Siphonophorae
Morphology

As siphonophores are comprised of many types of structures ("individuals"), each with a different function and some found only in these animals, a unique terminology for these structures has been developed. This can appear somewhat daunting to the non-specialist and necessitates a detailed introduction to it as it is critical to the identification of the species. In the following section the important terms and features are emboldened.

Siphonophores are cnidarians, and so basically consist of two cell layers, the outer ectoderm and the inner endoderm, with the latter forming the lining to the gastro-vascular cavity. These two layers are separated by a more or less thickened, amorphous mesogloea.

As noted above, the species in the 3 basic divisions (orders) are primarily distinguished on the basis of the presence or absence of either an apical, gas-filled float, the pneumatophore, or of swimming bells, nectophores. The latter, numbering from one to fifty or more, usually are grouped together at or toward the apex of the animal to form the nectosome. The pneumatophore (Physonect siphonophore) is produced, during development, by an invagination at the aboral end of the larva, which results in its cavity being lined by ectodermal cells. At the base of the cavity, there is a specialised area of cells, the gas-gland or pneumadenia, that secretes the gas to inflate the cavity. In some cystonect siphonophores this gland may bear processes, hypocystic villi, while in the benthic rhodaliids it is greatly expanded to form an aurophore.

Most siphonophores possess a long stem to which are attached the various "individuals". The latter usually are organised into two zones, the nectosome and siphosome, although only the latter is present in the Cystonectae.

—Nectosome
The nectosome, which lies immediately below the pneumatophore (Physonectae) (Physonect siphonophore) or is apical (Calycophorae) (Calycophoran siphonophore, Calycophoran siphonophore 2, Calycophoran siphonophore 3, Calycophoran siphonophore 4,), bears, with the exception of one family, only asexual, medusoid nectophores. These act in a co-ordinated way to propel the animal through the water. Within the Calycophorae, they may also contain large amounts of mesogloea, which contributes to the buoyancy of the animal. New or replacement nectophores are developed at the apex of the nectosome.

The morphology of the nectophores is important for specific identification. The nectophores of physonect siphonophores generally conform to a basic design (Physonect siphonophore 4, Physonect siphonophore 5). They do not possess a somatocyst (see below) and, with a practised eye, easily can be distinguished from calycophoran nectophores. The important taxonomic features of the physonect nectophores are:
(a) their general shape;
(b) whether the apico-lateral ridges have a notch or divide toward the abaxial (furthest from the stem) end of the nectophore;
(c) the course of the 4 (dorsal, ventral and 2 laterals) radial canals on the nectosac. The nectosac is equivalent to the sub-umbrella cavity of a medusa and, by means of contractions of its muscular walls, water is expelled through the narrowed, basal opening, or ostium, so that the animal is propelled by jet propulsion;
(d) the shape of the apico-lateral processes, or axial wings, which extend around the stem;
(e) the structure of the thrust block, which abuts the stem and through which passes the gastro-vascular canal; and
(f) the presence or absence of ridges on the sides of the nectophore. In this paper two types of these ridges will be referred to, in continuation of the scheme adopted by Pugh and Youngbluth (1988). Firstly, there are vertical lateral ridges, which are closer to the axis than any others, and run from the apico-laterals to or toward the infra-laterals. The course of this pair of ridges is not always vertical, and in two species, at least, more than one pair is present. Secondly, lateral ridges that usually run from the apico-laterals to the ostium. There are specific variations in their arrangement.

The nectophores of calycophoran siphonophores generally are more specialised, but there is a great deal of variability amongst the various families and it is difficult to give an overall picture. Six different types are illustrated. The first type (Calycophoran siphonophore) is basically a smooth, rounded structure containing large volumes of mesogloea, giving buoyancy to the animal. The extent of the nectosac can vary considerably between species. This type of nectophore is found in species of the Family Prayidae and Family Sphaeronectidae. In the latter family only a single nectophore, the larval one, is developed and is retained by the adult animal. In the former the larval nectophore may be retained in the adult, and a second, greatly reduced nectophore developed in addition; or the larval nectophore can be autotomised and replaced by one or, more usually, two adult or definitive nectophores. Further definitive nectophores may be developed in some species. When two definitive nectophores are present, they are identical in design and form an apposed pair.

The second type of nectophore (Calycophoran siphonophore 2), found in the Family Hippopodiidae, is flattened in the axial-abaxial direction and usually bears spines or protuberances. The nectosac is widely open, but shallow, giving the impression that the animals must be weak swimmers. On the ventral radial canal of the nectosac there is a rete mirabile, a dilation of the ventral canal, which is particularly obvious in younger nectophores. Up to fifteen nectophores may be developed, which have a tight, biserial arrangement.

In the remaining calycophoran families the nectophores are more streamlined, and the animals themselves usually are active and rapid swimmers. Generally a larval nectophore is developed, which is then replaced by one or, more commonly, two definitive nectophores. However, it is believed that the larval one may be retained in certain species. When two nectophores are present, they differ in structure and do not form an apposed pair, rather one is wholly (Family Diphyidae and Family Abylidae, Calycophoran siphonophore 3, Calycophoran siphonophore 4, Calycophoran siphonophore 6) or partially (Family Clausophyidae, Calycophoran siphonophore 5) superimposed above the other. The upper, anterior nectophore, often has a pointed apex, while the lower, posterior nectophore often is truncated apically so as to fit snugly with the other. These nectophores frequently bear patterns of external ridges and teeth, which are useful diagnostic features. Most of the ridges run longitudinally, from the apex to the base of the nectophore, but are not necessarily complete. In some species a transverse velar ridge, lying close to the ostium, is present. In addition, many species have a basal lamella or mouth plate that extends below the ostium, the opening of the nectosac, on its ventral side. The angle at which the mouth plate is joined to the ventral surface of the nectophore, where it forms the basal facet, also can be of taxonomic importance.

The structure of the hydroecium is only of taxonomic importance in calycophoran siphonophores (Calycophoran siphonophore, Calycophoran siphonophore 3). In species with stream-lined nectophores, the hydroecium of the anterior nectophore may be greatly reduced or absent. However, in the posterior nectophore it usually extends the full length of the nectophore and the siphosomal stem may be partially or wholly retracted into it for protection and to reduce drag during locomotion. In the genus Sulculeolaria the nectosac of the anterior nectophore may have additional commissural canals connecting the laterals to the dorsal radial canal (Calycophoran siphonophore 3).

In calycophoran siphonophores a somatocyst lies above the apical wall of the hydroecium. This may be a relatively simple tube, as in certain species of the family Prayidae, forming part of the gastro-vascular canal system, and connecting with the radial canals on the nectosac via a pedicular canal (Calycophoran siphonophore). It may have basal (descending) or apical (ascending) extensions, or branches. In the other calycophoran families, the somatocyst tends to form a caecal extension to the gastro-vascular canal, of taxonomically variable shape and design (Calycophoran siphonophore 3). In those species with nectophores designated anterior and posterior the somatocyst is present only in the anterior one, except for the family Clausophyidae where it is present in both. The morphological features of importance for the taxonomy of calycophoran nectophores are:
(a) their basic shape and design;
(b) the presence and pattern of ridges; and
(c) the structure of the somatocyst.

—Siphosome
The siphosome generally is much longer than the nectosome, and in some physonect species can extend to several tens of metres. Its zone of proliferation lies immediately below the nectosome. The siphosome bears a succession of different polypoid and medusoid structures, with different functions, arranged into a succession of groups or cormidia (Calycophoran siphonophore). Each cormidium consists of a single gastrozooid, the digestive organ or stomach, to the base of which is attached a tentacle. The tentacle bears numerous nematocysts, usually on side branches called tentilla (Physonect siphonophore 3), that are discharged on contact with a prey item and either paralyse or ensnare it. Many large nematocysts are often arranged into a cnidoband that may be covered by an involucrum. Frequently a terminal filament is present. The structure of the tentillum can be useful in identifying some species, particularly physonect ones.

The cormidia of physonects usually include a number of palpons, which are reduced gastrozooids and bear a small tentacle or palpacle (Physonect siphonophore 2). The palpacle probably has a sensory function, while the palpon aids in digestion, and accumulates and disposes of waste products. Palpons usually are attached directly to the stem, but a specialised type, the gonopalpon, may be budded from a gonodendron. The latter itself may be a specialised palpon (Totton, 1965). The gonodendron bears the developing sexual medusoids, the gonophores, and in some species asexual nectophores.

Another cormidial structure is the bract (Physonect siphonophore, Calycophoran siphonophore), whose medusoid or polypoid origin is uncertain. These structures have a protective function, overlying the other cormidial elements. Often they contain large amounts of mesogloea and play an important role in flotation. They are absent in cystonects, while in physonects each cormidium usually contains several bracts, which are leaf-like with a simple bracteal canal. In calycophores each cormidium has a single bract, except in the family Hippopodiidae where it is absent. The bract may have a complex structure, with a divided canal system or a phyllocyst (equivalent to the somatocyst of the nectophore), whose arrangement is of taxonomic importance. The bracts of some species are rounded, while others are angular, conical or helmet-shaped. The latter types usually possess a basal process called the neck shield. Those calycophores with stream-lined nectophores usually release individual cormidia, which are then known as eudoxids. Each eudoxid, thereby, comprises a bract, a gastrozooid and tentacle, a succession of sexual gonophores and, possibly, an asexual nectophore. The specific identification of this stage often is difficult, and depends largely on the structure of the bract.