Chap. III. HYDRACTINIA POLYCLINA. 229 



so as to form a network. Here and there this network rises, bearing with it 

 the overlj-ing soft layers, as if pushed up from beneath, into more or less elevated 

 jiillars, of a cylindrical or conical shape (PI. XVI. Figs. 1, *•, 2, s, and 6), thus 

 adding another diversity to the j^olymorphism of the colony. In some instances, 

 where the colony is situated on Buccinum undatum, the spines are arranged in 

 rows along the spiral ridges of the shell, with such perfect regularity' that one might 

 at first sight suppose he had a different species before him. There is considerable 

 variation in regai'd to the proportionate size of the three different forms of a 

 colony. In some colonies the reproductive hydroids (PI. XYI. Fif/. 1, A B) are 

 nearly as tall as the sterile forms (D), and in others they are hardly one third 

 as high (PI. XVI. Fig. 2, B), but yet they bud as plentifully as the largest ones. 



The hydroids, of all forms, are as closely crowded together as are the indi- 

 viduals of a colony of Bryozoa, among the Flusti-as and Lepralias, but not with 

 any such regularity. We have observed an instance where a colony of Hydrac- 

 tinia had settled upon the calcareous habitation of a dead Flustra, and nearly every 

 Hydroid had chosen a cell of the Bryozoan for a basement, into which it withdrew 

 itself almost entirely when touched. This adaptation of our Hydroid to the nature 

 of its habitat reminds one of a similar phenomenon which occurs among Oysters, and 

 in the genus Crepidula among Gasteropods. Fossil Oysters, for instance, attached 

 to Ammonites, frequently assume the form of the ornaments of the latter, along 

 their growing edge ; the Crepidula of our shores, when growing upon Pectens, 

 becomes plicated ; when growing upon Natica or Pyrula, it is smooth ; and those 

 which are attached to the outside of these shells are convex, while those growing 

 upon the inside of empty shells become concave. These different forms have been 

 described as distinct species. 



The fertile Hgdroid. — In general outline the fertile hydroid (PI. XVI. Fig^. 2" 

 and 2'') may be compared to a club, gradually thpering from a broad, more or 

 less globular head, to a slender base. In a contracted state {Fig. 1^) the stem 

 swells in the middle, so that, on the whole, it resembles a figure 8- When the 

 stem is loaded with medusoids, it is almost invariably thicker at the point of 

 attachment of these buds than elsewhere {Figs. 1, 2, and 3). This is not caused 

 by a thickening of the walls, but by the expansion of the digestive cavity {Fig. 

 3, d). The head is as changeable in shape as that of the sterile form. In a 

 natural state, or rather in that state in which it is seen most frequently, it is glob- 

 ular {Figs. 1, B C, //, 1*-', 2, C K, 3, 4", and 4"^, h), the whole spherical mass seeming 

 to be composed of the conglomeration of the tentacles ; but that this is not so, may 

 be seen when, as frequently happens, the tentacles are spread apart at the extreme 

 tip of the head, and a broad, thick proboscis {Figs. 2, A p, 2", m, 2, *■, m, and 2% p) 

 is j)rotruded for a considerable distance. There can be no doubt that this is a 



