THE TROCHOPHORE STAGE AS A FREE-SWIMMING LARVA. 37 



In the Annelidas Trochophore, a nerve-ring is found beneath the pre-oral 

 ciliated ring (Vol. I., p. 266). Such a nerve-ring is, as far as we know, not yet 

 demonstrated in the Laniellibranch larva, but, considering the great agree- 

 ment in other respects between the two larval forms, it is very probable that 

 it is present. 



Besides the neural plate, there is, according to Hatschek, in the 

 larva of Teredo, another constituent part of the nervous system, viz., 

 the rudiment of the ventral, sub-oesophageal ganglion, lying as a 

 large ectodermal thickening between the mouth and the anus (Fig. 

 18, g) ; this becomes the pedal ganglion of the Mollusca. Com- 

 missures between the two central organs of the nervous system which 

 would make the comparison with the supra-oesophageal ganglion and 

 the chain of ventral ganglia of the Annelida still more striking, have 

 not been found at this stage (Hatschek). 



On either side of the ventral ganglionic mass, the otocysts arise 

 as small ectodermal invaginations in the same position as in the 

 Annelidan Trochophore (Fig. IIS B, p. 265). Fine hairs are attached 

 to their walls, and in the centre of each is a strongly refractive round 

 otolith (Fig. 18, ot)* 



The eye-spots with lenses embedded in them, which Loven observed in a 

 few pelagic Laniellibranch larvae (e.g., Mytilus), apparently arise at a later 

 stage of development, i.e., after the larva has passed out of the actual Trocfaj- 

 phore stage. They then lie near the oesophagus, and thus behind the pre- 

 oral ciliated ring, and cannot therefore be compared with the eyes of the 

 Annelidan larva which lie on the cephalic area, i.e., in front of the pre-oral 

 ciliated ring. 



[Pelseneer (No. IV.) has recently discovered that these eyes are retained 

 in the adult Mytilidae and in Arvicula, where they are situated at the base 

 and under cover of the anterior filament of the internal branchial lamella ; 

 they are innervated from the brain. They are not homologous with the larval 

 eyes of the Gastropoda, which occur on the velum, and are therefore true 

 cephalic eyes, but are possibly homologous with the larval eyes of Chiton 

 IP- 14)-] 



The oesophagus and the base of the intestine of Teredo arose, as 

 has already been mentioned, as ectodermal invaginations (Fig. 15 

 A and C). Before they both become finally connected with the 

 enteron, the latter assumes a sac-like shape, through the active 



* [Although arising near the pedal ganglia, the otocysts are innervated from 

 the cerebral ganglia, as in the Gastropoda. This primitive condition is still 

 to be seen in Xiicula and its allies, where also the otocysts retain, even in the 

 adult, their connection with the exterior by a long canal opening on the surface 

 of the foot. In other forms the nerve of the otocyst is bound up with the 

 cerebro-pedal commissure so as to be indistinguishable in the adult. — Ed.] 



