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PLATE XXXIX. 



HALICHONDRIA PANICEA, Johnston. 



Vol. ii, p. 229, ' Mon. Brit. Spongiadse.' 



I have devoted two plates to the illustration of this 

 remarkably protean sponge, not only because it varies 

 in its form and habit to an extraordinary extent ac- 

 cording to differences in the nature of its localities 

 and other circumstances attending its growth and 

 development, but also as a general illustration of the 

 little dependence that can be placed on the characters 

 of form and colour in the specific descriptions of these 

 extraordinary animals. 



The variations in form influence also other impor- 

 tant specific and anatomical characters. Thus, in figs. 

 1 and 2, Plate XXXIX, the anatomical and physiolo- 

 gical characters are modified in accordance with its 

 peculiarities of form. 



The oscula, which, in its massive forms, are on its 

 external surface as in figs. 3, 4, and 5, are in figs. 1 

 and 2 within the large cloacal tubes, and this appears 

 to be always the case where the sponge puts forth such 

 organs. This necessarily modifies the arrangement of 

 interstitial canals and cavities. Such an extreme 

 variation in form and structure, forcibly illustrates the 

 futility of depending on external form in the discrimi- 

 nation of species, and teaches us, that our only safe 

 guides are to be found in their anatomical structures. 



This great range of variation of form necessarily 

 leads to a simulation of other sponges of a totally dif- 

 ferent anatomical structure, and this is well illustrated 

 by a comparison of fig. 4 in Plate XXXIX with that 

 of Isodictya indistincta, Plate LI, fig. 2, in the present 

 volume, and of Ophlitaspongia papillata, Plate LXXXI, 

 fig. 1. 



Fig. 1 . Represents a specimen which has developed 

 itself in the form of a singular large fistula with a few 



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