SPONGES 



41 



of the endoderm into the mesodenn, which, together with 

 the ectoderm, exhibits an independent growth of its own ; 

 and this results in the formation of a thick investment, 

 known as the cortex (fig. 5), to the whole exterior of the 



Fio.5. UteArgenlea, Pot Part of a transverse section. The concentric circles, 

 indicating transverse sections of spicnles, lie within the cortex. After Pole- 

 jaeff, " Challenger " Report ( x 100). 



sponge. The radial tubes may branch, Heteropegma (fig. 

 4). If the branches are given off regularly, as the radial 

 tubes were in the first plan, and if at the same time the 

 original radial tube exchanges its flagellated for a pave- 

 ment epithelium, a structure as shown in fig. 6 (Polejna 



Rhagon 



type. 



Fio. 6. Polejna connexlva, Pol. Part of a transverse section. E, excnrrent 

 canals, into which the flagellated chambers open. After Polejaeff, " Challenger" 

 Report (x 50). 



connexiva, Pol.) will result. This form might also be 

 brought about by unequal growth of the gastral endoderm 

 leading to a folding of the inner part of the sponge-wall. 

 Very little direct evidence exists as to which of these two 

 plans has actually been followed. Phylogenetically the 

 transition from a simple Ascon to the most complicated 

 Sycon can be traced step by step ; and ontogeny shows 

 that such a Sycon form as Grantia raphanus passes through 

 an Ascon phase in the course of its larval development. 



Returning to the ancestral form of sponge, Olynthus, 

 let us conceive the endoderm growing out into a number 

 of approximately spherical chambers, each of which com- 

 municates with the exterior by a prosopyle and with the 

 paragastric cavity by a comparatively large aperture, which 

 we may term for distinction an apopyle; at the same time 

 let the endoderm lose its flagellated character and become 



converted into a pavement epithelium, except in the 

 spherical chambers. Such a form, called by Haeckel 

 " dyssycus," may be more briefly named a Rimy on from 

 the grape-like form of its flagellated chambers, which differ 

 from those of a Sycon both by their form and their smaller 

 dimensions. The Ehagon occurs as a stage in the early 

 development of PlaJcina motvolopha (Schulze) and Reniera, 

 fertilis (9) (fig. 7) ; a calcareous sponge which appears to 



Fio. 7. Vertical section of a Rliagon, partly diagrammatic, o, oscule ; p, 

 paragaster. After Keller (X about 100). 



approach it somewhat is Leucopsis pedunculata, Lfd. By 

 the folding of the wall of a Ehagon, or by its outgrowth 

 into lobes, a complicated structure such as that of Plakina, 

 monolopha (20) (see fig. 26 /) results. This is character- 



Fio. 8. Transverse section across an excurrent canal and surrounding choanc- 

 some of Cydonium eosaster, Soil, e, excurrent canal ; /, flagellated chambers 

 communicating with it by aphodal canals ; i, an incurrent canal cut across ; s, 

 a sterraster ; o, an oxea cut across. After Sollas, * ' Challenger " JKeport ( x 1 25). 



ized by the chambers retaining their immediate communi- 

 cation with the incurrent and excurrent canals, opening 

 into the latter by the widely open apopyle and receiving 

 the former by one ( 

 several prosopyles. This J 

 may be termed the eury- \ 

 pylous type of Rhagon K 

 canal system. The fold- - 

 ing of the sponge-wall 

 may be simple, as in the 

 example given, or too 

 complex to unravel. In 

 higher forms of sponges ' 

 (Geodinidx, Stellettidx) 0' 

 the chambers cease to 

 open abruptly into the 

 excurrent canals : each is ; 

 prolonged into a narrow 

 canal, apkodus, or abitiis, 

 which usually directly, ; 

 sometimes after uniting 



with one Or more of its Flo . 9 ._ D iplodal canal system in Corticium 



fellows, Opens into an candelabrum, O.S. f, excurrent canal ; the 



. ,, incurrent canal is shown on the left-hand 



excurrent Canal. Ine side, near its commencement in the cortex. 



prosopyles, now restrict- After F - E - Schulze <> 

 ed to one for each chamber, may remain unchanged in 

 character, or at the most be prolonged into very short 



F 



