Page:Encyclopædia Britannica, Ninth Edition, v. 22.djvu/445

 SPONGES 421 Classification. The phylum Parazoa or Spongiae consists of two main kation. branches, as follows : Branch A. MEGAMASTIC- Branch TOR A. Class CALCAUEA, Grant. Order 1. Homoccela, Pol. Order 2. Heteroccela, Pol. Position in animal ungdom. B. MICROMASTIC- TORA. Class I. MYXOSPONGI*:, Haeckel. Order 1. Halisarcina. Order 2. Chondrosina. Class II. SILICISPONGI.S:. Sub-class i. HEXACTINELLIDA, 0. Schmidt. Order 1. Lyssacina, Zittel. Order 2. Dictyonina, Zittel. Sub -class ii. DEMOSPONGI.E, Sollas. Tribe a. Monaxonida. Order 1. Monaxona. Order 2. Ceratosa, Grant. Tribe b. Tetractinellida, Marshall. Order 1. Choristida, Sollas. Order 2. Lithistida, O.S. By the possession of both sexual elements and a complex histo- logical structure, and in the character of their embryological devel- opment, the sponges are clearly separated from the Protozoa ; on the other hand, the choanoflagellate character of the endoderm, which it retains in the flagellated chambers throughout the group without a single exception, as clearly marks them off from the Metazoa. They may therefore be regarded as a separate phylum derived from the choanoflagellate Infusoria, but pursuing for a certain distance a course of development parallel with that of the Mctazoa. Different views have been propounded by other authors. Savile Kent regards the sponges as Protozoa (/o) ; Balfour suggested that they branched off from the Metazoan phylum at a point below the Cwlentera, and considered them as intermediate between Protozoa and Mctazoa ; Schulze regards them as derived from a simple ancestral form of Ccelentera (23) ; Marshall advocates the view that they are degraded forms derived from Coelenterates which were already in possession of tentacles and mesenteric pouches (14). As a phylum the Spongix are certainly divisible into two branches, one including the Calcarea and the other the remaining sponges, which Yosmaer has termed Non-Calcarea, and others Plethospongise,. Since, however, the choanocytes of the Calcarea are usually, if not universally, larger than those of other sponges, we may make use of this difference in our nomenclature, and distinguish one branch as the Mcgamastictora (juwH/crwp, "scourger") and the other as the Micromastictora. Branch A. MEGA MA STICTORA. Sponges in which the choanocytes are of comparatively large size, 0'005 to 0'009 mm. in diameter (Haeckel, 6). Class CALCAREA. Calcarea. Mcgamastictora in which the skeleton is composed of calcareous spicules. Order 1. HOMOCOSLA. Calcarea in which the endoderm consists wholly of choanocytes. Examples : Lcucosolcnia, Bwk. ; Homo- di'rma, Lfd. Order 2. HETEROCOELA. Calcarea in which the endoderm is dif- ferentiated into pinnacocytes, which line the paragastric cavity and excurrent canals, and choanocytes, which are restricted to special recesses (radial tubes or flagellated chambers). Examples : Sycon, O.S. ; Granlia, Fl. ; Leuconia, Bwk. Branch B. MICR OMASTICTORA. (Non-Calcarea, Vosmaer ; Plcthospongito, Sollas.) Sponges in which the choanocytes are comparatively small, 0'003 mm. in diameter. Class I. MYXOSPONGI^!. Subdivi- sion in groups. Myxo- spongife. Micromastictora in which a skeleton or scleres are absent. Order 1. HALISARCIXA. Myxospongiee in which the canal system is simple, with simple or branched Sycon or eurypylous Rhagon chambers. An ectosome sometimes and a cortex always absent. Examples: Halisarca, Duj. ; Oscarella, Vosm. ; Bajalus, Lfd. Order 2. CHONDROSINA. Myxospongise in which the canal system is complicated, with diplodal Rhagon chambers and a well-developed cortex. Example : Chondrosia, O.S. The Halisarcina are evidently survivals from an ancient and primitive type. The simplicity of the canal system is opposed to the view that they are degraded forms ; we may therefore regard the absence of scleres as a persistent primary and not a secondary acquired character. They are as interesting, therefore, from one point of view (absence of scleres) as the Ascons are from another (undifferentiated endoderm). With the Chondrosina the case is different ; they differ only from Chondrilla and its allies by the absence of asters ; these differ only from the Tethyidfe by the absence of strongyloxeas ; and we may very reasonably assume that in these three groups we have a series due to loss of characters, the Chondrillse being reduced Tethyidx and the Chondrosina reduced Chondrillx. Still, as Huxley has well remarked, " classification should express not assumptions but facts " ; and therefore till we are in possession of more direct evidence it will be well to exclude the Chondrosina from the Silicispongise. Class II. SlLICISPONGUE. Micromastictora possessing a skeleton or scleres which are not calcareous. Sub-class i. HEXACTINELLIDA. Silicispongiw characterized by sexradiate silicious spicules. Hexacti- Canal system usually simple, with Sycon chambers. Sponge nettida. differentiated into ecto-, choano-, and endo-some. Order 1. LYSSACINA. Hexactinellida in which the skeleton is formed of separate spicules, or, if united, then by a subsequent not a contemporaneous deposit of silica. Examples : Euplectella, Owen ; Asconcma, S. Kent ; Hyalonema, Gray ; Rossella, Crtr. Order 2. DICTYONINA. Hexactinellida, in which sexradiate spicules are cemented together by a silicious deposit into a con- tinuous network pari passu with their formation. Examples : Farrca, Bwk. ; Eurete, Marshall ; Aphrocallistcs, Gray ; Myliusia, Gray ; Dactylocalyx, Stutchbury. The Hexactinellida are a very sharply defined group, impressed with marked archaic features. No other Silicispongise possess, so far as is known, so simple a syconate canal system. The oldest known fossil sponge is a member of the Lyssacina (7 and 24), viz., Protospongia, Salter, from the Menevian beds, Lower Cambrian, St David's Head, "Wales. The group is almost world-wide in distri- bution, chiefly affecting deep water, from 100 to 300 fathoms, but often extending into abyssal depths ; occasionally, however, though rarely, it frequents shallow water (Cystisjwngia siqierstes dredged off Yucatan in 18 fathoms). Sub-class ii. DEMOSPONGUE. Silicispongise in which sexradiate spicules are absent. Demo- Tribe a. MOXAXONIDA. Demospongise. in which the skeleton consists either of silicious spicules which are not quadriradiate, or of horny scleres or in- cluded foreign bodies, or of one or more of these constituents in conjunction. Order 1. MONAXONA. The skeleton is characterized by either uniaxial or polyaxial spicules. Examples : Amorphina, 0. S. ("crumb of bread" sponge); Spongilla, Link, ("freshwater" sponge) ; Chalina, Bwk. ; Tcthya, Link. Order 2. CEIIATOSA. The skeleton consists of horny scleres which never include "proper" spicules, or of introduced foreign bodies, or of both these in conjunction. Examples : Danvinella, F. Miiller; Euspongia, Bronn (the "bath" sponge). Tribe b. TETRACTINELLIDA. Dcmospongiie, possessing quadriradiate or trisene spicules or Lithistid scleres (desmas). Order 1. CHORISTIDA. Tetractinellida with quadriradiate or trirene spicules, which are never articulated together into a rigid network. Examples: Tetilla, O.S. ; Thenca, Gray ; Geodia, Lmk. ; Dercitus, Gray. Order 2. LITHISTIDA. Tetractinellida^ with branching scleres (desmas), which may or may not be modified tetrad spicules, arti- culated together to form a rigid skeleton. Triame spicules may or may not be present in addition. Examples : Theonclla, Gray ; Coral- listes, O.S. ; Azorica, Crtr.; Vctulina, O.S. This large sub-class embraces the great majority of existing sponges. Its external boundaries are fairly well defined, its internal divisions much less so, as its various orders and families pass into each other at many points of contact. Although there does not appear to be much resemblance between a Lithistid sponge, such as Theonclla, a Monaxonid such as Amorphina, and an ordinary "bath" sponge (Euspongia}, yet between these extremes a long series of inter- mediate forms exists, so nicely graduated as to render their dis ruption into groups by no means an easy task. If the delimitation of orders is difficult, that of genera is often impossible, so that they are reduced to assemblages depending on the tact or taste of the author. Thus Polejaeff states that with a single exceptK " none of the genera of Ceratosa are separable by absolute cha ters." The chief spicules of Mmiaxona are uniaxial, often ac __ j i -L._ ..Li.-, ^i^ ne , AltVmncrli distinguished as a ceptions are Known in wiucii uiesc u<^m v- 1 ">" -> Aearnus, Gray) ; these, however, present unusual characters which suggest an independent origin. The canal system of Monaxmm has not yet been fully investigated ; it appears usually to follow the