1901. J AND YOrNfi OF .V POLYCH.TTF, WdTJAr. 717 



caii be seen, this passage is probably only the space between the 

 masses of mesoblast. 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 filiated 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 work their way out of the mass of 

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

 water by means of their cilia. 



4. Very shortly after hatching (PI. XXXIX. tig. f.) the first 

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

 setae 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 seta? 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 larva) now show movements of the body of an " euglenoid '' 

 nature, and protrude the seta? 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 than at 

 batching, 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 hand 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 1 (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 truncal ed. The 

 larvffi 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 Buch swarms are beginning t<> 

 secrete webs of mucus 3 , in whicb they lie nearly motion! 



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



Tin- timet riven throughout ilii^ paper for the appearand of varioiu 

 organ* are only approximate and refer i<> ipeoimena in captivity. < m course, 

 ii by no meant followi thai they bold pood under natural conditions, 



I rmal and not a pathological phenomenon I am convinced 



i,v the fact thai i> occurred in ever) batch of Larvae under observation, and l>\ 

 the perfeotlj regular progress of the itibsequenl development. 

 pBOO. Zooi . So< . 1901, Vol. ||. So. XI. VII. -17 



