50 
THE VOYAGE OE H.M.S. CHALLENGER. 
flask-shaped, or barrel-shaped, or cylindrical, with the margin entire or variously cleft 
or dentate, and with the sides to a greater or less extent adnate to the hydrocaulus, 
orifice with or without an operculum. Hydranth with conical hypostome. 
Gonosome. — Gonophores adelocodonic, gonangia springing from one side of the 
hydrocaulus, each from a point near the base of a hydrotheca, and without a chitinous 
marsupium in either sex. 
The old Linnsean genus Sertularia included, as may well be expected, many forms 
which subsequent systematists have placed in distinct generic groups. Even after the 
Sertularia of Linnaeus had undergone the revision to which it had been subjected by 
Lamouroux and by Lamarck, there still remained species which later authors believed 
themselves justified in removing from it. 
J. E. Gray broke up the genus Sertularia as left by Lamarck into two genera, 
Sertularia and Sertularella, the former including such species as had their hydrothecae 
opposite, and the latter such as had them alternate. The insufficiency of the grounds 
on which this dismemberment was based soon became apparent, and Hincks, while 
accepting Gray’s genus Sertularella, attempted to give it a greater systematic value 
by connecting with it characters of more importance than those derived merely from the 
alternate disposition of the hydrothecae. The characters especially insisted on by Hincks 
as affording legitimate grounds for the dismemberment proposed by Gray are found in 
the condition of the margin of hydrotheca, which in Sertularella carries three or four 
denticles or cusps, and gives support to an operculum formed of three or four triangular 
membranous valves ; while in Sertularia proper the margin is either even or with 
a simple cleft, and according to Hincks is destitute of an operculum. 
Since the time when Hincks published his history of British Hydroid Zoophytes, 
many species from various parts of the world have come under examination, and show 
that the distinctions relied on in that valuable work have by no means the systematic 
importance which had been attributed to them. Some species which by their even 
or merely cleft hydrothecal margin would come under Sertularia proper have their 
hydrothecse as truly alternate as in the most typical representatives of Sertularella, 
and even the characters derived from the presence of an operculum in Sertularella, and 
its supposed absence in Sertularia, are found to be by no means of universal application. 
Some species whose hydrothecse in no essential point differ from those of the typical 
Sertularia jpumila, either in the condition of the rim or in their absolutely opposite 
disposition (e.g., Sertularia distans and others, see Hydroids of the Gulf Stream), are 
provided with opercular membranous valves, and yet few systematists would think of 
separating these generically from the closely allied species in which no valves are 
present. 
What has been called the operculum consists in most cases of three or of four very 
thin membranous triangular valves, composed, like the general perisarc, of chitin, and 
