156 



HARDWICKKS SCIENCE-GOSSIP. 



General Structure. — So far the sponge has been 

 regarded simply as a hollow sac, but the walls of the 

 sac possess a somewhat complicated structure, which 

 we must now describe. Commencing from the inner 



Fig. 94.— Successive stages in the segmentation of Sycnndra raphanus. (Copied 

 ^rom F. E. Sckulze.} A, Stage with eight segments still arranged in pairs, seen 

 from above ; B, side view of stage with eight segments ; C, side view of stage 

 with sixteen segments ; D, side view of stage with forty-eight segments ; E, view 

 from above of stage with forty-eight segments ; F, side view of embryo in the 

 blastosphere stage, eight of the granular cells which give rise to the ectoderm of 

 the adult are present at the lower pole ; cs, segmentation cavity ; ec, granular 

 cells, which form the ectoderm ; en, clear cells which form the endoderm. 



Fig. 95. — Larva of Sycu/tdra raphanus at pseudogastrula stage, in situ in the maternal 

 tissues. {Copied fron /•'. E. Sc/iulze.) vie, Mesoderm of adult ; hy, collared cells, or 

 endoderm, of adult ; 01, clear cells of larva, which eventually become involuted to form 

 the endoderm ; cc, granular cells of larva, which give rise to the ectoderm ; at this stage 

 they are partially involuted. 



face we find first a membranous lining, perforated 

 by a great number of small lioles, which are called 

 mouths, or ostia, and because they open into the 

 stomach, stomachal mouths or gastral ostia. Each 

 is the open cr.d of a tliin-wallcd tube, which is closed 



and conical at the other end, and except that it is 

 hexagonal in section, somewhat similar in form to a 

 chemist's test-tube. These tubes radiate from the 

 gastral ostia to the exterior of the sponge, and 

 constitute, lying side by side, 

 joined close together, the greater 

 part of the sponge wall. By 

 holding together a number of 

 test-tubes, and supposing them 

 to be joined along their lines 

 of contact, we shall gain a fair 

 idea of this arrangement. Fur- 

 ther, it will be seen that, how- 

 ever close the test-tubes lie to 

 one another, narrow three-sided 

 canals will remain between them, 

 one such canal between every 

 three mutually adjacent tubes. 

 Precisely similar canals are left 

 between the tubes of the sponge, 

 and are known as inter-canals, 

 whilst the tubes themselves are 

 termed radial tubes. The radial 

 tubes have not continuous walls 

 like those of a test-tube, but 

 are perforated all over by a 

 number of minute apertures, 

 or pores. Those pores which 

 occur over the projecting conical 

 ends of the tubes open im- 

 mediately to the surrounding 

 water ; those which occur along 

 the sides of the tubes, where 

 they are not in contact, open 

 into inter-canals, and so indi- 

 rectly into the outer water, while 

 those finally, which occur along 

 the line of union of the radial 

 tubes serve as a means of com- 

 munication between these tubes, 

 and do not open into the outer 

 water at all, except, of course, 

 by way of the stomach through 

 the mouth. 



Histology. — The proper wall 

 of the stomach, and alike of the 

 radial tubes, consists of three 

 layers of tissue, an outer, or 

 ectoderm ; an inner, or endo- 

 derm ; and a middle, or meso- 

 derm. 



The ectoderm, which, as it 

 forms the outer layer of the 

 sponge, extends over the ex- 

 terior of the radial tubes, and so lines the inter- 

 canals, consists of a single layer of plate-like poly- 

 gonal cells, 0'0i5 to 0'025 mm. in diameter. Each 

 contains a circular cake-like nucleus, bounded by a 

 nuclear membrane, and full of watery fluid, in which 



