THE ANATOMY OF ALCYONIUM DIGITATUM. 371 



for the study of the development of the spicules, as it is a 

 matter of very great difficulty to make a thin section of the 

 surface of the colony before decalcification. 



We have two very different accounts of the development of 

 these structures in Alcyonaria. According to von Koch 

 (20) the spicules of Clavularia prolifera are formed 

 within and continue to grow within a cell which is given off 

 from the ectoderm, the young spicules being always enclosed 

 in a nucleated protoplasmic sheath. In my decalcified sections 

 I have seen these sheaths (fig. 17, sp. c), which, I imagine, 

 contained young spicules, and I have also seen cells 

 containing a small vacuole lying close under the ectoderm. 

 I believe, therefore, that von Koch's account is substantially 

 correct. 



Schneider (35), on the other hand, if I understand his 

 description and figures correctly, believes that in Alcyonium 

 acaule one or more cells gradually assume the shape of the 

 future spicule, and then become calcified. I have seen nothing 

 in ray preparations to support this view. 



The Nervous System. — In longitudinal sections of the 

 tentacles a number of minute cells may be seen lying between 

 the epithelium and the layer of muscular fibres. These cells 

 are spindle-shaped, triangular, or star- shaped, and are con- 

 tinuous with a fine plexus of fibrils, some of which have a 

 beaded appearance (PI. 38, fig. 22 n. p.). 



Whether this system of cells and fibrils is really nervous in 

 nature and function has not been determined experimentally, 

 but it is undoubtedly homologous with the layer called 

 "Nervenschicht " by the Hertw^gs in the ectoderm of the 

 Actiniae. 



A similar plexus of cells and fibrils may be seen in the 

 endoderm, when suitably prepared sections are made which 

 cut the ccelenteric tubes longitudinally and slightly obliquely. 



I have represented in fig. 33 a portion of a tube that has 

 been cut somewhat obliquely, so that in the upper part of the 

 section the endoderm-cells may be seen, but in the lower part 

 these have been shaved off", and only the circular muscular 



