Sept. 7, 1883.] 



KNOWLEDGE . 



149 



For even in the clearest weather there is always enough 

 haze near the sea-level to affect the distinctness of the sea- 

 horizon, when, owing to the observer's ascent, it is thrown 

 twenty or thirty miles away. 



{To he continued.) 



THE FISHERIES EXHIBITION. 



By John Ernest Ady. 



LET us now look a little higher in the scale of sponge 

 life. We find sponges which strongly resemble the 

 simple Ascetta* already described ; they go a step further 

 to form the family Leucmies, of which Lexicon (Fig. 13, d) 

 is a type. The advance in structural complexity consists 

 in this, — that the ectoderm or syncytium becomes greatly 

 thickened, and, as a consequence, the simple pores which 



Pig. 13. — Diagrams to show the structure of Ascoxes and Lici:- 

 CON'ES. A, Ascetta primordialis, one of the AscoNES. B, Portion of 

 the body-wall of the same highly magnified to show e c, ectodermal 

 syncytium with spicules ; e, flagellate cells of endoderm ; e', endo- 

 dermal cells without flagella ; p, an inhalent pore. D, Leucon, to 

 show greatly thickened syncytium with branched anastomosing 

 canals, and ciliated chambers, c, One of the ciliated chambers of 

 Leucon highly magnified ; e c, the syncytium ; e, endodermal cells of 

 ciliated chamber. 



lead into the body cavity of Ascetta (which is typical of its 

 family, the Asconea) are here converted into long tubes, 

 which may branch, and unite at intervals. The endo- 

 dermal ciliated cells, moreover, do not now form a con- 

 tinuous layer lining the body cavity, but are parcelled out 

 into groups which everywhere stud dilatations in the 

 branching tubes. These dilatations are known as the 

 ciliated chambers (Fig. 13, c; Fig. 14, c) or ciliated baskets 

 of the sponge, and are the representatives of the once 

 continuous endoderm. 



"•^^^ 



Fig. 14. — Hypothetical sriiinTi ol a >, s, superficial 



layer; s', deeper substance of sponge; .-, iiihalent apertures; c, 

 exhalent aperture ; c, ciliated chambers. The arrows indicate the 

 direction of the currents. (After Huxley.) 



• Ut supra, p. 124. 



In Spongilla fliiviatilis, the common fresh-water sponge, 

 a growth something like that which has just been noted is 

 developed as shown in Fig. 11, but with this difiference, 

 that in place of the calcareous spicules of the Lev/ones 

 there is a fibrous skeleton of keratose strengthened with 

 spicules of silica, which are also scattered amongst the 

 sponge-cells, or sarcoids, as the latter are sometimes called. 

 In the curious Halisarca we have a sponge identically 

 similar, but entirely devoid of a skeleton ; it has therefore 

 been placed in a division by itself, the Mt/xosjionyici: In 

 the sponges of commerce also (Spongict), the main difference 

 lies in the nature of the skeleton, which is wholly fibrous 

 and composed of keratose. The beautiful siliceous sponges, 

 Euplectella and its allies, are likewise all resolvable into 

 the type of structure which has now been explained. 



There is one peculiar process which we have before 

 alluded to as an aberrant phase in the life-history of 

 Spongilla to which we would now draw attention, as of 

 importance to those who desire to follow its interesting 

 details practically.* During the autumnal months several 

 contiguous cells in the body of the sponge lose their ordi- 

 nary appearances, and acquire bright granules, which 

 finally obscure their other characteristics. The cells imme- 

 diately surrounding the group so formed, unite to produce 

 a kind of coat for them, and secrete in this a skeleton of 

 keratose. Each central cell now develops a peculiar sili- 

 ceous spicule, resembling two toothed-wheels united by an 

 axle (Fig. 15). A small opening, or hilum, is left at one 



Fig. 15. — Spongilla fluiiatilis. — A, so-called "seed" or "gem- 

 mule," external aspect showing hilum, h, and toothed tops of 

 spicules or amphidiscs. B, diagrammatic section of gemmule, 

 showing position of amphidiscs, a ; h, hilum. C, amphidisc seen 



in profile (after Nicholson). 



part of this amphidiscus ; its protoplasm dwindles away 

 till nothing but the outer keratose coat and the peculiar 

 spicules, arranged perpendicularly to the surface, as in Fig. 

 15, remains. This condition is arrived at in winter, and 

 the bodies, now known as sponge seeds, remain thus during 

 the cold months. On the advent of spring, however, the 

 internal cells resume their activity, are extruded from the 

 hilum, and settle down to develop into a young Spongilla. 



Lastly, there is a genus of sponges which bore into the 

 shells of molluscs by means of curiously-shaped siliceous 

 spicules, and live there parasitically ; these Clioiiida: have 

 been found in some of the oldest geological formations, e.g., 

 from the Silurian upwards. Their anatomy, and therefore 

 their atiinities, has not yet been determined. 



We cannot here enter into the details of the arguments 

 respecting the atiinities of the sponges. Suffice it to say, 

 that the weighty evidence of the formation of a blastoderm, 

 and its subsequent passages through the morula, gastrula, 

 and other stages, is sufficient to place the group of sponges 

 (Fori/'era) nearer the Culi'ittei-ala tlian the rroto^-ou. This 

 will become apparent when we have defined what a 

 cwlenterate animal is ; but, to avoid confusion, we must 



• Spnn[}ill(e may be obtained from Mr. G. H. King, 1G5, Great 

 Portland-street, W. ; or from Sir. Thomas Bolton, of 57, Xewhall- 

 strcet, Birminglmm, both of whom have shown specimens at their 

 stands in the Exhibition. — J. E. A. 



