42 



SPONGES 



Subder- 



mal 



cavities. 



Ecto- 

 some, 

 choano- 

 ^me. 



Cihone. 



tubes, each a prosodus or aditus (fig. 8). This may be 

 termed the aphodal or racemose type of Ehagon system, 

 since the chambers at the ends of the aphodi radiating 

 from the excurrent canal look like grapes on a bunch. As 

 Haeckel, however, has used "racemose" in a different sense, 

 we shall adopt here the alternative term. By the exten- 

 sion of the prosodal or adital canals into long tubes a still 

 higher differentiation is reached (fig. 9). This, which from 

 the marked presence of both prosodal and aphodal canals 

 may be termed the diplodal type of the Rhagon canal 

 system, occurs but rarely. Chondrosia is an example. 



The following scheme wUl render clear the foregoing 

 distinctions : — 



1. Asoon type : simple, ex. Ascetta, Hk. ; strobiloid, ex. Sonrn- 



derma, Lfd. 



2. Sycon type : simple radial tubes, ex. Sycetta, Hk. ; branobed 



radial tubes (cylindrical chambers), ex. Heteropegma, Fl. ; 

 cbamber-layer folded, ex. Polejna, Pol. 



3. Rhagon type : eurypylous, with several prosopyles to each 



chamber, ex. Spongelia ; with a single prosopyle to each 

 chamber, ex. Oscarella, Thenea ; aphodal, aphodal canals well 

 developed, ex. Geodia, Lmk. ; diplodal, with both aphodal 

 and prosodal canals well developed, ex. Chondrosia, O.S. 



In the case of the calcareous sponges Polejaeff has argued 

 forcibly that the eurypylous type arises directly from the 

 Sycon and not from the Rhagon. It is therefore doubtful 

 how far the Rhagon in other sponges is a primitive form 

 derived directly from an OlyntJms, or whether it may not 

 be a secondary larval state resulting from the abbreviated 

 development of a former Sycon predecessor. Whatever 

 may have been its past history, the Rhagon serves now at 

 all events as a starting-point for the development of the 

 higher forms of canal system. 



In the higher Rhagons, as in the Sycons, further com- 

 plications ensue, owing to an independent growth of the 

 external ectoderm and the adjacent mesoderm. While the 

 endoderm, with its associated mesoderm, is growing out 

 or folding to form the excurrent canal system, the super- 

 ficial mesoderm increases in thickness, and the ectoderm, 

 extending laterally from the sides of the incurrent sinuses, 

 burrows into it, parallel to the surface of the sponge. 

 Thus it forms beneath the skin (i.e., the layer of superficial 

 mesoderm and investing ectoderm) cavities which may be 

 either simple and spacious or be broken up into a number 

 of labyrinthine passages by a network of mesoblastic 

 strands (invested with ectoderm) which extend irregularly 

 from roof to floor of the chamber. These cavities are 

 known as suhdermal chambers. 



With the appearance of subdermal chambers the sponge 

 becomes differentiated into two almost independent regions, 

 an outer or ectosome and an inner or choanosome, which is 

 characterized by the presence of flagellated chambers. 

 The ectosome forms the roof and walls of the subdermal 

 chambers, and is in its simplest form merely an investing 

 skin ; but in a large number of sponges it acquires con- 

 siderable thickness and a very complicated histological 

 structure. It is then known as a cortex. The thickening 

 which gives rise to a cortex takes place chiefly beneath 

 those parts of the skin which are not furnished with pores. 

 Beneath the pores — in this case collected into sieve-like 

 areas — dome-like cavities are left in the cortex; they open 

 freely into the subdermal cavities below and their roof is 

 formed by the cribriform pore membrane above. In many 

 sponges (Geodia, Stelletta) the cortical domes are constricted 

 near their communication with the subdermal cavity (sub- 

 cortical crypt) by a transverse muscular sphincter, which 

 defines an outer division or ectochone from an inner or 

 endochone (fig. 10), the whole structure being a chone. 

 The endochone is frequently absent (fig. 10). The early 

 development of the cortex has scarcely yet been studied. 

 In Stelletta phrissens (Soil.), one of the " Challenger " Stel- 



lettidx, an early form of the sponge (fig. 11), shows the 

 choanosome already characteristically folded within the 



cortex, which forms a com- .^ , „ 



plete not-folded envelope ""^y** 

 around it. The roots of j''i# % 

 the incurrent sinuses form * ^ ^ , 



widely open spaces imme- 

 diately beneath the cortex 

 and are the rudiments of * 

 subcortical crypts. Again, -ft 

 in some sponges a part of >_ 

 the endoderm and asso- Sf -*?::,"' ,' 

 ciated mesoderm may like- 'f^'jf^ 

 wise develop independ-|f £..^«_i 

 ently of the rest of the / 

 sponge, as in the Hexac- 

 tinellida, where the choa- 

 nosome forms a middle 

 layer between a reticula- 

 tion of ectosome on the 

 one side and of endoderm i 

 and mesoderm, i.e., etido- 

 some, on the other. Fin- ! 



nllTT tbp aitarhpd nr Inwpr ^'o- 10.— Section through the cortex of Ci/- Eudo- 

 aiiy, ine axiacnea or lOWer clomum eosasUr, SoU., showing the pore- some, 

 half of a Rhagon may de- sieve overlying the chone, which com- 



velop in an altogether dif- 

 ferent manner from the 

 other or upper half, the 

 endoderm not producing 



any fiageUated chambers. In this case the upper portion 

 alone is characterized by the flagellated chambers, which 

 are the distinctive mark of a sponge, and hence may be 



raunicates through a sphinctrate aperture 

 with the subcortical crypt, lying in the 

 choanosome with its flagellated chambers. 

 The dotted circles in the cortex ^e sterr- 

 asters connected by iibrous strands. 

 After SoUas, '^ Challenger" Seport (x'lS). 



Pig. 11.— Young sponge of Stelletta phrissens, Soil. Longitudinal median sec- 

 tion, showing the choanosome folded within the cortex, o, oscule. After 

 Sollas, " ChaUenger" Report (x50). 



called the spongomere; the lower half, which consists of 

 all three fundamental layers, may be called the hypomere. 

 The form and general composition of sponges are ex- 

 ceedingly various and often difficult to analyse, presenting, 

 along vidth some important differences, a remarkable general 

 resemblance to the Gcelentera in these respects. Like Oscule. 

 them, some sponges are simple, and others, through 

 asexual multiplication, compound. The only criterion by 

 which the individual sponge can be recognized is the oscu- 

 lum ; and, as it is frequently difficult, and in many cases 

 impossible, to distinguish this from the gastric opening of 

 a large excurrent canal, there are many cases in which the 

 simple or compound nature of the sponge must remain 

 open to doubt. The oscule may also fail {lipostomosis), 

 and so may the paragastric cavity (lipogastrosis) ; the 

 problem then becomes insoluble. The loss of the oscule 



