Oval or bell-shaped capsules - Dawsonia - are often found mingled with the Graptolites, and have been supposed to be detached ovarial capsules (Nicholson). In the Retioloidea, the second section of Graptolithidae, there is no sicula; the outer layer of the perisarc is thin, the two inner reduced to a network; there are two opposite rows of thecae, but the coenosarcal tube is common to both; the two virgulae have either coalesced, or are opposed to each other, but in both instances situate in the perisarc. The colony is free in both sections.
There is no indication as to what the animal of the Graptolite was like. All-man has suggested that it resembled a Plumularian machopolype, inasmuch as the thecae, like those of machopolypes, are not constricted at their bases as are those of hydranths. This argument is not necessarily conclusive, for the thecae of Cuspi-della (Hincks, op. cit. p. 209), or of Trichydra (Id. p. 216, Fig. 26) are simple tubes. The virgula is a structure unknown in any living Hydrozoan. The second stem, found by Jickeli in Anisicola (Plumularid) Halecioides, which bears no hydranths itself, but supports the main stem to which it is connected at intervals, is hardly comparable to it; see M. J. viii. p. 636. Another difficulty is the fact that the virgula extends far beyond the theca-bearing region of the coenosarc. Jickeli suggests (loc. cit. pp. 668-71) that the Graptolites are possibly Octactinians. There are no known Antipatharians with a chitinoid perisarc covering the outside of the organism; but, supposing there were, the virgula is not altogether unlike the chitinoid tubular skeleton of Antipathes; cf. p. 737. On the other hand no known Anthozoa possess special enlarged generative capsules; nor do the Polyzoa, a group to which the Graptolites have been sometimes assigned.
1 Cf. note, p. 759, ante, and Haeckel, 'System,' pp. 101, 105.
The Siphonophora, the third order of Craspedota, are pelagic and colonial. The various parts which may enter into the composition of a colony1 are as follows, (I) The polypite or gastrozooid, universally present, usually attached to the coenosarc of the colony by a longer or shorter pedicle, and consisting of three regions, a basal with thickened ciliated ectoderm containing cnidoblasts, a central somewhat dilated, and a terminal oral ciliated portion, extremely changeable in shape. Cnidoblasts are to be found round the mouth. The endoderm cells are vacuolate, and those of the middle region are pigmented, produced into longitudinal ridges2 or villiform processes. (2) Hydrocysts or feelers ( = Taster of German writers), absent in Calycophoridae and Discoideae. These structures are polypites in which the distal or oral extremity is imperforate and usually armed with cnidoblasts3. The pedicle is absent or short, and the three regions (supra) are not differentiated, no trace of the basal ectodermic thickening being even discernible.
The endoderm is vacuolate and rarely elevated into ridges (Apolemia). The hydrocysts are represented in the Discoideae by small zooids (blastostyles) with mouths. (3) Tentacles. These structures in the Discoideae are simple and tubular; in Porpita dilated terminally and provided with a number of short capitate processes. The large tentacles of Physalia attain sometimes the length of many feet. Each tentacle, whether large or small, consists of a conical hollow sac (? = hydrocyst) covered with cnidoblasts, a long hollow filament attached to the base of the sac but connected by a membranous expansion with its side. The first section of the filament is coiled from side to side, but the greater portion of it hangs freely. One of its aspects is covered with transverse hollow reniform elevations in which cnidoblasts are aggregated. The tentacles of other Siphonophora are, with the partial exception of A thorybia and Abyla, attached to the pedicles of the polypites, on a special elevation of ciliated ectoderm containing cnidoblasts, from which new 'nettle batteries 'are derived in growth. They are branched except in Apolemia, and each branch terminates in a single nettle battery, the structure of which is often extremely complicated and characteristic of a genus, or even species.
The tentacles of the hydrocysts are unbranched. (4) The sexual zooids or gonozooids are medusiform. In the Discoideae they are medusae, and are borne in numbers upon blastostyles (gonoblastidia) which are disposed in a zone round the large central gastrozooid. The medusa of Velella is known as Chrysomitra striata; it is a quarter of an inch in diameter, has a single tentacle, and its generative products are developed in four groups on the manubrium as in Anthomedusae1. The female zooid of Physalia is probably detached as a medusa. In all other instances the zooid is a medusoid: it has typically four radial canals, a circumferential canal, and an imperforate manubrium or spadix which bears the sexual products. The Calycophoridae have many ova, the Physophoridae 2 a single ovum. The spadix in the latter grows round the ovum, and its cavity becomes reduced to a system of branched canals, sometimes confounded with the radial canals. A velum is present in the sexual zooid of the Diphyozooid (p. 773) but not in other instances3. In the Calycophorids Hippopodius and Epibulia ( = Galeolaria) and in Physo-phoridae, the bell itself consists only of three layers, an endodermal lamella bounded by two ectodermal, an outer and inner, and the bell cavity is small.
The zooids are numerous in Physalia, Physophoridae, and the Caly-cophorid Hippopodidae, developed in grape-like bunches on the pedicle, or as in Physalia and Physophora extending up the sides, of the hydrocysts, male and female in connection with the same hydrocyst, or as in Agalma rubrum with different hydrocysts. In the Diphyidae they are developed successively one after another on the pedicles of the polypites. The zooids, male and female, are detached in Hippopodius and Epibulia, or the male only is so, e. g. in Physophora, Halistemma, Forskalia, swimming by means of the cilia covering the bell. The sexual zooid in a Diphyozooid discharges the sexual products, and is detached when its successor is ready to replace it. The colonies are usually hermaphrodite. Abyla penta-gona and some species of Diphyes, however, are of separate sexes. (5) The hydrophyllia or bracts ( = Deckstiicke), absent in Discoideae, Physalia and Hippopodidae, are protective zooids of a leaf-like character but of various shapes, attached by a short pedicle either to the coenosarc (Physophoridae) or to the pedicles of the polypites (Calycophoridae). They consist of a lamina of mesoglaea covered by ectoderm, in which cnidoblasts are frequently to be found at the apex or projecting angles of the hydrophyllium.