14 Prof. W. C. M'liitosh OH certain 



foniier, the colour liarinoiiizes with the sand at and beyond 

 low-water mark, and a CL-rtain amount of translucency is pre- 

 sent, for the Cjuartzo.se sand-]jarticles are each fixed in the 

 cement like a jewel in its setting, so that there is little inter- 

 ference with the light. Moreover, the tube has only the 

 thickness of such grains — with a delicate coat of the secretion 

 internally. The smaller end of the tube has grains consider- 

 ably liner than the wider uj)per end, but they blend into eacii 

 other so insensibly that it is only by comparing the distant 

 parts that the difference is observed. Like many others, the 

 annelid works at its tube only during the night. 



Such is the ordinary condition of the home of Amphicteiie, 

 but occasionally circumstances alter the case very materially. 

 Thus, when amongst deep-sea mud it is debarred from 

 laying hold of sand-grains, it fashions a perfectly circulur 

 tube — tapered and curved as in the ordinary examples — out 

 of the only hard particles it can secui*e, viz. fragments of the 

 glassy spicules of sponges, which are placed with the utmost 

 neatness and regularity in horizontal rows from end to end, 

 somewhat after the manner of basket-work (fig. b). These 

 spicules consist, so far as observed, of short fragments, and 

 thus, with their minuteness, conduce to the perfect rotundity 

 of the tube, which only presents the slight convexity of each 

 spicule at the edge, the outline being minutely crenate under 

 the microscope (tig. 7). The cement fixing the spicules is in 

 small quantity, just sufficient to render the tube strong with- 

 out interfering witli its transparency* and smoothness. The 

 workmanship of these tubes of AmpJiictene, indeed, excites 

 admiration — no less for the skill and perseverance of the 

 architect than for the unfailing accuracy and beauty of the 

 result. No careless work betokens haste or weariness, but 

 from the first translucent circle to the last the same uniform 

 regularity is nuiintained. Moreover, on magnifying a portion 

 of the tube the beauty and com))lexity of its structure are 

 increased, for each of its fragments shows the central tube of 

 the s])icule either as a clear lumen or tilled with extraneous 

 granular matter (Hg. 8). 



Larger and stronger tubes than the foregoing are formed 

 by two northern species. The Hrst {Cistcnides (jrunuhita, 

 from Exeter Bay, CJreenland) constructs a slightly curved 

 tapering tube of rather coarse grains (the particles, iiowever, 

 being finer at the smaller end of the tube), which are promi- 

 nent, so that the surface is somewhat rough, yet the cement 



* The tniiisparem y of such a tube ii'iidily shows tho. coU-Hko bodies in 

 tlif sccretiuu whicli so lu-iitly lixcs the I'rajjcmeuls of the spicule.- aud 

 I'oruis a tuiu cuutiu-' iutcrmdJv. 



