196 Mr. H. J. Carter on the Polytremata, 



a Polytrema is easily explained by the fact that most sponges 

 overgrow every stationary object with which they come into 

 contact ; and as all kinds of sponges may do this, so the Poly- 

 trema may at one time be overgrown by one kind of sponge 

 and at another by another kind, this being, in the matter of 

 difference, purely accidental. But there is only one kind of 

 sponges that can become really parasitic, viz. the Clionida or 

 boring sponges ; and these do not grow upon but in their host, 

 and thus in the test of Polytrema. Of this I have but one 

 instance m Polytrema ^ although it is common enough elsewhere ; 

 and here the usual cavities in the test of the host, together 

 with their circular fenestral openings on the surface, have, by 

 the position and regular arrangement of the spicules, which 

 are always entire, at once pointed out their true nature, in 

 contradistinction to the spiculiferous accumulations within the 

 natural cavities of the Polytrema. As further means of dis- 

 tinction, it may be added that the pointed ends of the spicules 

 in sponges are always directed outwards, and the pin-like spi- 

 cules of a Cliona, which are chiefly confined to a bristling 

 coronal arrangement filling the circular fenestral apertures 

 which it has formed on the surface of its host, are arranged 

 after this manner ; 2nd, the spicules are entire and regularly 

 arranged ; 3rd, they are for the most part unaccompanied by 

 grains of sand and other foreign objects. On the other hand, 

 in Polytrema the pointed ends of the spicules are sometimes 

 outwards and sometimes inwards (fig. 6, n n &c,), there are 

 more fragments than entire forms (fig 6), and the whole is con- 

 fusedly arranged and mixed up together with sand-grains and 

 a variety of minute foreign objects (fig. 6, mmm). 



Then, again, the spicules in Polytrema, besides being for 

 the most part fragmentary, are of different forms, although 

 chiefly linear, as the furcate and radiated forms are more diffi- 

 cult of introduction. The latter seems to be proved not only 

 by their absence generally in the interior, but by their appearing 

 sometimes on the surface of the Polytrema with one arm fixed 

 in the aperture and the others outside (fig, Q,p). 



Again, the spicules may belong to different sponges. In 

 one mounted rounded embryonic specimen of Polytrema from 

 the Mauritius, whose rounded form had been occasioned by the 

 summit having been broken off, there are several hamates and 

 anchorates together with a linear spicule, all evidently dei-ived 

 from Halichondria incrustans ; while in the embryonic speci- 

 men figured, in which a portion of the side of the summit only 

 is broken away, three linear spicules may be observed, one of 

 which is pin-like and therefore not belonging to the sponge 

 mentioned (fig. 4), and so on. In another mounted but older 



