Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/457

Rh POLYZOA (Cristatella) having the form of a plano-convex ellipse and locomotive (fig. 3). In addition to the multiplication of polypides in a colony by budding, and to the annual production of new individuals from fertilized eggs which initiate new colonies, a reproduction by internal buds called &quot; statoblasts,&quot; comparable to the gemmae of Spon- gilla, has been observed in all the genera (fig. 3, I). The statoblasts are developed from the funiculus (mesentery), and are enclosed in ornate lenticular capsules of chitinous substance, characteristic in form in each species. The fertilized egg of the Phylactohema does not give rise to a zonociliate larva, but to a uniformly ciliate cyst- like diblastula, which develops directly and produces polypides by budding. The Phylactolaema are all inhabit ants of fresh water (lacustrine). FIG. 11. Semi-Ideal view of pait of the lophophore of Lophoptit and its tentacles intended to show the nerve-ganglion, nei ves, and parts around the mouth. The tentacles have been cut away all along the right arm of the lophophore and from the inner margin of the left arm. c, foramen .placing the cavity of the epistome in communication with the body-cavity; c, body-wall ; (/. mouth; e, the t-pistome or prae-oral lobe; /, wall of the pharynx ; f&amp;gt;, wall of the intestine; i, anus ; /-, lophophore ; I, a ciliated tentacle ; r, elevator muscle of the epistome; . the nerve-ganglion ; J-, .r , nerves to lophophore and tentacles: y, nerve to pharynx. The Phylactolrema include the genera Lophopus, Cristatella, Alcyonella, Plumatella, and Fredericella, which have been beauti fully figured and described in Allmau s classical Freshwater Polyzoa, Ray Society, 1856. The colonies of Lophopus are small, consist ing of half a dozen polypides embedded in a massive glass-like ceno3cium. Cristatella (fig. 3) is remarkable amongst all Polyzoa for its locomotive zoarium. Alcyonella forms massive coencecia of many hundred polypides, as large as a man s fist. Plumatella and Fredericella are delicate arborescent forms commonly encrusting stones and the leaves of water-plants. All the genera known are British. The Phylactolaema furnish a remarkable instance of a well- marked zoological group being confined to fresh water. Their reproduction by statoblasts (not known in the marine Polyzoa) appears to be related to the special conditions of lacustrine life, since it is also observed under the same exceptional conditions in the single freshwater genus of another great group of animals, viz., Spongilla. Also related to their non-mnrine conditions of life is the development of the fertilized egg, which, as in so many similar cases, does not produce the remarkable banded forms of locomotive larvaj which are characteristic of their marine congeners. Order 2. GYMNOL^MA, Allman. Ectoproctous Eupolyzoa in which the polypide is devoid of any trace of the prae-oral lobe or epistome, whilst the lophophore is perfectly circular. The polypides of a colony are frequently highly differentiated as avicularia, vibracu laria, ocecia (egg-receptacles), and even as root and stem segments. The neighbouring polypides of a colony communicate (?) with one another by &quot; rosette-plates &quot; or &quot; communication-plates &quot; perforated areas in the walls of contiguous zocecia. The greatest variety in the character of the cuticle forming the zorccia (gelatinous, horny, calcareous) and in the grouping of the polypides, as well as in the shape of their zooecia, is observed in different sub-orders and families. In addition to the ordinary sexual reproduction, there are various modifications of the process of budding, the full exposition of which would necessitate more space than is here allotted, and is not yet indeed within the possibilities of present knowledge. The fertilized egg of the Gymnolsema gives rise to remarkable ciliate larvae of various forms (figs. 19, 20, 21), from which the first polypide of a colony is developed by an extraordinary and unexplained series of changes. The Gymnolsema are, with the single exception of the genus Palu- dicella, inhabitants of the sea, The Gymnolsema are divided, accord ing to the system of Busk, into three sub-orders characterized by the shape of their zooscia, and the nature of the mouth-like margin which it presents when the exsertile portion of the poly pide is withdrawn within it. The Cyclostoma have long tubular zooacia, often of large size and often calcified, placed side by side in cylindrical bun dles, or in other definite grouping ; the mouth of the zocecium is circular and devoid of processes. There is little or no differentiation of the polypides con stituting a colony. Most of this group are fossil, and the living genera belong mostly to southern seas. The genera Crisia (fig. 13, A), Diastopora, Tubuli- pora, and Hornera are typical. The Ctenostoma have usually a soft zoce cium ; its orifice is closed by the folds of the retracted polypide or by a circlet of bristles which surround it. Alcyonidium yelatinosum is the com monest representative of this group on the British coasts. Bower ban kia (fig. 1, A) and Paludici-lla (fig. 1, E) also belong here. The Chilostoma form the largest and most varied sub order of Gvmnolrema. The zorecia are _ -, . c i, - , FIG. 12. Two zooecw of Aca- horny or calcified; their orifices can be marfllili &amp;lt;&amp;lt;,/) ar jc,,i&amp;lt;,rfa, closed by a projecting lip in the form i.mx. (Chilostoma), of which of an operculum. The operculum is a separable plate developed on the cuticle of the retractile part of the polypide, and has muscles attached to it (fig. 13, B, C, D). The surface of the zooecia is frequently sculptured, and its orifice provided with processes and spines (fig. 1, C, F). Very usually some of the polypides of a colony are modi tied as avicularia, vibracularia, radi cal fibres, and ooecia. The avicularium is a polypide reduced to a simple muscu lar apparatus workingupon the modified operculum and zocecium so as to cause these hard parts to act as a snapping apparatus comparable to a bird s head (fig. 12, o). They are frequently found regularly distributed among the normal cells of a colony, and probably have a cleansing function similar to that attributed to the Pedicelfarirc of the Echinoderms. &quot; Vibracularia are even more simplified polypides, being little more than motile filaments, probably tactile in function. The opercula of zorecia, ocecia, and avicularia have recently been used by Busk in character izing genera and species, in a systematic way. Stem-building and root-forming polypides are frequently found, being closed polypides which subserve anchoring or supporting functions for the benefit of the whole colony. The stem of Kinetoskias (fig. 14) is produced

the anterior contains a living polypide, whilst the posterior is empty. To cu* h is attached one of the characteristical ly modified polypides known as an &quot; avicularium &quot; o ; tho hinder of these has grasped and holds in its beak a small worm, a, anus; i, intestine ; r, stomach ; r, body-cavity (roelom); f, tentacular crown surrounding the mouth ; te, testis cells developed on the surface of the terminal mesen tery or &quot; funiculus &quot; ; o, o, avi cularia.