With Fig. n, illustrating the structure of a Craspedote Medusa.
THE Sea-fir belongs to the class Hydrozoa and order Hydroidea. It forms a fixed colony or hydrosoma, which is protected in all its parts by a yellowish chitinous perisarc. The hydrosome consists of a number of hydranths or nutritive zooids collectively forming the trophosome and connected to one another by a branching coenosarc. The hydranths are lodged in perisarcal cups ox hydrothecae ( = calycles) and are borne by a supporting plant-like structure or hydrophyton. This hydrophyton consists of the coenosarc above-mentioned and its perisarcal investment. It is divisible into a system of stems with branches, the hydrocaulus, and of rooting fibres, the hydrorhiza, by which the colony is attached to some foreign object. Numerous rooting fibres are to be seen at the base of this specimen. The stems, with their branches, rise vertically from the root: they are somewhat flattened and their two opposite edges are beset by the thecae for the hydranths. The thecae on one edge of a branch or stem alternate more or less accurately with the thecae on the other edge. The hydranths can be completely retracted into their thecae and may be seen in this specimen as opaque spots. The branching coenosarc is tubular.
The walls of the tube are composed of a delicate supporting lamina separating a layer of ectoderm and endoderm cells, the latter being ciliated. Consequently the products of digestion, a process which goes on only in the hydranths, are conveyed along the coenosarcal tube through branches, stems, and roots, in brief, through the whole hydrophyton. The ectoderm of the coenosarc is separated from the perisarc by a space, but the ectoderm of each hydranth remains in contact with its chitinous theca at the base of the cup. Wherever growth is taking place, i.e. at the tip of a root, or in a developing hydranth, the perisarc is in close contact with the ectoderm by which it is formed.
There may be noted in this specimen, towards the tip of the branches and upon the side turned uppermost, certain pear-shaped bodies, the gono-thecae, gonangia, or capsules. Each gonangium contains a modified hydranth, the blastostyle, which bears the generative zooids. These zooids are never set free in the family Sertularidae, and have, as in the family Plu-maridae, with few exceptions, the structure of a medusoid gonophore. The fertilised ovum in Serhdaria abietina and some of its allies passes into a small cyst, the acrocyst, formed at the apex of the gonangium. Here it undergoes development and is set free as a ciliated Planula which, after leading a wandering life, settles down, and by growth and budding establishes a new colony.
The hydranth resembles Hydra in all essentials (see Plate xiv. Fig. 6, and description). Like that organism it consists of a hydrocephalis ( = oral and stomachal regions) and a peduncle or hydrocope which is very short. The hypostome or oral cone is conical, the tentacles filiform and arranged in a single circle. They are solid and their axis is composed of a single row of vacuolated endoderm cells. The blastostyle is borne on a short peduncle or gonocope.
Representatives of two other Phyla may be seen to have affixed themselves to the main stems of this specimen. One of them is a Spirorbis, a small Tubicolar Annelid with a discoidal shell somewhat like that of the freshwater mollusc Planorbis: the other is a Cyclostomatous Polyzoan, Diastopora Patina, which with its aggregated calcareous cells presents an appearance not unlike that of a small tubiflorous flower belonging to a plant of the order Compositae. Coefenterata with an external perisarc and a single circlet of tentacles bear a superficial resemblance to many Polyzoa, a group formerly classed with them. A Hydroid may however be readily distinguished at once from a plant-like Polyzoan by the presence of thread-cells on the tentacles as well as by other anatomical features. Compare Fig. 10, p. 235, ante, and description.
The order Hydroidea is divisible into the sub-groups (1) Hydrocorallina, colonial organisms with an external calcareous skeleton; (2) Tubulariae or Gymnoblastea, with fixed plant-like colonies and all parts save the hydranths protected by a perisarc; (3) Campanulariae or Calyptoblastea, with fixed plant-like colonies the hydranths lodged in chitinous thecae formed by the perisarc; and (4) Trachy-medusae, a group of Medusae which have no fixed colonial forms, but develope as Medusae from the egg. The Sea-fir belongs to the Campanulariae.
In addition to the possession of hydrothecae, a Campanularian differs from a Tubularian in three important respects: the terminal hydranth of a stem or branch is the youngest and not the oldest in the stem or branch; the generative zooid is with few exceptions borne on a blastostyle; and when that zooid is a Medusa it is furnished, as a rule, with auditory sacs, and not with eye-specks. Hence the Medusae of Campanularians are classed as Vesiculatae, those of Tubularians as Ocellatae.
The Medusa-form in the order Hydroidea has the following general characters: -
Fig. .11, illustrating diagrammatically the structure of a Craspedote Medusa.
A, a radial, B, an inter-radial section. The ectoderm is indicated by a broken line. The dashes are thicker where it consists of columnar cells (over n in B), or where it developes muscular fibres. The endoderm is indicated by a thick dark line.
The shape is that of a bell, from the concavity of which depends a hollow tubular process, the manubrium (M.). At the free end of the manubrium is the mouth (pi.); at its base a cavity more or less dilated, the atrium (a.). From this atrium radial canals (A: r.c), varying in number, pass outwards in the substance of the bell, and are united by a circumferential canal (A: c.c), which runs circularly round its apparent edge. The true edge or rim of the bell is turned horizontally inwards, and forms the velum (v.). The whole system of cavities is lined throughout by a ciliated endoderm, represented by a thick dark line, the cells of which vary in size and character in different regions. This endoderm is everywhere separated from the ectoderm, represented as a broken line, by a supporting lamina.