Anatomy and Physiology of Cordylophora. 107 



possess the same essential structure as those of Hydra; 

 they are formed of a direct continuation of the ectoderm of 

 the polype, lined by a similar continuation of the endoderm. 

 The appearance of transverse septa at regular intervals, 

 which is so very striking in these tentacula, must not be at- 

 tributed to the existence of true septa. It is due to a pecu- 

 liar condition of the endodermal layer, but the author has 

 not been able to give a satisfactory explanation of it. |Through 

 the whole of the canal which pervades the axis of the stems 

 and branches, a constant though a regular rotatory move- 

 ment is kept up in the contained fluid ; this movement is not 

 due to the propulsive action of vibratile cilia, and is explained 

 by the author as the effect of the active processes going on 

 in the secreting cells of the endoderm, processes which can 

 scarcely be imagined to take place without causing local al- 

 terations in the chemical constitution of the surrounding 

 fluid, and a consequent disturbance in its stability. 



The reproductive system of Cordylophora consists of ovoid 

 capsules situated on the ultimate branches at some distance 

 behind the polypes ; some of these capsules contain ova, 

 others spermatozoa ; they are plainly homologous with the 

 ovigerous sacs of the marine Tubulariadm ; they present a 

 very evident, though disguised medusoid structure, having a 

 hollow cylindrical body, whose cavity is continuous with that 

 of the polype-stem, projecting into them below, and repre- 

 senting the proboscidiform stomach of a Medusa, while a 

 system of branched tubes which communicate at their origin 

 with the cavity of the hollow organ, must be viewed as the 

 homologues of the radiating gastro-vascular canals, and the 

 proper walls of the capsule will then represent the disc. 

 From comparative observations made on other genera of 

 Hydroida, the author maintains the presence of a true me- 

 dusoid structure in the fixed ovigerous vesicles of all the 

 genera he has examined, and he arrives at the generalization, 

 that for the production of true ova in the hydroid zoophytes, 

 a particular form of zooid is necessary, in which the ordinary 

 polype-structure becomes modified, and presents, instead, a 

 more or less obvious medusoid conformation, Hydra being 

 at present the only genus which appears to offer an excep- 



