210 J. H. ASHWOBTH. 



upon which cells (X.C.) are situated at intervals. The fibrils can be 

 best seen in the mesogloea, but can be traced close to the ectoderm and 

 endoderm. The cells of the nervous system are exceedingly small, and 

 usually fusiform, triradiate, or stellate in shape, the angles of the cell 

 beiug produced into nerve fibrils. 



On tracing the fibrils outwards from the mesogloea into the ectoderm, 

 they are seen to be in connection with small cells which are situated 

 in the deeper part of the ectoderm. Owing to the irregularity of the 

 inner face of the ectoderm, and to the presence of spicules in the 

 portion of the layer where the nerve-cells are situated, it is not possible 

 to obtain a section which shows the ectodermic nerve plexus clearly, 

 but small portions of it may be seen where the spicules are slightly 

 less numerous. 



In the case of the endodermic nerve plexus this difficulty does not 

 exist, and an oblique section through the wall of a polyp shows that 

 the plexus of fibrils in the mesogloea is connected with minute stellate 

 cells situated upon the outer face of the muscle processes of the 

 endoderm cells. Heeniatoxylin (especially Heidenhain'siron hsematoxy- 

 lin and Meyer's acid haemahim) stains the nerve-cells and fibres most 

 clearly. 



History of our Knowledge of the Buds of the Xentid.e. 



Besides the fully developed polyps, the description and measurements 

 of which are given above, there are on most of the branches of the 

 colony young polyps or buds in various stages of development. These 

 buds are invariably found at the edge of the umbrella-shaped area at 

 the end of the stem. On one stem (Fig. 1, left) there are ten small 

 buds varying in length from "5 mm. to 3 mm., and about fifty larger 

 polyps from 5 mm. to 10 mm. in length. The smallest buds have 

 simple tentacles devoid of pinnules, and all stages between these and 

 the adult polyps may be found. As the polyps (both young and old) 

 of Xenia are non-retractile, this genus offers considerable advantages 

 for the study of the development of the polyps, and the young polyps 

 have been noticed by many observers. 



Quoy and Gaimard (1833) first observed these small polyps in 

 the Xeniidce. They noticed them in Comidaria viridis, which appears 

 to belong to the genus Xenia, and they suggested that these small 



