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All zooids of the siphonophore colony are budded from and contiguous with a central stem (Figure 3.30). The stem is a robust tube that varies from a couple of centimeters to several meters in length depending on the species and is capable of expanding and contracting. It has a typical cnidarian two‐layered structure with a thick mesoglea and a central canal (Figure 3.30a). Canals of the siphonophore persons are all continuous with the central canal of the stem. The stem canal originates at the apex of the colony, either as a somatocyst (Figure 3.30b), a rounded end in the calycophorae, or as a continuation of the gastrovascular canal of the float (Figure 3.30c). Note the continuity of the stem canal with that of the persons in Figure 3.30c. In some species such as Physalia, the Portuguese man‐o‐war, the “stem” is really more of a disk that is contiguous with the wall of the float and the zooids are budded from beneath it (Figure 3.30d).

The stem is divided into two sections, the nectosome and siphosome (formerly siphonosome). In physonect siphonophores (Figure 3.25c and d), the nectosome extends from the base of the float to the bottom of the swimming bells or nectophores. In the calycophorans, lacking a float, the nectosome is apical (Figure 3.25e and f). The cystonects, which lack swimming bells altogether (Figure 3.25a and b), have no nectosome at all.

Figure 3.30 Siphonophore colony structure. (a) Cross section of a siphonophore stem; (b) helmet‐shaped bract. (c) Colony of the physoncect Agalma. (d) Colony of the cystonect Physalia.

Sources: (a) Hyman (1940), figure 151 (p. 476); (b) Hyman (1940), figure 148 (p. 470); (c) Redrawn from Mayer (1900), plate 32; (d) Adapted from Brusca and Brusca (1990), figure 9 (p. 224).

The remaining portion of the siphonophore stem is termed the siphosome. It makes up the majority of the stem in most species of physonects and calycophorans and the entire stem in the cystonects. Nectophores are budded from a zone of proliferation just below the float in the physonects. Similarly, the siphosome proliferates from a budding region just below the nectosome, producing repetitive groups of medusoid and polypoid persons called cormidia (Figure 3.31). Within the Calycophorae, a cormidium often consists of a single gastrozooid, to the base of which is attached a tentacle along with one to several gonophores (Pugh 1999). Overlying the cormidium is a bract that has a characteristic shape and is often useful in identifying species (Figure 3.31c). Cormidia within the physonects are larger and more complex, usually possessing several leaf‐like bracts, a gastrozooid, dactylozooids, and clusters of gonophores. The cystonects have simple cormidia consisting of a gastrozooid and clusters of gonophores radiating from a gonodendron or stalk. The cystonects have no bracts.

In the calycophorans, the terminal mature cormidia on the stem break away to form free‐swimming “eudoxids.” Until their true origins were resolved, eudoxids were thought to be a distinct group within the siphonophores (Hyman 1940). Limited propulsion within the eudoxids is achieved by the gastrozooids pinch‐hitting as nectophores. Shedding of cormidia for a brief free‐swimming existence is limited to the Calycophorae; neither the physonects nor the cystonects produce eudoxids.

Figure 3.31 Siphonophore cormidia. (a) Calycophoran Nectocarmen antonioi. (b) Diphyid Muggiaea sp. (c) Enlarged cormidium of Muggiaea.

Sources: (a) Adapted from Alvarino (1983), figure 1 (p. 342); (b) Kaestner (1967), figure 4‐34 (p. 74); (c) Adapted from Hyman (1940), figure 150 (p. 174).