SPONGES 



47 



cytes. 



by transverse diaphragms or vela, which contain myocytes 

 concentrically . and sometimes radiately arranged. The 

 excessive development of myocytes in such a velum gives 

 rise to muscular sphincters such as those which close the 

 chones of many corticate sponges, such as Fachpmatisma. 

 la this sponge, which occurs on the British shores, the 

 function of the oscular sphincters can be readily demon- 

 strated, since irritation of the margin of the oscule is 

 invariably followed after a short interval by a slow closure 

 of the sphincter. 



Supposed sense-cells or sesthacytes (fig. 22) were first 

 observed by Stewart and have since been described by 

 Von Lendenfeld {12). According to the latter, they are 

 spindle-shaped cells, O'Ol mm. long by 0'002 thick; the 

 distal end projects beyond the ectodermal epithelium in a 

 fine hair or palpocil ; the body is granular and contains a 

 large oval nucleus ; and the inner end is produced into 

 fine threads, which extend into the coUenchyme and are 

 supposed — though this is not proved — to become con- 

 tinuous with large multiradiate collencytes, which Von 

 Lendenfeld regards as multipolar ganglion cells (fig. 22). 



Proto- 

 plasmic 

 con- 

 tinuity. 



Fig. 22. — ^Transverse section through the edge of a pore in BendHlla cavernosa^ 

 Lfd. ; cells in the middle to the right, archseocytes ; fusiform cells on 

 each side of them, myocytes ; ff, ahove and helow these, with processes 

 terminating against the epithelium, gland cells ; fusiform cells terminating 

 against the epithelium at s, sesthacytes : at their inner ends these are con- 

 tinuous with ganglion cells. After Von Lendenfeld (x 800). 



More recently he has described an arrangement of these 

 cells curiously suggestive of a sense-organ. Numerous 

 sesthacytes are collected over a small area, and at their 

 inner ends pass into a granular mass of cells with well- 

 marked nuclei, but with boundaries not so evident ; these 

 he regards as ganglion cells. From the sides of the gan- 

 glion other slender fusiform cells, which Von Lendenfeld 

 regards as nerves, pass into the mesoderm, running tan- 

 gentially beneath the skin. The inner end of the ganglion 

 is in communication with a membrane formed of fusiform 

 cells which Von Lendenfeld regards as muscular. If his 

 observations and inferences are confirmed, it is obvious 

 that we have here a complete apparatus for the conversion 

 of external impressions into muscular movements. 



In most sponges a direct connexion can be traced by 

 means of their branching processes between the collen- 

 cytes of the mesoderm and the cells of the ectodermal 

 and endodermal epithelium and the choanocytes of the 

 flagellated chambers. As the collencytes are also united 

 amongst themselves, they place the various histological 

 constituents of the sponge in true protoplasmic continuity. 

 Hence we may with considerable probability regard the 

 collencytes as furnishing a means for the transmission of 

 impulses : in other words, we may attribute to them a 

 rudimentary nervous function. In this case the modifica- 

 tion of some of the collencytes in communication with the 

 ectoderm might readily follow and special sesthacytes arise. 

 Fusiform collencytes perpendicular to the ectoderm, and 

 with one end touching it, are common in a variety of 

 sponges; but it is diflacult to trace the inner end into 

 connexion with the stellate collencytes, so that precisely in 



those cases in which it would be most interesting to find 

 such a connexion absolute proof of it is wanting. 



The colour of sponges usually depends on the presence Pigment 

 of cells containing granules of pigment ; though dispersed cells, 

 generally through the mesoderm, these cells are most richly 

 developed in the ectosome. Pigment granules also occur 

 in the choanocytes of some sponges, — Oscarella lobularis 

 and Aplysina aerophoha, for instance. In the latter the 

 pigment undergoes a remarkable change of colour when 

 the sponge is exposed to the air, and finally fades away. 

 In many cases sponges borrow their colours from parasitic 

 algse (Osdllatoria and Nostoc) with which they are infested. 

 The colours of sponge-pigments are very various. They 

 have been examined by Krukenberg and Merejknovsky. 

 Zoonerythin, a red pigment of the lipochrome series, is one 

 of the most widely diifused ; it is regarded as having a 

 respiratory function. Reserve cells or thesoeytes (fig. 21/) 

 have been described in several sponges as well as amylin 

 and oil-bearing cells. 



Each spicule of a sponge originates in a single cell Sclero- 

 (fig. 21 h-n), within which it probably remains enclosed tilasts. 

 until it has completed its full growth ; the cell then prob- 

 ably atrophies. During its growth the spicule slowly 

 passes from the interior to the exterior of the sponge, and 

 is finally (in at least some sponges, Geodia, Stelletta) cast 

 out as an effete product. The sponge is thus constantly 

 producing and disengaging spicules ; and in this way we 

 may account for the ex-traordinary profusion of these struc- 

 tures in some modern marine deposits and in the ancient 

 stratified rocks. Within the latter these deciduous spicules 

 have furnished silica for the formation of flints, which have 

 been produced by a silicious replacement of carbonate of 

 lime (26). 



The horny fibres of the Geratosa are produced as a 

 secretion of cells known as sponginblasts, which surround 

 as a continuous mantle the sides of each growing fibre, and 

 cover in a thick cap each growing point (fig. 23). The 



Pig. 23.— Section through the homy fihre and associated tissues of a homy 

 sponge (D&ndrilla). A, longitudinal section ; s, layers of spongin, surrounded 

 at the sides by the lateral mantle of sponginblasts, and at the ends by the 

 terminal cap. A desmachymatous sheath, a, surrounds the whole (xl50). 

 B, transverse section ; in the centre is the soft core, surrounded by wavy 

 spongin layers, the outermost being surrounded by sponginblasts, and these 

 by a fibrous sheath ; i, part of an incuiTent canal lined by ilagellated epi- 

 thelium ; e, part of an exourrent canal ; /, part of a flagellated chamber ( x 150). 

 After Von Lendenfeld. 



lateral sponginblasts are elongated radially to the fibre ; 

 the terminal cells are polygonal and depressed. The latter 

 give rise to the soft granular core and the former to the 

 spongin -walls of the fibre. Cells similar to the lateral 

 sponginblasts, and regarded as homologous with them, 

 occur in a single layer just below the outer epithelium of 

 some horny sponges (Aplysilla and Dendrilla), and under 

 certain circumstances secrete a large quantity of slimy 

 mucus {11). 



