352 THE SPONGTDA OR PORIFERA. 



Defejisive Spiculce surrounding the pores are also shown in the 

 diagram o{ Sycandra compressa (Fig. 2, d, Plate XVI.), As in the 

 latter sponge, the pores of S. ciliata open into radial tubes, which 

 pass through the wall of the sponge to the central cavity. 



We have before us a microscopical slide, showing a fine section 

 of the radial tubes ; and the accompanying illustration (Fig. i, 

 Plate XV.) shows exactly and truthfully what is to be seen with a 

 one-twelfth inch objective. The tubes are lined, it will be seen, 

 mt\\ flagellate collared cells, a few of which have become detached 

 from the others. The cutting of sections for the proper examina- 

 tion of the collared cells is by no means an easy matter, as, 

 however well hardened and prepared, the structural elements of this 

 delicate and fragile sponge are very readily displaced. From the 

 examination of several slides, however, we are convinced that the 

 collared cells are arranged in nature side by side with almost exact 

 mathematical regularity, such as represented in the diagram of 

 Syca?idra compressa (Fig. 2, e, Plate XVI.). We have drawn, on 

 a greatly enlarged scale, two of the detached collared cells (Fig. 2, 

 Plate XV.) just as we saw them, except that the flagella are curved 

 so as to be included in the plate. We saw all the parts represented, 

 viz., the whip-like yf^^^//?/w (J, b), the collar {a, a),- the neucleus ic), 

 and neucleolus, \}ci^ granular protoplasm {e, e), and (as far as one 

 could judge from a prepared slide, and the description given by 

 others) also the contractile vesicles (d, d). 



The flagellate collared cells form a very essential part of 

 sponge structure ; through their action the circulatory water- 

 currents, necessary for respiration and feeding, are kept up ; and 

 they line the canal system, in greater or less degree, and speaking 

 generally, according to one or other of two broad plans, through- 

 out the whole of the Spongida. In some Sponges, the flagellate 

 collared cells line more or less completely the entire canal system; 

 in a great number of others, they line only rounded cavities 

 within the sponge, to which the water runs from the pores through 

 " afferent " channels, and frojn which the currents are conveyed 

 by " efferent " canals to the oscula. Sycandra compressa and 

 Sycandra ciliata (Figs. 4 and 5, Plate XIV.) illustrate the first 

 plan, the entire length of the radial tubes being lined by the 

 collared cells (Fig. i, Plate XV., and Fig. 2, Plate XVI.). As 



