,46 



SPONGES 



lated chamber (fig. 20, ii.). The presence of this membrane 

 enables us readily to distinguish the excurrent from the 



r ' 



Pia. 20.— Choanoeytes -with coalesced collars, (i.) Longitudinal section through 

 two flagellated chambers of Anthastra communis. Soil. ; i, prosopyles ; c, 

 aphodal canals leading from the flagellated ohamhers ; e, excurrent canal ; 

 the tissue surrounding the chamhers is sarcenchyme ( x 360). (ii.) Diagram 

 showing the fenestrated membrane (m) produced by coalesced collars of 

 choanoeytes. After SoUas, " Challenger " Report. 



incurrent face of the chamber, since its convex surface is 

 always turned towards the prosopyle. In sponges with an 



Fio. 21.— Histological elements, a, collencytes, from Thenea mwieata; h, 

 chondrenchyme, from cortex of Corticium candelabrum (the unshaded bodies 

 are microscleres); c, oystenohyme, from Pachymatisma johnstoni (partly dia- 

 grammatic) ; d, desmacyte, from Dragmastra norrtuini ; e, myocytes in con- 

 nexion with collencytes, from Cinachyra larbata ; f, thesooyte, from Thenea 

 murimta ; g, choanocyte, from Sycandra raphanus ; h-n, scleroblasts— A, of 

 a young oxea, from an embryo of Craniella cranium ; », of a fully grown oxea, 

 from an adult C. cranium ; j, orthotriiene, with associated scleroblast from 

 SicUeita ; fc, of a tetracladine desma, from Theonella swinhoei ; i, of a sigma- 

 spire, from Craniella cranium ; m, of an orthodragma, from Disyringa dis- 

 similis ; n, of a sterraster, from Geodia barretti. Figs. 6 and g after Schul^e, 

 the others after SoUas. 



aphodal canal system the flagellated chambers usually pass 

 gradually into the aphodal canal, but the incurrent canal 



enters abruptly. This abrupt termination of the incurrent 

 canal appears to mark the termination of the ectoderm 

 and the commencement of the endoderm. The flagellated 

 chambers differ greatly in size in different sponges, and 

 evidently manifest a tendency to become smaller as the 

 canal system increases in complexity; thus Sycon are always 

 larger than Ehagon chambers, and eurypylous than aphodal 

 Rhagon chambers. In most sponges except the Ascons the 

 mesoderm is largely developed, and in many it undergoes Meso- 

 a highly complex histological- differentiation. In its com- derm, 

 monest and simplest form it consists of a clear, colourless, 

 gelatinous matrix in which irregularly branching stellate 

 cells or connective tissue corpuscles are embedded ; these 

 may be termed collencytes (fig. 21 a) and the tissue collen- 

 chyme. In the higher sponges {Geodia, Stelletta) it consists 

 of small polygonal granular cells either closely contiguous 

 or separated by a very small quantity of structureless jelly, 

 and in this form may be termed sarcenchyme (fig. 20). 

 Collenchyme does not originate through the transformation 

 of sarcenchyme, as one might expect, for it precedes the 

 latter in development. Schulze {so), who has compared 

 coUenchjrme to the gelatinous tissue which forms the chief 

 part of the umbrella of "jelly-fish," describes it as becoming 

 granular immediately in the neighbourhood of the flagel- 

 lated chambers in the bath sponge, the granules becoming 

 more numerous in sponges in which the canal system 

 acquires a higher differentiation, till at length the collen- 

 cytes are concealed by them. According to this view, 

 sarcenchyme would appear to originate from a densely 

 granular collenchyme. Amceboid wandering cells or archer 

 ocytes (fig. 22) are scattered through the matrix of the 

 collenchyme. They evidently serve very different purposes : 

 some appear to act as carriers of nourishment or as 

 scavengers of useless or irritant foreign matter; others 

 may possibly contribute to the formation of higher tissues, 

 some certainly becoming converted into sexual products. 

 Their parentage and early history are unknown. 



A tissue [cystenchyme) whicli in some respects resembles certain 

 fonns of vegetable parenchyme occurs in some sponges, particularly 

 GeodimidsR and other TetractinelUda. It consists of closely ad- 

 jacent large oval cells, with thin well-defined walls and fluid 

 contents. Somewhere about the middle of the cell is the nucleus 

 with its nucleolus, supported by protoplasm, which extends from 

 it in fine threads to the inner side of the wall, where it spreads out 

 in a thin investing film (fig. 21 c). Cystenchyme very commonly 

 forms a layer just below the skin of some Geodinidse, particularly of 

 Pachy7riaHsma, and, as on teasing the cortex of this sponge a large 

 number of refringent fluid globules immiscible with water are set 

 free, it is just possible that it is sometimes a fatty tissue, and if so 

 the contained oil must be soluble in alcohol, for alcoholic prepara- 

 tions show no trace of it. A tissue resembling cartilage, chondren- 

 chyme, occurs in Corticidse (fig. 21 6). 



Connective-tissue cells or desmacytes are present in most Desma 

 sponges ; they are usually long fusiform bodies, consisting ^y'^^- 

 of a clear, colourless, often minutely fibriUated sheath, 

 surrounding a highly refringent axial fibre, which stains 

 deeply with reagents (fig. 21 d). In other cases the des- 

 macyte is simply a fusiform granular cell, with a nucleus 

 in the interior and a fibriUated appearance towards the 

 ends. The desmacytes are gathered together, their ends 

 overlapping, into fibrous strands or felted sheets, which in 

 the ectosome of some sponges may acquire a considerable 

 thickness, often constituting the greater part of the cortex. 

 The spicules of the sponge often furnish them with a sur- 

 face of attachment, especially in the Geodinidss, where each 

 sterraster of the cortex is united to its neighbours by des- 

 macytes, in the manner shown in fig. 10. 



Contractile fibre cells or myocytes occur in all the higher Myo- 

 sponges. They appear to be of more than one kind. Most <=y^^- 

 usually they are fine granular fusiform cells with long 

 filiform terminations, and with an enclosed nucleus and 

 nucleolus (fig. 21 e). In the majority of sponges both ex- 

 current and incurrent canals are constricted at intervals 



