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The Siphonophora comprise an order in the class Hydrozoa, subclass Hydroidolina, as shown in the classification scheme. They are quite distinct, differing from their hydrozoan brethren and the remainder of the pelagic Cnidaria in general morphology and basic organization. Three major biological groups are widely recognized within the Siphonophora, but there are disagreements about their relative taxonomic rank within the order. In this case we are following the World Register of Marine Species (WoRMS) and many decades of history in placing them each in their own suborder. The three suborders are the Cystonectae the Physonectae, and the Calycophorae (Figure 3.25). Altogether, the three suborders contain about 190 species, with the lion’s share of them, about 60%, in the suborder Calycophorae. Suborders within the siphonophores are discriminated initially by the presence or absence of a gas‐filled float or pneumophore. The suborders Cystonectae and Physonectae (Figure 3.25a–d) both have a gas‐filled float, whereas the calycophorans do not (Figure 3.25e and f). The second division for classification is in the presence of swimming bells beneath the float. The physonects have swimming bells, or nectophores, beneath the float; cystonects do not. Table 3.5 is a synopsis of siphonophore classification. The siphonophores are more diverse at the family level than the rest of the Cnidaria.

Figure 3.24 Basic statocyst structure showing the calcium carbonate statolith resting on sensitive sensory cells, which respond to changes in position of the statolith.

Source: Tschachotin (1908), text figure 5 (p. 358).

Siphonophores are possibly the most confusing group in the animal kingdom. A free‐floating individual siphonophore is considered to be a colony of various individuals working together to feed, reproduce, and move about, and within each siphonophore colony are individuals with either medusoid or polypoid affinities. Once the concept of a floating colony of individuals working together as a single entity is mastered, a lexicon of terminology replete with historical changes needs to be assimilated before life‐history questions can be resolved. How does a colony develop from a single propagule? How do the various bits and pieces work together? Fortunately, some of the great minds in the history of biology have been fascinated by the group: Ernst Haeckel for example. And two very cogent reviews of the group by some of the best talent in gelatinous zooplankton biology (Mackie et al. 1987; Pugh 1999) are invaluable resources.