THE ANATOMY OF SCALIBREGMA INFLATUM. 301 



6. The uerve-cord is situated close to tlie epidermis, and is 

 non-ganglionated. The ganglion cells are distributed along 

 the whole length of the cord on its lateral and ventral faces. 

 The cord gives off in each segment a pair of nerves lying in 

 each interannular groove, and a pair lying in the chaeti- 

 gerous segment. The latter supplies the cirri, sense organs, 

 and seta] sacs. 



7. A pair of lateral sense organs is present in each chteti- 

 gerous segment. Each sense organ is a small eminence 

 rising from the base of a shallow depression bordered by 

 prominent lips of epidermis. From a darker area in the 

 centre of the papilla the delicate sense hairs arise. They are 

 implanted in exceedingly slender columnar cells, closely and 

 regularly arranged. These cells are in connection at their 

 inner ends with pyriform or fusiform ganglion cells, which 

 occupy the axis of the sensory papilla. Around and below 

 these are numerous deeply-staining nuclei, which are prob- 

 ably, as Eisig showed, the nuclei of multipolar ganglion cells, 

 the protoplasm of which forms the fine network upon which 

 the nuclei are situated. The sense organ receives a mode- 

 rately stout branch from the spinal nerve, which runs along 

 the middle of the chajtigerons annulus. The sense organ 

 may be withdrawn into the depression in the epidermis by 

 the contraction of a retractor muscle attached to its base. 

 In very young sense organs, such as are found in the last two 

 or three segments of a specimen about 15 mm. long, the rods 

 Avhich occupy the space of only one or two epidermal cells 

 are exceedingly small, and do not bear sense hairs. There 

 are only two or three small ganglion cells at their bases, 

 accompanied by about twenty of the deeply staining nuclei 

 mentioned above. In the next segment antei-ior to this the 

 rods are more obvious, and one segment further forward the 

 sense hairs may be seen on their distal ends. In old sense 

 organs the axial part of the organ is more fibi'ous, and the 

 deeply staining nuclei are very numerous. 



8. Similar sense organs are present in E urn en i a crassa 

 and Lipobranchius jeffreysii. -. .. 



