ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 203 



Remarkable Organ in Eudendrium ramosum.*— When engaged 

 in his investigations into the mode of origin of the generative 

 products of this hydroid, Dr. A. Weismann observed remarkable 

 outgrowths on the head of the hydranth, of which, at first, he took no 

 especial notice, as he regarded them as pathological products. They 

 are horn-shaped stout processes growing out laterally from the head 

 of the polyp ; in form, though not in dimensions, they resemble a 

 tentacle, with the exception that they are not thinner, but, as a rule, 

 swollen at their free ends ; they are formed by the two body-layers, 

 and contain a continuation of the body-cavity ; they are not found 

 on all the hydranths of a colony, and this might lead us to think 

 that they are degenerated structures ; that, however, they are not 

 degenerated gonophores is shown by the fact that, while all gono- 

 phores arise below the circlet of tentacles, or in the upper half of 

 the hydranth, these are always developed below the hydranth ; again, 

 their structure shows that they have a definite function, they are 

 actively motile, have a well-developed muscular layer, and are so 

 remarkably welh provided with urticating organs that they might be 

 spoken of as cnidophores. 



If we enter into the details of their structure, we find that the 

 ectoderm only differs from that of the hydranth in its much richer 

 supply of urticating organs ; while there is nothing remarkable in 

 the supporting lamella, there are, in addition to the epithelial cells of 

 the endoderm, subej)ithelial cells lying on the supporting membrane 

 and giving origin to circularly arranged muscular fibres : as yet 

 circular muscles have only been observed in Tuhularia among the 

 Hydroid Polyps. In Eudendrium the circular muscular layer of the 

 cnidophores is strongly developed and consists of very fine long fibres, 

 which frequently exhibit a delicate transverse striation. After this 

 description it can hardly be doubted that we have to deal with an 

 offensive organ ; the power of active movement, and the notable supply 

 of stinging organs of colossal size sufficiently demonstrate the 

 correctness of this view of their function. 



The cnidophores always arise from a circular but indistinct 

 wall of ectoderm, which is separated off by a circular groove from the 

 wall of the stalk ; this groove may be known as the glandular ring, 

 and the wall as the urticating wall. In the region of the former the 

 ectoderm cells are in one layer only, or, the glandular cells reach to 

 the surface. 



Viewed morphologically, the cnidophores are seen to be processes 

 of the body-wall; in their earliest stages they are blunt, broad, solid 

 processes of the urticating wall developed by a thickening of the 

 ectoderm. In the next stage they contain an endodermal process, and 

 thence to the complete condition there is every kind of intermediate 

 stage. It is important to note that they only arise on developed 

 hydranths, for this shows that they are, phylogenetically, relatively 

 young organs. Their presence on some hydranths only presents 

 some difficulties, and we can only suppose, till they shall have been 

 studied during life, that they are developed as a protection against 

 * MT. Zool. Stat. Neapel, iii. (1881) pp. 1-14 (1 pi.) 



