108 M.E.lIiickcl on the Ort/aui'zation of SjiongeSj 



of all nn'\ma\ J )Iif/It I J -will be fxplaincd more fully in my ^lono- 

 graph of the Calcispongia?. 



1 will admit that this law, which a])j)('ars to me to l»e of 

 hiiih importance, is suhject to certain mcidifications in many 

 individual cases, and that ])erhaps here and there, in Loth the 

 ►Sponges and Acalcphs, the two gerni-lamclhv or formative, 

 membranes (the entoderm and ectoderm) may rejjlacc each 

 other by locaJ suhsti'tution. Not luifrequently the entoderm is 

 lost over large spaces, and is re])laced by the ectoderm. In 

 some, perhaps in many cases (both in Sponges and Acalejihs), 

 the ditlcrent signification of the two divergent germ-lamcllfe 

 is, in ]iarticular parts of the body, not clearly recognizable, or 

 even actually changed. Thus, for instance, jyerhajyfi in both 

 groups of animals, sexual products may sometimes be deve- 

 loped from the ectoderm and muscles from the entoderm. But 

 then, probably, these deviations and local substitutions of the 

 two lamella? are to be regarded as secondary modijicationsj 

 only produced at a late period by adaptation. The original 

 jyrimary relation inherited by all Sj)07i(jies and Acalephs from 

 the common trunh-form (Protascus) is probably that described 

 above : the entoderm^ as the inner j vegetative germ-lamella^ 

 fonns the nutrient cells of the canal-epithelimn, and the cells 

 produced from these, by division of labour, serving for the pur- 

 pose of reproduction (germ-cells or spores, ova andzoospennia); 

 Avhilst the ectoderm, us the outer^ animal germ-lamellay forms 

 the muscles, nerves, skeletal parts, outer covering, &c. 



This law finds its strongest support in the structure of the 

 young fonns of the two groups of animals, which have been 

 already referred to. The cup-shaped young state, produced 

 from the ciliated larva, which possesses a simple stomachal 

 cavity (or digestive body-cavity) with a single, simple aper- 

 tiire (or mouth), and which, in the living Prosycum, still re- 

 calls to lis the long-lost picture of the ProtascnSj shows us its 

 simple solid body-wall (or stomach-Avall) composed throughout 

 of the two distinctly differentiated formative membranes, the 

 entoderm and the ectoderm, and, indeed, equally in the corre- 

 sponding young states of the Spongia? as in those of the corals 

 and the Acalephs generally. Here, again, however, the Calci- 

 spongife serve as admirable elucidatory objects, because, on 

 the one hand, of all Sponges they approach nearest to the 

 corals, and, on the other, in the graduated evolution of their 

 simple organization, from the very simple Prosycum and 

 Olynfhus, up to the highly developed Dunstervillia and Cya- 

 thiscusj they bring wonderfully before our eyes the continual 

 separation of the two originally divergent formative mem- 

 branes, the vegetative entoderm and the animal ectoderm, 



