Sub-class II. Siphonophora

The members of this subclass constitute the so-called "Oceanic Hydrozoa;" and are characterised by the possession of a "free and oceanic hydro-soma, consisting of several polypites united by a flexible, contractile, unbranched or slightly-branched coenosarc, the proximal end of which is usually furnished with 'nectocalyces,' and is dilated into a 'somatocyst' or into a 'pneumatophore'" (Greene).

All the Siphonophora are unattached, and permanently free, and all are composite. They are singularly delicate organisms, mostly found at the surface of tropical seas, the Portuguese man-of-war (Physalia) being the most familiar member of the group. The sub class is divided into two orders - viz., the Catycophoridae and the Physophoridae.