REPORT ON THE HYDROIDA. 
63 
occurs on the British coast, where it is perhaps the most abundant of all the large Hydroids 
there found, while it is also one of the most widely distributed throughout the North 
Atlantic region. It is of rather robust habit, and is at once distinguished from every 
other species by its flask-shaped hydrothecse, with their attenuated diverging necks. 
Dipliasia, Agassiz, 
Sertularia , Linn, (in part). 
Diphasic, Agassiz, Nat. Hist. U.S., vol. iv. p. 355. 
Generic Character. Trophosome . — Colony dendritic ; hydrocaulus divided by equi- 
distant joints into internodes, each of which carries a pair of hydrothecse. Hydrothecse 
sessile, more or less adnate by their walls to the hydrocaulus, distichous, opposite or sub- 
opposite, with entire or emarginate rim, and with a lid-like operculum formed by a single 
valve. 
Gonosome. — Gonophores adelocodonic. Gonangia in female colonies crowned by 
a marsupial chamber enclosed within chitinous walls, gonangia in male destitute of 
marsupium. 
The genus Dipliasia was founded by Agassiz for the purpose of including species 
hitherto placed in Sertularia, but which differ from the true Sertularise in the gonangia 
of the female colony carrying on their summit a special chamber enclosed within lobe-like 
extensions of the chitinous walls of the proper gonangium. Into this chamber the ova are 
at an early stage expelled from the gonangium in order to undergo further development 
before the escape of the embryo into the surrounding water. No marsupium is present 
in the male. 1 
The hydrothecse are provided with a peculiar membranous lid, which springs by a 
hinge-like joint from one point of the rim, and forms when depressed a transverse septum 
just within the margin. 
Diphasia pinaster (Ellis and Solander) (PI. XXX. figs. 2, 2 a, 2b, 2c). 
Sertularia pinaster, Ellis and Solander, Zooph., p. 55, pL vi. 
Diphasia pinaster, Agassiz, Nat. Hist. United States, vol. iv. p. 355. 
„ „ Hincks, Brit. Hydroid Zooph., vol. i. p. 252, pi. 1. fig. 1. 
Trophosome . — Stem monosiphonic, unbranched, set with alternate pinnae. Hydro- 
thecse borne both by stem and pinnae, subopposite, cylindrical, adnate to the hydrocaulus 
for about two-thirds of their height, then diverging at a wide angle ; orifice oblique, 
crowned by a membranous valve-like lid. 
1 The “ acrocyst” which in certain species of Sertularia (e.g. Sertularia pumila) is formed on the summit of the 
female gonangium, differs from the marsupium of Dipharia in the fact of its being never included within an external 
chamber bounded by chitinous walls. 
