This order comprises those Hydrozoa "whose hydro soma is fixed by a hydrorhiza, and consists o several polypites, protected by hydrothecae, and connected by a coinosarc, which is usually branched and invested by a very firm outer layer. Reproductive organs in the form of gonophores arising from the coenosarc or from gonoblastidia" (Greene).
The Sertularida resemble the Corynida in becoming permanently fixed after their embryonic condition by a hydrorhiza, which is developed from the proximal end of the coenosarc; but they differ in the fact that the polypites are invariably protected by "hydrothecae," or little cup-like expansions of the polypary (fig. 46, a, b); whilst the hydrosoma is in all cases composed of more than a single polypite. The mouth of the hydrotheca is generally furnished with an operculum or valve for its closure. Owing to the presence of "hydrothecae," the name of "Calyptoblastic Hydroids" has been proposed by Professor Allman for the Sertularians and Campanularians. In all these forms, also, the generative buds are similarly enclosed in chitinous receptacles - the so-called "gonothecae" or "gonangia." The coenosarc generally consists of a main stem - or "hydrocaulus" - with many branches; and it is so plantlike in appearance that the common Sertularians are almost always mistaken for sea-weeds by visitors at the seaside. It is invested by a strong corneous or chitinous covering, often termed the "periderm."
Fig. 46. - a Sertularia (Diphasia) pinnata, natural size ; a' Fragment of the same enlarged, carrying a male capsule (p), and showing the hydrothecae (h); b Fragment of Campanularia neglecta (after Hincks), showing the polypites contained in their hydrothecae (h), and also the point at which the coenosarc communicates with the stomach of the polypite (c).
The polypites are sessile or sub-sessile, hydra-form, and in all essential respects identical with those of the Corynida, though usually smaller. Each polypite consists of a soft, contractile and extensile body, which is furnished at its distal extremity with a mouth and a circlet of prehensile tentacles, richly furnished with thread-cells. The tentacles have an indistinctly alternate arrangement. The mouth is simple or lobed, and is placed, in many cases, at the extremity of a more or less prominent extensile and contractile proboscis. The mouth opens into a chamber which occupies the whole length of the polypite, and is to be regarded as the combined body-cavity and digestive sac. At its lower end this chamber opens by a constricted aperture into a tubular cavity which is everywhere excavated in the substance of the coenosarc (fig. 46, b). The nutrient particles obtained by each polypite thus serve for the support of the whole colony, and are distributed throughout the entire organism. The nutritive fluid prepared in the interior of each polypite gains access through the above-mentioned aperture to the cavity of the coenosarc, which by the combined exertions of the whole assemblage of polypites thus becomes filled with a granular nutritive liquid. The coenosar-cal fluid is in constant movement, circulating through all parts of the colony, and thus maintaining its vitality, the cause of the movement being probably due in part, at any rate, to the existence of vibrating cilia. The generative buds (gonophores or ovarian vesicles) are usually supported upon gonoblastidia, and do not become detached in the true Sertularids. They are developed in chitinous receptacles known as "gonothecae" (figs. 47, 48).
Fig. 47. - Diagrams of the gonothecae, with their contents, of the Sertularians and Campanularians. n Chitinous envelope ; g Central gonoblastidion or blastostyle; e Medusiform gonophores carried upon the blastostyle, each with a central manubrium, in the walls of which the generative elements are produced ; s Sporosacs carried upon the blastostyle, each with a central pillar (spadix), round which the ova are developed. (Altered from Allman.)
Fig. 48. - Ovarian capsule of Diphasia (Ser-tularid) operculata, Linn. (after Hincks). Greatly enlarged.
Sometimes the "gonangium" or "gonotheca" contains only a single gonophore, but more commonly it contains several, which increase in maturity as we recede from the base of the gonoblastidion (or blastostyle) and approach its summit (fig. 47, B). The buds carried on the sides of the blastostyle may have the form either of sporosacs or of medusoids. The ova may be directly discharged into the surrounding water, or may be retained for some time in a peculiar receptacle, "where they undergo further development, and which is supported upon the summit of the gonangium, and lies entirely external to its cavity" (Allman).
In Plumiilaria and some of its allies there occur certain peculiar structures, to which the name of "nema-tophores" has been applied. Each of these consists of a process of the coenosarc, which is invested by the horny polypary, with the exception of the distal extremity, which remains open. The nematophores are sometimes fixed, sometimes movable. They "constitute cup - like appendages (fig. 49, n n) formed of chitine, and filled with protoplasm, which has the power of emiting pseudopodia or amoeboid prolongations of its substance, and having their cavity in communication with that of the common tube of the hydro-caulus" (Allman). Whilst part of the sarcode in each nematophore is capable of being extended in long filaments resembling the pseudopodia of an Amoeba, another portion is charged with large thread-cells, and is not capable of emission in this way. The function of these extraordinarily modified zooids is uncertain.
Fig. 49. - Portion of a branch of Antennularia antennina, enlarged. (After Allman.) p One of the polypites ; n n n Nematophores emitting pseudopodial filaments of sarcode; n' Nematophore with its sarcodic contents quiescent; c Coenosarc enclosed within the polypary.