40 



SPONGES 



of the Ascetta, as of all sponges, is most obviously mani- 

 fested, as Grant (j) first observed, by a rapid outflow of 

 water from the oscule and a gentle instreaming through 

 the pores, a movement brought about by the energetic 

 action of the flagella of the 

 endoderrnic cells. The in- 

 streaming currents bear with 

 them into the cavity of the 

 sac (paragastric cavity) both 

 protoplasmic particles (such as 

 Infusoria, diatoms, and other 

 small organisms) and dissolved 

 oxygen, which are ingested by 

 the flagellated cells of the en- 

 doderm. The presence of one 

 or more contractile vacuoles in 

 these cells suggests that they 

 extricate water, urea, and car- 

 bonic acid. The insoluble re- 

 sidue of the introduced food, 

 together with the fluid excreta, 

 is carried out through the os- 

 cule by the excurrent water. 

 New individuals are produced 

 from the union of ova and 

 spermatozoa, which develop 

 from wandering amoeboid cells 

 in the mesoderm. The walls 

 of Ascetta are strengthened by 

 calcareous scleres, more especi- 

 ally designated as spicules, FIG. 2. Homoderma sycamim, Lfd. 

 which have the form of tri- m edian a ^ectlon aW Afte/v a Lend'en! 



radiate needles. If we make feid(xabout 6). 



abstraction of these we obtain an ideal sponge, which 



Haeckel has called Olynthus (6), and which may be re- 



Canal System. We shall now trace the several modifi- 

 cations which the Olynthus has undergone as expressed in 

 the different types of canal system. 



The simple paragaster of Ascetta may become compli- Ascon 

 cated in a variety of ways, such as by the budding off tJT 6 - 

 from a parent form of stolon-like extensions, which then 

 give rise to fresh individuals, or by the branching of the 

 Ascon sac and the subsequent anastomosis of the branches; 

 but in no case, so long as the sponge remains within the 

 Ascon type, does the endoderm become differentiated into 

 different histological elements. The most interesting 

 modification of the Ascon form occurs in Homoderma sy- 

 candra (i2\ in which from the walls of a simple Ascon 

 csecal processes grow out radiately in close regular whorls, 

 each process reproducing the structure of the parent 

 sponge (figs. 2, 3). From this it is but a short step to 

 the important departure which gives rise to the Sycons. 



In the simplest examples of this type the characters of Sycon 

 Homoderma sycandra are reproduced, with the important type, 

 exception that the endoderm lining the paragastric cavity 

 of the original Ascon form loses its primitive character, 



FIG. 3. Humodcrma syctiiulm, Lfd. Transverse section, showing radial tubes opening 

 into central naragustric cavity. After V. Lendeiiteld (xabout !-) 



garded as the ancestral form from which all other sponges 

 have been derived. To give greater exactness to our ab- 

 straction we should perhaps stipulate for the Olynthus a 

 somewhat thicker mesoderm and more spherical form than 

 a decalcified Ascon presents. 



FIG. 4. Heteropegnwnodus-gordii, Pol. Part of a transverse section. The straight 

 lines indicate spicules ; the poriferous surface is uppermost; the branching 

 radial tubes are rendered dark by numerous small circles representing 

 clioauocytes. After PolejaefT, ' ' Challenger " Report ( X 50). 



and from a layer of flagellated cells becomes converted 

 into a pavement epithelium, not in any distinguishable 

 feature different from that of the ectoderm. The 

 flagellated cells are thus restricted to the caecal 

 outgrowths or radial tubes. Concurrently with 

 this differentiation of the endoderm a more abun- 

 dant development of mesoderm occurs. In some 

 Sycons (Sycaltis, Hk.) the radial tubes remain 

 separate and free; in others they lie close together 

 and are united by trabeculte, or by a trabecular 

 network, consisting of mesodermic strands sur- 

 rounded by ectoderm (fig. 4). The spaces between 

 the contiguous radial tubes thus become converted 

 into narrow canals, through which water passes 

 from the exterior to enter the pores in the walls 

 of the radial tubes. These canals are the " inter- 

 canals " of Haeckel, now generally known by their 

 older name of incurrent canals. The openings of 

 the incurrent canals to the exterior are called 

 pores, a term which we have also applied to the 

 openings which lead directly into the radial tubes 

 or paragastric cavity; to avoid ambiguity we shall 

 for the future distinguish the latter kind of open- 

 ing as a prosopyle. The term "pore" will then be 



restricted to the sense in which it was originally used by 

 Grant. The mouth by which a radial tube opens into the 

 paragaster is known as a gastric ostimn. In the higher 

 forms of Sycons the radial tubes nolonger arise as simple out- 

 growths of the whole sponge- wall, but rather as outgrowths 



