NOTES ON PLYMOUTH HYDROIDS. 151 



around the stem, branches, hydrotheca, and gonangia, in a perfect 

 maze of threads, or even flattened lobate masses. 



2. In retraction, the movement is not comparable to the flowing of 

 pseudopodia, but is effected by decided, quick, jerky retraction, giving 

 an idea of definite outlines and cohesion. To use a crude comparison, 

 the sarcostyle contracts much as if it was made of stretched india- 

 rubber and not of a fluid. It it also worthy of note that there is no 

 mechanical entanglement of the various extensions of the sarcostyles, 

 although they appear to be hopelessly intertwined. 



3. The sarcostyles are particularly active in the vicinity of mutilated 

 or dead hydranths and gonophores, particularly the latter, and seem to 

 have a definite object in climbing over the sides and into the interior 

 of hydrothecae and gonangia. There is no evidence that they are able 

 to repair damaged parts. 



4. An examination of living sarcostyles, under a high magnification, 

 disclosed certain cells on the distal surface which had the characteristic 

 outlines and movements of amoeboid cells, and contained foreign 

 particles. 



It would seem from the foregoing observations that the sarcostyles 

 of P. pinnata are primarily neither fighting persons, nor persons con- 

 cerned in the repair of mutilated or diseased parts. It is probable, 

 on the other hand, that they do remove extraneous matter, or dead 

 organic material from the interior of the hydrothec?e* and gonangia, 

 and that they may aid in the capture and ingestion of food for the 

 colony. 



Origin of the sex-cells. This species is an excellent one for the de- 

 monstration of the coenosarcal origin of the sex-cells in the Plumularidcc 

 as first announced by Weismann.f The gonangia are so excessively 

 numerous that a single series of sections may often be obtained which 

 will show nearly all stages of this interesting process. The course of 

 events in P. pinnata agrees very closely with Weismann's description 

 of the origin of the sex-cells in P. echinulata, both ova and spermato- 

 blasts, arising in the endoderm of the stem and afterwards migrating 

 into the gonophores, ultimately appearing as ova, or sperm, masses in 

 the matured structures. The ova break through the " stutzlamella " 

 and are fertilized and undergo segmentation between the stutzlamella 

 and the ectoderm. Although the ultimate division of the spermato- 

 blasts may take place in the ectoderm, the primary divisions occur in 

 the endoderm. I have been unable to find any cells recognisable 

 as spermatoblasts in the ectoderm, although very satisfactory serial 



* E. Metsohnikoff, Quart. Jour. Micr. Sci. no. 93. 



t See Die Enstehung der Sexuahellcn bei den Hydromeduseu , by Dr. August Weismann. 

 The first announcement was in Zool. Anzeiy. no. 75, 1880. 



NEW SERIES. — VOL. IV. NO. 2. M 



