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158
[SIPHONOPHORA
HYDROMEDUSAE


3. Tentacles (“Fangfäden”), always present, and implanted one at the base of each siphon (fig. 68, f). The tentacles of siphonophores may reach a great length and have a complex structure. They may bear accessory filaments or tentilla (f′), covered thickly with batteries of nematocysts, to which these organisms owe their great powers of offence and defence.

4. Palpacles (“Tastfäden”), occurring together with palpons, one implanted at the base of each palpon (fig. 68, h). Each palpacle is a tactile filament, very extensile, without accessory filaments or nematocysts.

5. Bracts (“hydrophyllia”), occur in Calycophorida and some Physophorida as scale-like appendages protecting other parts (fig. 68, l). The mesogloea is greatly developed in them and they are often of very tough consistency. By Haeckel they are considered homologous with the umbrella of a medusa.

EB1911 Hydromedusae - structure of Velella.jpg

From G. H. Fowler, after A. Agassiz, Lankester’s Treatise on Zoology.

Fig. 70.—Diagram of the structure of Velella, showing the central and peripheral thirds of a half-section of the colony, the middle third being omitted. The ectoderm is indicated by close hatching, the endoderm by light hatching, the mesogloea by thick black lines, the horny skeleton of the pneumatophore and sail by dotting.

BL,  Blastostyle.
C, Centradenia.
D, Palpon.
EC, Edge of colony prolonged beyond the pneumatophore.
G, Cavity of the large central siphon.
M, Medusoid gonophores.
PN, Primary central chamber, and PN′, concentric chamber of the pneumatophore, showing an opening to the exterior and a “trachea.”
S, Sail.

6. Gonostyles, appendages which produce by budding medusae or gonophores, like the blastostyles of a hydroid colony. In their most primitive form they are seen in Velella as “gonosiphons,” which possess mouths like the ordinary sterile siphons and bud free medusae. In other forms they have no mouths. They may be branched, so-called “gonodendra,” and amongst them may occur special forms of palpons, “gonopalpons.” The gonostyles have been compared to the blastostyles of a hydroid colony, or to the manubrium of a medusa which produces free or sessile medusa-buds.

7. Gonophores, produced either on the gonostyles already mentioned or budded, as in hydrocorallines, from the coenosarc, i.e. the stem (fig. 68, i.). They show every transition between free medusae and sporosacs, as already described, for hydroid colonies. Thus in Velella free medusae are produced, which have been described as an independent genus of medusae, Chrysomitra. In other types the medusae may be set free in a mature condition as the so-called “genital swimming bells,” comparable to the Globiceps of Pennaria. The most usual condition, however, is that in which sessile medusoid gonophores or sporosacs are produced.

EB1911 Hydromedusae - Upper surface of Velella.jpg

From G. H. Fowler, after G. Cuvier, Lankester’s Treatise on Zoology. Fig. 71.—Upper surface of Velella, showing pneumatophore and sail.

The various types of appendages described in the foregoing may be arranged in groups termed cormidia. In forms with a compact coenosarc such as Velella, Physalia, &c., the separate cormidia cannot be sharply distinguished, and such a condition is described technically as one with “scattered” cormidia. In forms in which, on the other hand, the coenosarc forms an elongated, tubular axis or stem, the appendages are arranged as regularly recurrent cormidia along it, and the cormidia are then said to be “ordinate.” In such cases the oldest cormidia, that is to say, those furthest from the nectosome, may become detached (like the segments or proglottides of a tape-worm) and swim off, each such detached cormidium then becoming a small free cormus which, in many cases, has been given an independent generic name. A cormidium may contain a single nutritive siphon (“monogastric”) or several siphons (“polygastric”):

The following are some of the forms of cormidia that occur:—

1. The eudoxome (Calycophorida), consisting of a bract, siphon, tentacle and gonophore; when free it is known as Eudoxia.

2. The ersaeome (Calycophorida), made up of the same appendages as the preceding type but with the addition of a nectocalyx; when free termed Ersaea.

3. The rhodalome of some Rhodalidae, consisting of siphon, tentacle and one or more gonophores.

4. The athorome of Physophora, &c., consisting of siphon, tentacle, one or more palpons with palpacles, and one or more gonophores.

5. The crystallome of Anthemodes, &c., similar to the athorome but with the addition of a group of bracts.

EB1911 Hydromedusae - Diphyes campanulata.jpg

Fig. 72.—A, Diphyes campanulata; B, a group of appendages (cormidium) of the same Diphyes. (After C. Gegenbaur.)

a, Axis of the colony.
m Nectocalyx.
c, Sub-umbral cavity of nectocalyx.
v, Radial canals of nectocalyx.
o, Orifice of nectocalyx.
t, Bract.
n, Siphon.
g, Gonophore.
i, Tentacle.

Embryology of the Siphonophora.—The fertilized ovum gives rise to a parenchymula, with solid endoderm, which is set free as a free-swimming planula larva, in the manner already described (see Hydrozoa). The planula has its two extremities dissimilar (Bipolaria-larva). The subsequent development is slightly different according as the future cormus is headed by a pneumatophore (Physophorida, Cystophorida) or by a nectocalyx (Calycophorida).

(i.) Physophorida, for example Halistemma (C. Chun, Hydrozoa [1]). The planula becomes elongated and broader towards one pole, at which a pit or invagination of the ectoderm arises. Next the pit closes up to form a vesicle with a pore, and so gives rise to the pneumatophore. From the broader portion of the planula an outgrowth arises which becomes the first tentacle of the cormus. The endoderm of the planula now acquires a cavity, and at the narrower pole a mouth is formed, giving rise to the primary siphon. Thus from the original planula three appendages are, as it were, budded off, while the planula itself mostly gives rise to coenosarc, just as in some hydroids the planula is converted chiefly into hydrorhiza.

(ii.) Calycophorida, for example, Muggiaea. The planula develops, on the whole, in a similar manner, but the ectodermal invagination arises, not at the pole of the planula, but on the side of its broader portion, and gives rise, not to a pneumatophore, but to a nectocalyx, the primary swimming bell or protocodon (“Fallschirm”) which is later thrown off and replaced by secondary swimming bells, metacodons, budded from the coenosarc.