain their independence is very striking. When once a persistent character like this is disturbed, excessive variability may be predicted, as in the Anomocladine scleres. Classifi- The classification of the sponges into families is shown in the cation in following scheme. families. P1 f,. Tf,. T, r. Class CA LCA HE A. Order 1. HOMOCCELA, Pol. Family 1. ASCONID^E, Hk. Homocala which are simple or com- posite, but never develop radial tubes. Examples : Ascctta, Hk. (fig. 1) ; Lcucosolenia, Bwk. Family 2. HOMODERMID*, Lfd. Homoccela with radial tubes. Example : Homoderma, Lfd. (figs. 3, 4). Order 2. HETEROCCELA, Pol. Tribe a. tSvcoNAiuA. 1 The flagellated chambers are either radial tubes or cylindrical sacs. Family 1. SYCONID^:. The radial tubes open directly into the paragastric cavity. Sub-family a. Syconina. The radial tubes are free for their whole length, or at least distally. Examples : Sycetta, Hk.; Sycon, O.S. Sub-family b. Uteina, Lfd. The radial tubes are simple and entirely united. The ectosome is differentiated from the choanosome and sometimes develops into a cortex. Examples : Grantissa, Lfd. ; Ute, O.S. (fig. 5); Sycortu-sa, Hk.; Amphoriscu$, Pol. Sub-family c. Grantina, Lfd. The radial tubes are branched. The incurrent canal system is consequently complicated. An ecto- some is present. Examples : Grantia, Fl. ; ffeterqpcgma, Pol. (fig. 4); Anamaxilla, Pol. Family 2. SYLLEIBID.E, Lfd. The choanosome is folded. The flagellated chambers (which are partly rhagose in Vosmacria) communicate with the paragastric cavity by excurrent canals. Examples : Polejna, Lfd. (fig. 6) ; Vosmacria, Lfd. Family 3. TEICIIOKELLID^:, Carter. Composite Sy1leibid& with the oscules and pores occurring on different parts of the surface. Example : Teichonclla, Crtr. Tribe b. +LETJCOXARIA. The canal system belongs to the eurypylous Rliagon type. Family 1. LEUCONID>E, Hk. The outer surface is not differentiated into osculiferous and poriferous areas. Examples : Lcucetta, Hk. ; Leucaltis, Hk. ; Lcucortis, Hk. Family 2. EILHARDID.E, Pol. Composite Leuconaria, with the outer surface differentiated into special osculiferous and poriferous areas. Example : Eilhardia, Pol. The arrangement adopted above is founded on Von Lendenfeld's revision (//) of the classification propounded by Polejaeff (16), who in a masterly survey has thrown an unexpected light on the struc- ture and inter-relationships of a group which Haeckel has rendered famous. It should not be overlooked that Vosmaer ( j/) had pre- viously explained the structure of the Leucones. However errone- ous in detail, Haeckel's views are confirmed in their broad outlines, and it was with true insight that he pronounced the Calcarca to offer one of the most luminous expositions of the evolutional theory. In this single group the development in general of the canal system of the sponges is revealed from its starting-point in the simple Ascon to its almost completed stage in the Leucon, with a complete- ness that leaves little further to be hoped for, unless it be the re- quisite physiological explanation. Class MYXOSPONGIJE. Order 1. HALISARCINA. Family 1. HALISARCID.E, Lfd. The flagellated chambers arc syconate. Examples: Halisarca, Duj. (with branched chambers) ; Bajalus, Lfd. (with simple chambers). Family 2. OflCAEBZitlDJjL Lfd. The flagellated chambers are eurypylous and rhagose. Example : Oscarclla, Vosm. Order 2. CHONDROSINA. Family 1. CHONDROSIID.E. With the characters of the order. Example: Chondrosia, O.S. Class SILICISPOXGI&. Sub-class I. HEXACTINELLIDA. Order 1. tLYSSACINA. Family 1. Eun.ECTELLiD^E. The spicules of the dermal mem- brane are "daggers" (fig. 15 ). Examples: Evplcdclla, Owen; Holascus, E. Sch.; Habrodictyum, W. T. Family 2. ASCONEMATIIXE. The dermal spicules are " pinnuli " (fig. 15 b, c). Examples: Asconcma, S. Kent; Sympagclla, O.S. ; CaulophsRiis, Schulze. Family 3. HYALOXEMATIDJE. The dermal spicules are pinnuli and amphidisks (fig. 15 d). Example : Hyaloncma, Gray. Family 4. tRossELiD.E. The dermal spicules are ;omphi, stauri (fig. lo/), and oxeas. Examples: Rossclla, Crtr.; Crateromorpka, Gray ; Aulochona, E. Sch. Family 5. *RECEPTACULIDJE, Hinde. The distal ray of the dermal spicules is expanded horizontally into a polygonal plate. Example : * Rcccptaculitcs, Defr. Order 2. +DICTYONINA. Sub-order 1. UXCIXITARIA. Uncinate spicules are present. Tribe a. CLAVULARIA. Clavulaj (fig. 16 c) are present. Family 1. FAUREID.E. Characters those of the tribe. Example : Farrca, Bwk. Tribe b. SCOPULARIA. The dennal spicules are scopularine (fig. 16 b). Family 1. fEuRETiD/E. Branched anastomosing tubes, or goblet- shaped, with lateral outlets. Examples : Eurcte, Marshall ; Peri- phrayella, Marshall ; Lefroyclla, Schulze. Family 2. +MELI.ITTONID^;. Tubular or goblet-shaped, with honeycomb-like walls. Example : Aphrocallistes, Gray. 1 An * indicates that the group is only known in the fossil state, a f that it is both recent and fossil.