_Fig._ 1. A portion of sponge, Halichondria simulans, showing silicious spicula imbedded in the sarcode matrix--2. Spicula divested of its matrix by acid--3. Gemmule Spongilla fluviatallis enclosed in spicula--4. Birotulate spicula from same--5. Gemmule after being steeped in acid showing reticulated coating of birotulate spicula--6. Gemmules of Geodia--7. Gemmule in more advanced stage of growth--8. Skeleton of the acerate form covered by rows of spines--9. Showing rings of growth and horny covering, and bundles of spicula of the genus Verongia--10. Sphero-stellate spicula of Tethya--11. Tricuspidanchorate and sphero-stellate spicula--12. Acuate-bi-clavate and other forms of spicula from Geodia--13. Clavate spicula covered with short spines. PLATE XVII.--_Page_ 518. ZOOPHYTES, ASTEROIDS, NUDIBRANCHS, AND ECHINOIDS. _Fig._ 1. _a._ Astrophyton scutatum--_b._ Doris pinnatifida, back and side view--_c._ Æquorea Forbesina--_d._ Medusæ bud--_e._ Thaumantias corynetes--_f._ Echinus in an early free stage--_g._ Echinus sphæra--_h._ Cydippe pyleus--_i._ Ascidiæ--_k._ Botryllus violaceus, on a Fucus--_l._ Corystes cassivelaunus--_m._ Eurynome aspera--_n._ Ophiocoma rosula--_o._ Pagurus Prideauxii--_p._ Ebalia Permantii. PLATE XVIII.--_Page_ 558. SHELLS OF MOLLUSCA. _Fig._ 1. Transverse section of spine of Echinus--2. Another section of Echinus, showing reticulated structure, the calcareous portion dissolved out by acid--3. Horizontal section of shell of Haliotis splendens, showing stellate pigment--4. Shell of crab with granules in articular layer--5. Another section of same shell, showing hexagonal structure--6. Horizontal section of coach-spring shell, Terebratulata rubicunda, showing radiating perforations--7. Transverse section of shell of the Pinna ingens--8. Crystals of carbonate of lime, from oyster shell. PLATE XIX.--_Page_ 636. VERTEBRATA.