Cluetoptcnis \^n'iupt'(latus. 523 



the large-bodied lai'va becomes more slng-iiish in its movements and 

 settles down, at first becoming attached by the holdfast. Later it 

 creeps among the diatoms of the bottom, with its large post-oral lobe 

 protruded forwards like a scoop. In young specimens the creeping 

 is accomplished chiefly by the movements of the ciliated rings, and 

 the axial contraction of the body, and, in part, by use of the hold- 

 fast as a prop ; but in older lan^se chiefly by the powerful cilia of the 

 ciliated rings and the greater contractility of the body. When they 

 move among the diatoms of the aquaria they leave a slight trail of 

 mucus. Later they make short, horizontal, mucus-coated tunnels into 

 the mass of diatoms and sand. One of these tunnels may be extended 

 to several times the length of its body and from this simple tunnel 

 of agglutinated sand and diatoms the larva may build the L^-shaped 

 tube within which it remains confined. 



FORMATIOX AND ExLARGF.MEXT OF THE TuBES. 



The first tube in which the larva lives and feeds for several days 

 is nearly a millimeter in diameter and from eighteen to twenty-two 

 millimeters long. It is either a straight tube or a shallow U whose 

 curved portion is downward. 



After an interval of a day or two in the mucus-coated tunnel the 

 young worm, for it is now an adult in miniature, has outgrown it 

 and a new tube is constructed. This is done by splitting the tube 

 at a point where the upright arm meets the horizontal portion, in a 

 U-shaped tube, or near one end, in a straight tube, and then excavat- 

 ing a tunnel obliquely do^vnwards and, after nearly doubling the 

 length of the basal portion, upwards to the surface. This is its first 

 lateral enlargement. The sand which the worm excavates is expelled 

 from the opposite end of its first tube. The walls of the tunnel are 

 coated with mucus as the tunnel advances, so that the U-shaped tube 

 is completed when the excavation reaches the surface. The tube 

 becomes strengthened from time to time by additional layers of 

 mucus that harden to form a parchment-like material that gives older 

 tubes a laminated structure. They are enlarged in the same vertical 

 plane as the original tube unless prevented from doing so by some 

 obstruction, as a shell, when they turn obliquely along the surface 



