154 HUGH WATSON. 



nerves from the different parts of the olfactory organ 

 converge and unite behind the level of the eye to form the 

 large olfactory nerve, which runs back for a considerable 

 distance in the centre of the posterior part of the tentacle. 

 The much smaller optic nerve is situated laterally, being- 

 embedded in the muscle-fibres on the outer side of the olfactory 

 nerve. These longitudinal muscle-fibres make their appear- 

 ance just behind the sense-organs, and are chiefly restricted 

 to the peripheral part of the retracted tentacle ; but as they 

 ai'e traced backwards they gradually increase in number, and 

 become united into muscular strands, which coalesce with one 

 another to form the retractor of the tentacle . In the spaces 

 between these strands, before they all coalesce, large cells 

 occur, as will be seen from fig. 84. These large cells stain 

 rather deeply, and have rounded nuclei, containing prominent 

 nucleoli. They are evidently homologous with the somewhat 

 similar cells that Beutler^ found in the tentacles of Pary- 

 phanta hochstetteri [Pfr.). In this region the tentacle is 

 rather darkly coloured ; yet the pigment-granules are not 

 contained in these large cells, but are confined to narrow 

 filaments which surround the cells and the different strands 

 of muscle. Moreover the pigment extends a little further 

 back, even slightly beyond the point shown in fig. 86, where 

 the olfactory and optic nerves emerge from the tentacular 

 retractor and curve over to the cerebral ganglion (PL XIII, 

 figs. 52-55). 



Not more than 2 or 3 mm. further back the retractor of the 

 upper tentacle is joined by that of the lower tentacle, which 

 is narrower and lies more ventrally. The common retractor 

 thus formed is of no ffreat lenarth. It arises from the side of 

 the body-wall between a quarter and a third of the distance 

 from the head to the hind end of the animal. The common 

 tentacular retractors of each side have no connection Avith 

 each other or with the buccal retractor ; on the contrary, the 

 three retractors have become unusually widely separated in 

 their origin. On the right side the retractors of the upper 

 ' ' Zool. Jahrb..' 19()1. vol. xiv, p. 404, pi. xxix, figs. 63, 64. 



