40 



SPONGES 



of the Anre.Ua, as of all sponges, is most obviously mani- 

 fested, as Grant (5) first observed, by a rapid outflow of 

 water from the osculo and a gentle instreaming through 

 the pores, a movement brought about by the energetic 

 action of the flagolla of the 

 eiidodennic cells. The in- 

 streaming currents boar with 

 them into the cavity of the 

 sac (paragastric cavity) both 

 protoplasmic particles (such as 

 Infusoria, diatoms, and other 

 .small organisms) and dissolved 

 oxygon, which are ingested by 

 the flagellated colls of the on- 

 dodorm. The presence of one 

 or more contractile vacuolos in 

 thoso colls suggests that they 

 extricate water, urea, and car- 



liiinic arid. Thr insolnlilii re 



sidtio of the introduced food, 

 together with the fluid excreta, 

 is carried out through the os- 

 cule by the excurrent water, 

 Now individuals are produced 

 from the union of ova and 

 spermatozoa, winch develop 

 from wandering aiiuuboid colls 

 in the mesoderm. The walls 

 of Amrttd are strengthened by 

 calcareous scloros, more especi- 

 ally designated as spicules, Pm. 2 nomoOtrma upaitdra, Lfd. 



wliii-li linvn tlin form nf tri- " lmlf rut awftv 1>v tt vortical 



I,..-. I IIMI HOOtloil. Ml.'. V. I .,.!, 



radiate needles. If wo make fuiu(xaixmt<)). 



abstraction of tliose wo obtain an ideal sponge, which 



Haockel has called Olynthus (6), and which may bo ro- 



Canal System. Wo 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- Acoa 

 cated in a variety of ways, such as by the budding off tyl 16 - 

 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 typo, does the endoderm become differentiated into 

 different histological elements. The most interesting 

 modification of the Ascon form occurs in J/omoderma sy- 

 candra (is), in which from tho walls of a simple Ascon 

 ctecal processes grow out radiately in close regular whorls, 

 each process reproducing tho 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 tho simplest examples of this typo tho characters of Sycon 

 Homoderma sycandra are reproduced, with the important 

 exception that tho endoderm lining the paragastric cavity 

 of tho original Ascon form loses its primitive character, 





Km. 3. Houuxlerma lyoumim, I.fd. Transvi<nw sort Ion, showing radial tubes opening 

 Into coutml iwrtigaslrlc cavity. Aftor V. LoiKlciifuia (x about 12). 



gardod as tho ancestral form from which all other sponges 

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

 straction wo should perhaps stipulate for tho Olynthw a 

 somewhat thicker mesoderm and more spherical form than 

 a decalcified Ascon presents. 



T 



Fio. 4. Httenptgrnawxlut-gardll, Pol. Partofatransvorsoscctlon. Thostraight 

 lines incliiMti- s|iiriilrs : tin' pnrit'iTcms MirfiuT is uppermost ; the branching 

 milial tubes lire rendered dark by nunummM small circles representing 

 clioiinocytes. After FoleJaelT, " 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. Tho 

 flagellated cells are thus restricted to the cajcal 

 outgrowths or radial tubes. Concurrently with 

 this differentiation of tho endoderm a more abun- 

 dant development of mc.soderm occurs. In some 

 Sycons (Sycaltis, Hk.) the radial tubes remain 

 separate and free ; in others they lie close together 

 and are united by trabeculse, or by a traliecular 

 network, consisting of mesodermic strands sur- 

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

 the contiguous radial tubes thus become converted 

 into narrow canals, through which water passes 

 from the exterior to outer the pores in tho walls 

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

 canals " of Haeckel, now generally known by their 

 oklor name of incurrent canals. Tho openings of 

 the incurrent canals to the exterior are called 

 pores, a term which vr lia\r also applied to the 

 openings which U-ad 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 ]>i\>s<>i>i/l?. The term "pore" will then bo 

 restricted to the sense in which it was originally used by 

 (3 rant. Tho mouth by which a radial tube opens into tho 

 paragastor is known as a gastric ostium. In the higher 

 forms of Sycons tho radial tubes no longer arise as simple out- 

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



