1901.] AND YOUNG OF A POLYCIL^TB WORM. 717 



can be seen, this passage is probably only the space between tbe 

 masses of niesoblast. A clearer area of squarish shape marks the 

 position of the fore-gut. 



The mouth cannot yet be seen in a surface view, and its relation 

 to the ciliated band is therefore uncertain, but the fore-gut rudi- 

 ment stands in the middle of the band. 



On each side of the backward projection of the mid-gut rudi- 

 ment is a space in which the first seta-sac will shortly arise. 

 Behind this, on each side of the anal site, is a small mass of meso- 

 blast, in the middle of which lies a rounded object, perhaps a 

 pole-cell. The larvae now vi-ork their way out of the mass of 

 mucus, and swim about in a lively manner in tlie surrounding 

 water by means of their cilia. 



4. Very shortly after hatching (PI. XXXIX. fig. G) the first 

 pair of seta-sacs appear towards the hinder end of the body. The 

 setse are at first only two in number and of simple form, one pointed 

 at the end and the other somewhat blunter. 



5. To these two setse there is added, in the course of an hour or 

 two, a third of a different form. This is a compound seta, having 

 at its free end an oval swelling, flattened at one side, and bearing 

 on the flattened surface a triangular end-joint, articulated to the 

 seta by one angle of its base. When placed under a coverslip 

 the larvaB now show movements of the body of an " euglenoid " 

 nature, and protrude the setse on a parapodium-like prominence, 

 which, however, is very contractile and can be completely with- 

 drawn. The form of the body is already more elongate i3han at 

 hatching, owing to the gradual lengthening of the region between 

 the main band of cilia and the perianal patch. It is in this region 

 that the successive bundles of setae will arise, the latest always at 

 the hind end. As this process continues, the ciliated band becomes 

 narrower and narrower, both absolutely and relatively to the whole 

 length of the body, and is thus less and less efficient as an organ 

 of locomotion. 



6. About eighteen hours after hatching' (PI. XXXIX. fig. 7) 

 a second pair of seta-sacs appears behind the first, with a single 

 seta in each. This seta is of the same compound form as the 

 third one of the anterior sac. The body is steadily elongating, 

 and the hind end is now no longer rounded but truncated. The 

 larvae begin to show a preference for the sides and bottom of the 

 vessel, and have a power of adherence to objects due, as it seems, 

 to the secretion of a sticky mucus by the epidermis, which contains 

 numerous unicellular glands for this purpose. They show a strong 

 tendency to swarm together, and such swarms are beginning to 

 secrete webs of mucus ', in which they lie nearlv motionless. 



7. About forty hours after hatching (PI. XXXIX. fig. 8) the 



1 The times given throughout ihis paper for the appearance of various 

 organs are only approximate and refer to specimens in captivity. Of course, 

 it by no means follows that they hold good under natural conditions. 



2 That this is a normal and not a pathological phenomenon I am convinced 

 by the fact that it occurred in every batch of larvee under observation, and by 

 the perfectly regular progress of the subsequent developnient. 



Proc. Zool. Soc— 1901, Vol. II. Xo. XLVII. 47 



