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The fresh-water Polyzoa admit of being divided into eight generic groups. The discrimination of these is very easy in the living animal; but as some of the genera are founded on the form of the polypide alone, the canccium not presenting one character which can be employed for the purposes of distinction; while in others the form of the entire animal is so changed by death, as to render it almost utterly incapable of conveying any idea of what it had been, we are in most instances obliged to examine the animal in a living state before we can form any safe opinion as to even the genus to which it belongs.

The following is a condensed synopsis of the orders, sub-orders, families, and genera of fresh-water Polyzoa :

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Genus I. CRISTATELLA, Cuvier, 1798.

Name.-A diminutive noun formed from crista, a crest, in allusion to its tentacular plume.

The genus Cristatella was established by Cuvier, for the little animal originally described by Rösel under the title of “ Der Kleinere Federbusch Polyp mit dem ballenförmigen Körper." Its characters were given in the 'Tableau Elémentaire des Animaux, published in 1798, and it is the first generic group constituted for any of the fresh-water Polyzoa, hitherto all confounded together in discordant and unzoological associations. Rösel's animal, however, was unknown to Cuvier, except from the figures and descriptions left us by its discoverer; and it was not until Turpin and Gervais many years afterwards rediscovered it, having hatched it from statoblasts found in the Canal d'Ourcq, in the city of Paris, that any additions were made to the original account left us by Rösel. Still, however, nothing was known of the animal but in its young and partially developed state, and it was only afterwards, when adult and fully formed specimens were met with, that the real nature of this beautiful Polyzoon was properly understood.

The genus, which includes as yet but a single species, may be thus characterised :

Generic character.—Cænæcium sacciform, hyaline, with a common flattened discadapted for locomotion; orifices placed on the surface opposite to the disc, and arranged in several concentric marginal series. Statoblasts orbicular, with an annulus and marginal spines.

Species unica. Cristatella mucedo, Cuvier. Pl. I.

Specific character. --Same as that of the genus.

SYNONYMS.-1755. Der Kleinere Federbusch-Polyp. Rösel, Insect. Belustig. Supp., p. 559,

tab. 91. (Original figure.) 1766. La seconde sorte de Polypes à Bouquets. Ledermuller, Amusm. Mic. 2de cinq.,

p. 94, pl. 87. (The figures are imperfect copies from Rösel.) 1798. Cristatella mucedo. Cuvier, Tab. Elém., p. 656. 1816. Cristatella vagans. Lamk., An. sans Vert., 1st edit., vol. ii, p. 97. 1817. Cristatella mucedo. Cuv., Règne A., 1st edit., vol. iv, p. 68. 1820. Cristatella vagans. Schweigger, Handbuch der Naturg. p. 423. 1824. Cristatella vagans. Lamouroux, Enc. Méth. Zooph., p. 226, pl. 472.

(Figures copied from Rösel.)

1824. Cristatella vagans. Goldfuss, Naturhistorisch Atlas. (Figures copied from

Rösel.) 1828. Alcyonella, secundus evolutionis gradus. Raspail, Hist. Nat. de l’Alcyon. fluv.,

Mém. de la Soc. d’Hist. Nat. de Paris, vol. iv, p. 129. 1830. Cristatella mucedo. Cuvier, Règ. An., 2d edit., vol. iii, p. 296. 1831. Cristatella mirabilis. Dalyell, Rep. Brit. Assoc., p. 601; and Edinb. New

Phil. Journ., vol. xviii, p. 414. 1831. Cristatella vagans. De Blainville, Man. d'Act., p. 489, pl. 85, fig. 7. (Figure

copied from Rösel.) 1831. Cristatella vagans. De Blainville, Dict. Sc. Nat., art. Cristutelle, fig. 7.

(Figure copied from Rösel.) 1836. Cristatella vagans. Lamarck, An. sans Vert., 2d edit., vol. ii, p. 110. 1837. Cristatella mucedo. Turpin, Ann. Sc. Nat., 2d series, tom. vii, p. 65, pl. 2, 3.

(Original figures.) 1837. Cristatella mucedo. Gervais, Ann. Sc. Nat., 2d series, tom. vii, p. 77, pl. 4.

(Original figures.) 1838. Cristatella mucedo. Johnston, Brit. Zooph., 1st edit., p. 308, pl. 43. (Figures

copied from Turpin.) 1839. Cristatella mucedo. Gervais, Ann. Franç. et Etrang. d'Anat., tom. iii, p. 133. 1810. Cristatella moisissure. Gervais, Dict. Sc. Nat. Suppl., art. Alcyonelle, Planches

Supplémentaires, Pol, fluviatiles. (Original figures.) 1813. Cristatella mucedo. Thompson, Rep. Brit. Assoc., p. 285. 1811. Cristatella mucedo. Allman, Ann. of Nat. Hist., vol. xii, p. 330. . 1816. Cristatella mucedo. Allman, Rep. Brit. Assoc., Trans. of Sect., p. 88. 1847. Cristatella mucedo. Johnston, Brit. Zooph., 2d edit., p. 387, pl. 73. (Figures

copied from Turpin.) 1818. Cristatella mucedo. Van Beneden, Bryoz. Fluv. de Belg., p. 16; Mém. de

l'Acad. Roy. de Belgique. 1848. Cristatella mirabilis. Dalyell, Rare and Remarkable Animals of Scotland,

vol. ii. (Original figures.)

Iconography.The original figures are those of Rösel, Turpin, Gervais in. Ann. Sc. Nat.,' Gervais in Dict. Sc. Nat.' Suppl., and Dalyell.

Habitat.—In clear lakes and ponds, where it creeps slowly over the upper side of submerged stones and the stems of aquatic plants, delighting in sunlight.

LOCALITIES.British : A millpond near Little Baddow, Essex; Union Canal, Edinburgh ; a beautiful little subalpine lake near Glandore, County Cork ; Lakes of Killarney, Grand Canal, Dublin ; a lake near Armagh, and several other Irish localities. G. J. A.Duddingston Loch, near Edinburgh ; Coldingham Loch, Berwick; a garden pond at Binns House, Linlithgowshire. Sir J. G. Dalyell. Foreign : Lake of Lucerne; “Grand Etang,” Fontainbleau.

“Grand Etang,” Fontainbleau. G. J. A.–Near Paris. Gervais and Turpin.—Belgium. Van Beneden.—Germany. Rösel.

The first perfectly developed specimen of Cristatella mucedo was described in 1834, by Sir John Graham Dalyell, who discovered it near Edinburgh, and perceiving it to be so very

different from previous descriptions and figures, looked upon it as a new species, and named it C. mirabilis. * Adult specimens have since been found by M. Gervais, and I have met with them in abundance in Ireland, and in other parts of the British Isles.

A more interesting and beautiful animal than a fully developed specimen of Cristatella mucedo can scarcely be imagined. The entire colony is of an oval shape, convex above and flat below, where it attaches itself to neighbouring objects. Upon the convex surface are arranged the orifices through which the polypides emerge, they are placed near the margin, and run round the entire cænæcium in three regular concentric series, which alternate with one another, and leare an oval space in the centre where no orifices exist.

In the middle of the flattened under surface is an oval disc, resembling the foot of a gasteropodous molluse; on this dise, which is contractile, and admits of frequent change of shape, the colony adheres to neighbouring objects, or creeps about on the submerged leaves and stems of aquatic plants. From the edges of the disc a flat space extends outwards, passing beyond the external series of orifices in the form of a projecting margin, whose interior is occupied by a series of tubular cells or chambers, visible through the translucent skin, and extending in a radiating direction from the disc outwards, but possessing no external opening.

The tentacula are about eighty in number, being more numerous than in any other known Polyzoon, except, perhaps, Pectinatella, Leidy. The cænæcium is of a dull yellow, or sienna colour. The polypide is nearly of the same colour, with the exception of the intestine, which, in well-fed, healthy specimens, is light bluish-green.

The largest specimens measure about two inches in length and a quarter of an inch in breadth, and, with the polypides extended, are, at the first glance, not unlike certain hairy caterpillars, or, as M. Gervais has aptly enough suggested, the silk fabric known by the name of Chenille. Such large specimens are very sluggish, and change their place with reluctance, at least when kept in confinement; but specimens of about half an inch in length creep about on the sides of the jar in which they are preserved at the rate of several inches in the day; they generally prefer keeping near the surface of the water, and seem to be much under the influence of light; indeed, while the greater number of the fresh-water Polyzoa lurk on the under surface of stones and in dark recesses, Cristatella loves to expose itself to the full light and warmth of the sun. It differs, moreover, from all the Polyzoa with whose habits I am acquainted, in the constant pleasure it takes in maintaining its polypides in the exserted state; these, indeed, must be very roughly handled to cause them to withdraw into their cells; and the annoyance is no sooner removed than they again emerge. This exquisite little Polyzoon seems, in fact, capable of existing only under the full influence of light, and in the midst of the innumerable vortices excited in the surrounding water by the vibratile cilia of its tentacles.

The statoblasts are very characteristic. They are about it's of an inch in diameter, exclusive of the marginal spines, and, with the exception of the statoblasts of Pectinatella, which they closely resemble, are larger than those of any other fresh-water Polyzoon. They are also, with the same exception, the only ones having an orbicular shape. One face is a little more convex than the other. The annulus is wide, very distinctly cellular, and of a light yellow colour. The disc is deep reddish-brown, and elegantly mamillated. The spines

* Report of Meeting of British Association, held at Edinburgh, 183 1.'

spring from both faces of the disc, just within the annulus, and thence radiate outwards, extending for some distance beyond the margin. The spines springing from the more convex face are somewhat longer and more numerous than the others, and alternate with them. All the spines are terminated by two, three, or four curved hooks resembling grappling-irons. Towards the end of summer, the statoblasts occur in considerable numbers in the interior of full-grown specimens, and are visible through the transparent tissues of the animal. On the death and decay of the cænæcium they are liberated, when they become attached, by means of their hooked spines, to various aquatic plants, and ultimately open for the escape of the young, by the separation of the two faces, at the commencement of the following summer.

, on its escape from the statoblast, is at first solitary, but is rapidly multiplied by the production of

gemmæ. I have never met with Cristatella later than the autumn; it seems to be strictly annual in its duration.

The young

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