20 Mr. H. J. Carter on the Relationship 



so thoroughly convinced of this homology that I (with Mik- 

 lucho) designate the largest cavity into which that canal- 

 system is dilated in the sponge-body, and which is usually 

 called the excurrent tube or flue (caminus) as the stomach, or 

 digestive cavity, and its outer orifice, which is usually called 

 the excurrent orifice or osculum, as the buccal orifice or 

 mouth." 



As may be perceived from these quotations, Hackel's views 

 of the organization of the Spongiadge (which also form the 

 basis of his classification of the Calcispongise) do not accord 

 with the facts which I have stated. Hence, our premises 

 being diiferent, it is useless to raise any argument against his 

 hypothesis : the facts must speak for themselves. 



But, as regards the inflow of the water into the osculum or 

 vent, which, as before stated, is only occasional, abnormal, 

 and not the rule but the exception (for even Hackel observes, 

 at p. 10, that it is " generally (but not always!) the case "), no 

 one well acquainted with the habits of the sponge would ex- 

 pect to see any thing but an exhalant current from this 

 orifice. 



Relative to this, Hackel adds, at p. 11 : — " The difierence 

 in the direction of the current of water which is usually ad- 

 mitted in the two classes is a matter of perfect indifference in 

 this close morphological comparison. Even if this diff'erence 

 was really constant, general, and thoroughgoing, it ^ould not 

 be capable of invalidating om' notion of the homology of the 

 canal-system in the body of the sponge and coral." 



The necessitous adaptation, however, of the vent in the 

 sponge to an inflow instead of an outflow of water is only 

 temporary, and, not being constant, seems to me of no value 

 in establishing an homology. 



Thus, neither the prehensile extremity of the elephant's 

 trunk nor that of the spider monkey's tail can make these two 

 organs homologous with each other, or with the finger, al- 

 though aU three are used for similar purposes and in a similar 

 way. Again, although a human being may be nourished 

 through the rectum, it does not make the latter homologous 

 with the stomach ; neither does the casual inflow of the water 

 through the vent of the sponge make this aperture homologous 

 with the mouth of an Actinia ; while in all these instances 

 it seems to me more essential to know what their respective 

 functions may be than their homologies, albeit the latter, 

 when based on facts and not fancies, are equally essential as 

 the basis of true classification. It is not difiicult to assume 

 that a spider monkey should have a tail, but it is much more 

 useful in natural history to know how it differs from tails in 



