SPONGIAD^E. 193 



Suborder III. Spiculo-reticulate skeletons. Skeletons 

 continuously reticulate in structure, but not fibrous. 



Halichondria. 

 Hyalonema. 

 Isodictya. 

 Sponyilla, 



The sponges of this suborder vary in the different genera 

 to a great extent in the mode of the construction of the 

 skeleton, but in all cases the spicula are the dominant 

 material; their terminations overlap each other, and they 

 are cemented together by keratode. The reticulations thus 

 formed sometimes consist of a single series of spicula, at 

 other times they are very numerous, and are crowded 

 together in the manner of elongated fasciculi. 



The genera Halichondria and Isodictya are exceedingly 

 rich in species, but the inconvenience attending their dis- 

 crimination arising from their number may be remedied to 

 a great extent hereafter by subdivisions of each genus, based 

 on the characteristic forms of the spicula of their respective 

 skeletons. The structural distinction between Halichondria 

 and Isodictya is so well marked as to render the recog- 

 nition of each comparatively certain and easy. The skeletons 

 of the species of the latter genus, generally speaking, are 

 very much more slight and fragile than those of the former 

 one, and the same rule obtains to a great extent as regards 

 the comparative size of their spicula, and in many species 

 of Isodictya they are very minute. Hyalonema and Spon- 

 gilla are readily to be distinguished by the peculiarities of 

 their structure and localities. 



The genus Halichondria, as constituted by Dr. Fleming 

 in his ' History of British Animals/ and adopted by Dr. 

 Johnston in his ' History of British Sponges,' contains 

 species which differ exceedingly in their mode of organiza- 

 tion. Thus, if we take H. panicea of Johnston, which is 

 undoubtedly the " sponge-like crumb of bread " of Ellis, 



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