SPONGES. 63 



Sub-Oeder III. — Leucones. 



The higher jjosition of this group is shown by its greater complexity. The meso- 

 derm is thicker than in the last sub-order. The canals of the supply system are irregu- 

 larly branched and frequently anastomose with each other, forming cavities near the 

 outer surface. The collared cells are disti'ibuted along the smaller canals in the lower 

 forms, while in the higher they are confined to the ampullaceous sacs. The Leucones 

 represent tlie massive growths of the Keratosa and Silicea, but usually have a common 

 or united cloacal aperture and are composed of consolidated tubes. Zittel considers this 

 group a lineal descendant of the next, a view which does not seem to be justified by the 

 morphology or the mode of development of the individuals of the group. No fossil forms 

 are known. 



Sub-Oeder IV. — Phaeetrones. 



This division was established to contain a number of forms which occur as fossils 

 in the rocks between the Devonian and the end of the cretaceous period. The author 

 is inclined to consider the genus, Trichonella, which is represented by a species in 

 Australia, as a living member of this group. The spicules are so united as to form 

 irregular threads, and sometimes a very intricate network. The canal system was 

 branching and irregular,, while the mesoderm must have been very thick. 



Class II. — CARNEOSPONGI^. 



With increasing knowledge the multitude of forms comprised in this class will 

 doubtless be separated. The common characters are a very thick mesoderm, the ecto- 

 derm and endoderm similar to that of the Leucones, and the supply and drainage 

 system as described above in the commercial sponge. The skeleton may be either com- 

 posed of horny material (keratode) or partly or entirely of silicious spicules. The 

 skeletal elements are radiately or irregularly arranged according to the plan of canal 

 system which it supports. One order has no skeleton, but the form and structure 

 show it to belong to this class. 



Oedee I. — HALISARCOIDEA. 



This order, which Haeckel calls Myxospongiee, embraces but a single genus of 

 fleshy sponge, known as Halisarca. One species is common on our shores, and also on 

 those of northern Europe. The animals grow usually in flat masses or little bunches 

 of a dull color, coating rocks or surrounding the stems of marine plants. The general 

 structure can be seen in Fig. 49. With a fleshy nature, of course no fossils of this 

 order can occur. 



Order II. — GUMMININ^. 



These are tough and leathery sponges, the external layer forming a cortex which is 

 partly composed of fibres, which also permeate the central mass surrounding the canals, 

 and also penetrate the mesoderm. Their composition is still unknown. The genus 

 Chondrilla has star-shaped silicious bodies in the cortex which are not found in Gum- 

 minia and in the other genera. No fossils are known. 



