42 AMERICAN HYDKOIDS. 



phenia lielleri the present writer found the two layers to be formed by delamination, as described 

 above. 



Reproduction. As in all other hydroids, the common method of propagating the species is by 

 means of a true sexual reproduction involving ova and spermatozoa. With rare exceptions, for 

 example, Antenniilnria, each colony is unisexual; that is, all of the gonophores of a given colony 

 will contain sexual elements of one kind only. 



When the spermatozoa have reached maturity they go forth in countless numbers from the 

 male gouophore, pass through an aperture in the top of the gonangium in a compact stream of 

 rapidly moving bodies lashed along in their course by the vigorous movements of the tiagella, the 

 heads moving from side to side as they progress in what appear to be definite directions. How 

 long the spermatozoa are capable of living this free life in sea water we do not know. A few 

 fortunate ones find their way into the gonangia of a female colony and succeed in impregnating 

 the mature ova while the latter are still within the gonophores. I do not know that the cytological 

 phenomena immediately following the impregnation of the plumularian ovum have ever been care- 

 fully worked out. What is known of the segmentation and succeeding events has already beeu 

 described under the head of Embryology. 



STOLONIFEROUS REPRODUCTION. 1 



Plumularia pinnata is the most abundant plumularian at Plymouth, affording ample material 

 for satisfactory study. The first specimens with young gonangia were brought to the laboratory 

 on May 2. Ten days before this I noticed that several fresh specimens were peculiar in having a 

 number of the hydrocladia greatly produced into thread-like extensions ending in a clavate 

 enlargement (fig. 117). Neither hydrauths nor uematophores grew upon these processes, although 

 the usual number were found in their normal position on the unmodified portions of the hydro- 

 cladia. 



These specimens were kept alive in a separate jar, and three days later it was found th.it the 

 curiously lengthened hydrocladia had continued their abnormal growth, and that some of the 

 enlarged ends had become forked. A microscopic examination showed that the hydrocladia] 

 extensions were almost or entirely destitute of nodes, the whole structure being a simple tube, 

 with perisarc, ectoderm, and endoderm, inclosing the axial cavity in which the life currents were 

 moving in unusual activity. The most notable histological feature was the surprising number of 

 uematocysts embedded in the cosnosarc. The colony seemed in good condition, the hydranths 

 being fully expanded and active. 



Four days later I noticed some delicate, thread-like lines adhering to the inside of a jar 

 containing living colonies of P. pinttata (fig. 119). Upon moving a piece of stone, it was found 

 that these lines were the long, thread-like processes or continuations of hydrocladia noticed 

 several days before. Upon close investigation hydranths were seen fully expanded arising from 

 these processes attached to the glass, and one small colony with the pinnate branching of 

 Plumularia had advanced so far as to show seven hydranths on branches (tig. 120). The original 

 process from the hydrocladium of the parent colony had become a creeping stolon attached to 

 the glass. It was sending up the new colony on the one hand, and giving forth delicate rootlets 

 on the other. A single hydranth growing on the stolon a little to the right of the incipient 

 colony already described seems to indicate the starting of a second colony. Several other stolons 

 (derived in the same way from greatly elongated hydrocladia) were giving off little colonies. 

 There had been no other plumulariaus in this jar, and the original colonies were without gouangia. 



These new colonies were kept alive for a week longer, by which time their connection with 

 the parent stocks had been destroyed by atrophy of the hydrocladial extensions from which the 

 new colonies arose, and the daughter colonies had attained considerable size and all the charac- 

 teristic features of /'. pinnata (fig. 121). 



In another jar a colony showing the hydrocladial extensions was purposely placed so that they 

 could reach neither the side of the jar nor any other point of support. This did not interfere with 

 the asexual reproduction, however, as the processes became forked at their distal ends, and from 



1 The description under Stoloiiiferous Reproduction is from au article by the author in the American Naturalist for 

 Novernurr, 18115. The observations described were made at Plymouth, England. 



