Development of the Marine Sponges. 403 



accomplished) — I hope to put forth that classification of the 

 sponges generally which has chiefly resulted from my ex- 

 amination and arrangement of the collection in the British 

 Museum. 



EXPLANATION OF THE PLATES. 



N.B. All the figures iu Plates XX. and XXL, with the exception of the 

 first and a little minor detail in figs. 14 & 17 which will he otherwise 

 mentioned, are drawn, as near as possible, to the scale of l-12th to 

 l-1800th of an inch, in order that their relative sizes may be at once 

 appreciated. 



The first figure, although drawn to a much smaller scale, viz. l-48th to 

 l-1800th of an inch, has its detail also drawn to this scale. 



Plate XX. 



Fig. 1. Halisarca lobular is, Schmidt, vertical section (diagrammatic) of 

 an oviparous portion, with detail relatively magnified to the scale 

 of l-48th to l-1800th of an inch or thereabouts : a «, sponge 

 reduced to a thin layer, covering a heap of ova in different stages 

 of development ; b b, ova ; c c, surface of the rock on which they 

 rest ; d, ampullaceous sacs, or groups of spongozoa, of a red- 

 violet colour ; e e, layer of cilia on the surface of the sponge ; 

 f, osculum or vent, provided with a sphinctral diaphragm of 

 sarcode ; g, sac-like membrane common to the heap of ova, 

 opening (Y) at the vent ; h h h, spheroidal ova in different stages 

 of development, showing the segmentation of the yelk; i, ellip- 

 tical embryo, whose ciliated ectoderm is marked by the dotted 

 line ; k, its capsule. 



N.B. Each ovum, although spheroidal, and, for convenience, 

 represented without capsule, has nevertheless its proper one, and 

 gradually passes from this form into the elliptical one of the 

 embryo. 



Fig. 2. The same, vertical section of the ampullaceous sac, showing the 

 position of the pavement-layer of spongozoa : a, separate spon- 

 gozoon. Scale l-12th to l-1800th of an inch. 



Fig. 3. The same, ova at an early period : a, ovum when first recogni- 

 zable, in a passive or spheroidal form, l-3000th of an inch in 

 diameter ; b, ovum when further advanced, viz. 6-GOOOths inch in 

 diameter, showing distinctly the yelk, nucleus, nucleolus, and 

 germinal vesicle, together with the polymorphic locomotive 

 envelope c. 



Fig. 4. The same, ovum in which the yelk has undergone the first dupli- 

 cative division, now 31-G000ths of an inch in diameter: «, cap- 

 sule ; b, membrane of the yelk ; c, line dividing the yelk into 

 two equal parts or cells ; dd, nuclei. 



Figs. 5, 6, 7, 8, 9, & 10. The same, showing the second, third, fourth, 

 fifth, sixth, and seventh degrees of duplicative subdivision in 

 the ovum respectively. The nuclei are omitted in all of these 

 for reasons mentioned in the text. 



Fig. 11. The same, embryo : a, ectodermal layer, represented by the dots 

 over the surface of the body ; b, long cilia ; c, short cilia ; d, tuft 

 at posterior extremity. 



N.B. After this the monociliated cells which compose the 

 ectoderm will be only represented by the dotted line at the 

 margin of the embryo. 



