SPONGES 



41 



of the emloderm into the mesoderm, \vliirh, 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 



' 



Kb. agon 

 type. 



Fio. 5. Utc Argcnted, Pol. Fart of a transverse section. The concentric circles, 

 indicating transviTsr sections of spicules, lie within the cortex. After Pule- 

 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 



Fio. 6. Polejna connexiva, Pol. Part of a transverse section. E, excurrent 

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

 Effort (KM). 



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 Grant in r<iphanus 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 apapyle; 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 llaeckel 

 "dyssyciis," maybe more briefly named a ]{lni<i<,ii 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 Khagon occurs as a stage in the early 

 development of Plakiiia monoloji/ui (Sehulxe) and llt'iiiera, 

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



FIG. 7. Vertical section of a Rhagon, partly diagrammatic, o, oseulc ; p, 

 paragaster. After Keller (X about 100). 



approach it somewhat is Leucopsis pedunndatn, 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 choano- 

 some of Cydoniitm eosuster, Soil, e, excurreut canal ; /, flagellated chambers 

 communicating with it. by aphodal canals ; ;., an incurrent canal cut across ; 5, 

 a sterraster ; o, an oxea cut across. After SoUas, " Challemjcr " .Report (x 125). 



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 or 

 several prosopyles. This ',' 

 may be termed the e ury- 

 pylous type of Ehagon 

 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, Sttlhttidx) 

 the chambers cease to 

 open abruptly into the 

 excurrent canals : each is 

 prolonged into a narrow 

 canal, apfiodus, or alii(a, 

 which usually directly, 

 sometimes after uniting 

 with one or more of its PI Q. 9. mplodaTcanal system in Cort;?;,n. 



fellows Opens into an candelabrum, O.S. e, excutrent canal ; the 

 . ,, incurrent canal is ><li"\ni <*n the k-ft-hand 

 excurrent canal. llie side, near its commencement in the cortex, 

 prosopyles, now. restrict- After F - E - Scllulze < x : 

 ed to one for each chamber, may remain unchanged in 

 character, or at the most be prolonged into very short 



F 



