l62 



MARINE BIOLOGICAL LABORATORY. 



TROCHOPHORE 



nal opening at the posterior end of the body. There is 

 a brain (supra-oesophageal ganglion) dorsal to the oesoph- 

 agus, connected by commissures with the first pair of 

 ganglia of the ventral chain. An eye lies in the mid- 

 dorsal line over the brain. The first pair of appendages 

 is simple and uniramous and receives its nerves from 

 the brain. The second and third pairs of appendages 

 are each biramous, and the first and second pairs are 

 innervated from ventral ganglia. These I believe are 

 the essential features of the Nauplius. It swims freely 



in the water, and lives a per- 

 fectly independent life in those 

 forms in which it is best repre- 

 sented. 



The Trochophore of the 

 Annelids is likewise a free- 

 swimming pelagic animal, and 

 is diagrammatically represen- 

 ted by Fig. 6. The larva has a 

 large oval anterior end, and a somewhat funnel-shaped 

 posterior part. In the centre of the head-region lie 

 two eye specks, and under this the beginnings of the 

 brain. The middle of the animal is encircled by a 

 band of ciliated cells which form the locomotor ap- 

 paratus. Just beneath this band, or where the band 

 is double within it, is the large mouth opening. The 

 oesophagus is short and leads into the large mesen- 

 teron, and from this runs posteriorly the proctodaeum 

 to end in the anus. Beneath the mouth, and lying 

 between it and the anus, is a bilateral plate of cells 

 which is to form the nervous system. In the earliest 

 stages the Trochophore represents a single segment — ■ 



Fig. 6 



