SPONGES 



41 



type. 



of the endoderm into the mesoderm, 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 



Fig. 5. — TJte Argentea^'PQi, Part of a transverse section. The concentric circles, 

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

 jaeff, "Challenger" Beport (xlOO). 



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 



Fig. 6. — Poli^na cownexi/oa, Pol. Part of a transverse section. E, excurrent 

 canals, into which tiie flagellated chambers open. After Polejaeff, " CMUenger" 

 Seport(xm). 



■ 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 

 8ycon 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. 



Eeturning 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 Bhagon 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 Plakina monolopha (Schulze) and Beniera 

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



Fig. 7.— Vertical section of a Bhagon, 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 Khagon, or by its outgrowth 

 into lobes, a complicated structure such as that of Plakina 

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



FiG. 8. — Transverse section acioss an excurrent canal and surrounding choano- 

 some of Cydonium eosasfer, Soil e, excurrent canal , /, flagellated chambers 

 communicating with it by aphodal canals , ^, an mcurrent canal cut across ; s, 

 a sterraster ; o, an oxea cut across. After Sollas, " Challenger " Meport ( 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 Cfg 

 several prosopyles. This '^ '^ 

 may be termed the eury- 

 pylous type of Ehagon % 

 canal system. The fold- ;K#| 

 ing of the sponge -wall ^|* 

 may be simple, as in the .iiQ 

 example given, or too l;|^ 

 complex to unravel. In ^ 

 higher forms of sponges ^^ 

 {Geodinidx, Stellettidx) 'pC^. 

 the chambers cease to i Oi 



cease 

 open abruptly into the 

 excurrent canals : each is 

 prolonged into a narrow ^ 

 canal, aphodus, or abittis, Rp 

 which usually directly, 

 sometimes after uniting K 

 with one or more of its 

 fellows, opens into 



Fig. 9. — ^Diplodal canal system in Cortictum 

 an candelabrum, O.S. e, excurrent canal ; the 

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



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



prosopyles, now restrict- •"*™ ^- ^- ^=''"1^° '■^ ^°°^- 

 ed to one for each chamber, may remain unchanged in 

 character, or at the most be prolonged into very short 



F 



