200 Proceedings of the 



four small pullulating tentacles. When perfectly formed, these 

 polyps are about half an inch in length, club-shaped, and fur- 

 nished with from eight to thirty tentacles, placed in two alter- 

 nating rows. Those of the upper row, which are about twice the 

 length of the lower row, are held nearly parallel with each other 

 above the mouth, while the lower row are extended at right angles 

 to the axis of the body, — an exaggerated state of a disposition 

 frequently observed in Sertularian and Campanularian polyps. 

 The mouth opens into a buccal cavity, contained within a more 

 or less elongated papilla, which rises in the centre of the circle 

 of tentacles. This papilla is exceedingly distensible, frequently 

 expanding itself into a wide discoid sucker, as shown in the 

 figures of Quatrefages, and even folding itself backwards, so as 

 to conceal the tentacles, and completely evert the body. A 

 similar habit has been observed by Dr Coldstream in the 

 polyp of Clava, and by myself in Goryne pusilla. 



19. The body of the polyp consists of the same elementary 

 tissues as the polypary, with the addition of a muscular tunic, 

 which is interposed between the endoderm and ectoderm. 



20. The Ectoderm of the alimentary polyp is a transparent, 

 homogeneous membrane, containing molecular matter, also 

 transparent, but of higher refractive power; so that expansions 

 of this tissue have a finely- dotted appearance under the high- 

 est powers of the microscope. The ectodermic layer also con- 

 tains multitudes of thread- cells of the smaller kind. These 

 are amassed in the greatest numbers at the upper parts and 

 ends of the tentacles. Over the site of each thread-cell a very 

 delicate soft spine or cilium may be observed projecting from 

 the ectoderm, and apparently springing from the thread-cell 

 itself. Similar spines exist over the smaller thread-cells of 

 Hydra vulgaris, and those of Sertularian and Campanularian 

 polyps. Now, when the thread-cells of these hydroiclse are 

 displaced by gentle pressure, so as not to burst them, they are 

 found to have a perfectly smooth contour. Moreover, the 

 ectoderm of the planarioid larva of Hydractinia is thickly 

 studded with these spines before any development of thread- 

 cells has taken place. These spines also occur in various 

 tribes of the mollusca ; for instance, on the very adhesive 

 tentacles which fringe the inner edge of the mantle of Lima, 



