CANAL SYSTEM OF THE SPOXGE. 



319 



most widely distributed amongst the Sponges, it is, at the same time, the most complicated. In its 



simplest expression, the canal system is found amongst the lower members of the Calcispongise, as in 



the little A scetta blanca (Fig. 8), discovered by Micklucho Maclay. This is simply an oval sac, with a 



laro-e internal cavity, and very thin walls, opening at one end by an osculum, attached at the other, 



and perforated all over by numerous short pore 



canals. The endoderm consists entirely of 



flagellated cells, so that these line the whole 



interior, and driving the water out at the mouth, 



cause an influx through the pores, which are 



mere fluctuating apertures, with no constancy 



in position. There are here no true incur- 



rent canals, and the whole Sponge might be 



compared to a magnified flagellated chamber 



#ith a surrounding layer of mesoderm and 



ectoderm. Haeckel regards it as similar to a 



Fig. 9. A, EUPLECTELLA SUBEREA (After Thomson] ', B, HYALONEMA SIEBOLDII (After Scliuhe) ' C, POTERIOX (After Hartiny} ; 

 D, SIPHOMA PY1UFOUMIS (After Sotwby) ; E, PACTYLOCALYX STUCHBUKYI (After Solids). 



single Hydrozoon. Neglecting the spicules which are embedded in its mesoderm, Ascetta might be 

 regarded as an embodiment of just so much as is common to all the Sponges, a concrete definition 

 of the group. 



From this simple stage the more complicated appear to arise in two ways ; in one, which is 

 characteristic of the small group of Sycones, buds, repeating in every way the structure of the parent, 

 sprout out at right angles from the wall of an Ascon. such &s Ascetta; to the central cavity of these the 

 flagellated cells become restricted, those of the original Ascon becoming converted into polygonal pave- 

 ment cells ; the central cavities of the buds remain in free communication with that of the parent. The 

 latter is now the excurrent canal, the former the flagellated endings or branches of it. (Plate 71, 

 Fig. G.) In more integrated forms the buds grow close together, touch, and unite along the lines 

 of contact, the narrow canal-like interspaces left between them serving for the conduct of water to 

 the pores, and constituting an incurreut canal system. (Plate 71, Figs. H, i.) 



Precisely how complication ensues in the other case, which is that of the great majority of 

 Sponges, is not quite so clear ; but it would appear that from the endoderm of a sac resembling an 

 Ascon hollow buds are formed, which project into the mesoderm. These are the flagellated chambers. 



