Mr. R. Q. Couch on the Morphology of Zoophytes. 165 



From the foregoing observations it will appear, that in the 

 earliest stages of growth, the stem, the polype cells and the ovarian 

 vesicles of the Sea-threads, Laomedea, are precisely alike, and that 

 at a particular point of their development each assumes its indi- 

 vidual character. The stem advances to one point and is there 

 arrested in its organization ; the polype cell advances to the same 

 point, but instead of being arrested, the pulp becomes developed 

 into a polype and the sheath into a cell. This however seems to 

 be only the case when the termination is bulbous ; for in many 

 instances I have seen that where the pulp was not bulbous, but 

 of the same diameter throughout, and about the size of the stem, 

 that no polype has been developed, but merely a distorted branch. 

 Where specimens grow in unfavourable situations, such distor- 

 tions are not unfrequently to be found ; and most of them I be- 

 lieve are attributable to this cause. This malformation is most 

 frequently to be seen in Sert. polyzonias, rosacea, and a few other 

 kindred species. 



The ovarian vesicle also advances as the branch and cell, but 

 instead of being of equal diameter throughout, as the former, or 

 bulbous at its extremity, as the latter, it is enlarged or bulbous 

 throughout its extent, and is united to the branch or trunk by a 

 narrow and short peduncle. Instead of the vesicle being arrested 

 in its growth at the same point as the stalk, or organized into a 

 polype as in the cell, it becomes developed into numerous minute 

 globes covered with vibratory cilia as previously mentioned. Here 

 then we see a great similarity between the different organs of 

 these creatures, — a similarity so great as to warrant the supposi- 

 tion of their primary identity and subsequent individualization, 

 even if there were no others. In Sertularia polyzonias I have 

 several times seen a polype cell terminate in a distorted branch ; 

 and on the other side I have seen a branch terminate in a polype 

 cell, showing a convertibility Into each other. 



In all my examinations I have never seen the ovarian vesicle 

 occupied by a polype. Ellis, however, has figured something like 

 this with the polype protruding, but he says nothing of the kind 

 in his text, and I am unacquainted with any one who has wit- 

 nessed anything of the sort, though observers have become nu- 

 merous since Dr. Johnston^s work has been published. I have 

 however seen a cell, apparently designed for a vesicle, small in 

 its growth and occupied by a polype. This form of vesicle has 

 been selected for my illustration, because, if I understand Prof. 

 Forbes, it is the one about which he had doubts. At some future 

 time, when I have a little more leisure, I should like to offer a few 

 observations on others and diversified forms of these transitory 

 cells. 



