224 



HYDROIDiE. 



Part IV. 



In the male medusoids the spermatic particles (PI. XX. Fiy. IG, 1-) are devel- 

 oped in a position which is homologous to the place where the eggs are developed 

 in the females. As the wall of the disk recedes from the proboscis, the increasing 

 space which lies between them is constantly kept filled by the growing mass of 

 spermatic material. In the eai-liest stages, this mass is transparent, so that the 

 medusoid, at first sight, appears to be empty {Fitj. 16, B), but gradually it becomes 

 granular, and the color changes to an orange tint, and finally, at maturity, to a 

 deep, dull orange, and withal very opaque. The fully-developed particles keep 

 up a constant and very lively agitation within the cavity of the medusoid, but 

 do not appear to move from place to place. They escape from the medusoid 

 through an aperture in the disk opposite the end of the proboscis. In shape, the 

 head of the spermatozoa is ovate {Fig. 16"), and, at its narrower end, a slender 

 tail, about a dozen times the length of the head, is attached. 



SECTION IV, 



RHIZOGETON FUSIFORMIS AG. 



The Adult Hi/dromediisarium. — Among the pools left between the rocks by the 

 receding tide on the promontory of Nahant, near Boston, red, velvet-like patches, 

 varying in size from a mere point to several inches in breadth, may be found 

 incrustinff the stones beneath the surface of the water. Without close examination 

 these may be mistaken for Hydractinia, which has an identical habitat, and can be 

 found even upon the same stone.' The whole length and breadth of the colony is 

 traversed by creeping tubes (PI. XX. Fig. 17, /), from which arise two difterent 

 kinds of individuals ; the ones, thick cylinders (B), tapering to a blunt point [m) 



' He who would make a successful search after 

 these delicate specimens, and discriminate carefully 

 between them, must not be over fastidious in his 

 examination of the puddles and tide-pools among 

 the rocks. He must go prepared to lie down, 

 sometimes to stand almost upon his head, to creep 

 up and down through wet and slimy crevices, and 

 over the surfaces of treacherous rocks, covered with 

 sea-weed. It will not do to remove these Hydroids 

 from their foundation, and transfer them to a bot- 

 tle, in order to ascertain their nature, inasmuch as 



they contract and disguise their shape, to such an 

 extent that one might bring home Hydractinia 

 when he wanted Rhizogeton, and vice versa, unless 

 he had patience to wait until the animals expanded 

 again. The only ready method of getting at these 

 sensitive creatures, without disturbing them, is to 

 observe them with lenses fixed in a long tube, 

 that may be plunged into the water. The sliding 

 tube of a common pocket telescope may be used, 

 if one does not wish to have a special apparatus 

 constructed. 



