534 G, C. BOURNE. 



riorly. The second of the two pairs of prominences above 

 mentioned is the foundation of the first pair of permanent 

 antennae. 



In the oral field the ventral plates increase greatly in size, 

 extend anteriorly and posteriorly, and become divided by a 

 number of clear transverse lines into segments, but as yet there 

 are no external furrows marking off one segment from another. 

 Usually as many as eight segments make their appearance at 

 once, and the number increases up to sixteen, but is very in- 

 constant. In the next stage the number of segments is not 

 added to, but each segment grows greatly in size, and soon the 

 hinder part of the ventral plate begins to project beyond the 

 oral lobe of the larva. Already in each segment a median, and a 

 distal prominence, have made their appearance on each side of 

 the body. These are the foundations of the dorsal and ventral 

 cirrhi of the parapodia, and before long a third prominence 

 appears between each pair of cirrhus-foundations, which is the 

 foundation of the chsetopodium. In further growth the vermi- 

 form segmented body of the Annelid, derived from the ventral 

 plates, becomes more and more important, and preponderates 

 over the oral lobe of the larva. The cheetopodial elements 

 grow out into the parapodia of the adult, and in so doing carry 

 with them the dorsal and ventral cirrhi which, at first separate 

 from the chsetopodia, are eventually carried on the ends of the 

 latter. At this stage the pre-oral lobe and the prototroch retain 

 their primitive characteristics, and the larva has a broad, dome- 

 like trochosphere head, with a vermiform body bearing para- 

 podia. The foundations of the two pairs of antennas have, 

 however, grown out into processes projecting from the ventral 

 surface of the pre-oral field, and the olfactory pits are developed 

 as deep sacs, with a narrow, slit-like opening, which can be 

 evaginated and drawn in again. 



The final changes consist in the atrophy of the preoral lobe, 

 the loss of the ciliated ring, the further growth of the two pairs 

 of antennas, and the completion of the parapodia with their 

 setse, &c. 



The internal organs of the adult Annelid are developed by 



