Page:EB1911 - Volume 22.djvu/58

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44
PoLYZoA


through perforated “ rosette-plates " in the .dividing walls. In the Phylactolaemata a single definite funiculus passes from the bodywall to the apex of the stomach. This latter organ is pigmented in all Polyzoa, and is produced, in the Ectoprocta, beyond the point where the intestine leaves it into a conspicuous caecum (fig. 6, v). The nervous system is represented by a ganglion situated between the mouth and the anus. The ovary (o) and the testis (t) of Ecto rocta are developed on the body-wall, on the stomach, or on the fifiniculus. Both kinds of reproductive organs may occur in a single zooecium, and the reproductive elements pass when ripe into the body-cavity. Their mode of escape is unknown in most cases. In some Gymnolaemata, polyp ides which develop an ovary possess a fiask-shaped “ intertentacular organ, "»situated between two of the tentacles, and affording a direct passage into the introvert for the eggs or even the spermatozoa developed in the same zooecium. In other cases the reproductive cells perhaps pass out by the atrophy of the polypide, whereby the body-cavity may become continuous with the exterior. The statoblasts of the Phylactolaemata originate on the funiculus, and 'are said to be derived partly from an ectodermic core possessed by this organ and partly from its external mesoderm (Braem), the former giving rise to the chitinous envelope and to a nucleated layer (fig. 7, ect), which later invaginates to form the inner vesicle of the polypide-bud. The mesodermic portion becomes charged with a yolk-like material (y), and, on the germination of the statoblast, gives rise to the outer layer (mes) of the bud. The production of a polypide by the statoblast thus differs in no essential respect from the formation of a polypide in an ordinary zooecium. The statoblasts require a period of rest before germination, and Braem has shown that their property of floating at the surface may be beneficial to them by exposing them to the action species of Membranipara the " frontal membrane/1 or membranous free-wall, is protected by a series of calcareous spines, which start from its periphery and arch inwards. In Cribrzlina similar spines

FIG. 9.-Diagrammatic Longitudinal Sections of Cheilostomatous Zooecia.

A, Membranipora (after Nitsche); B, Cribrilina; C, Some of the Lepralioid forms. b.c., Body-cavity. cr., Cryptocyst. Ls., Compensation-sac. jim., Frontal membrane. o., Orifice, through which the tentacles are protruded. op., Operculum. p.m., Parietal FIG. 10.-Zooecium

M of frost, which in some

5 "§ °“ cases improves the ger-1, , minating power. The

fp ., " " ° occurrence of Phylac-3 gg; tolaemata in the tropics

ss ef 'l" a em W9“1d Sh°W» h<2W<='V@f» § .gt ' without further evidence, % ' ' -@' that frost IS not a factor -1 " ' =': ' essential for germination. sp The withdrawal of the

(After Braem.)

FIG. 7.-Section of a Germinating Statoblast of Cristatella mucedo. ann, Chitinous annulus, containing air cavities which enable the statoblast

to fioat.

f-ct, Thickened part of the ectoderm, which will give rise to the inner layer of the polypide- bud.

mes, Mesoderm, forming the outer layer of the bud.

sp, Anchoring spines of the statoblast. y, The yolk-like mesodermic mass. body-wall exerts a pressure on the fluid extended polypide is

effected by the contraction

of the re tractor

muscles (fig. 6, mr), and

must result in an increase

in the volume of

the contents of the body cavity. The alternate

increase and diminution

of volume is easily understood

in forms with flexible

zooecia. Thus in the

Phylactolaemata the contraction of the muscular

of the body-cavity and is

the cause of the protrusion of the polypide. In the Gymnolaemata protrusion is effected by the contraction of the parietal muscles, which pass freely across the body-cavity from one part of the body-wall to another. In the branching Ctenostomes the entire body-wall is flexible, so that the contraction of a parietal muscle acts equally on the two points with which it is connected. In encrusting Ctenostomes and in the Membranipora-like Cheilostomes (hgs. 8 A, 9 A) the free surface or frontal wall is the only one in which any considerable

amount of movement

can take place. The parietal

muscles (p.m.), which

pass from the vertical walls

to the frontal wall, thus

act by depressing the latter

and so exerting a pressure

muscles. Ls., Tentacle-sheath.

are developed in the young zooecium, but they soon unite with one another laterally, leaving rows of pores (fig. IO). The operculum retains its continuity with the frontal membrane (Hg. 9, B) into which the parietal muscles are still inserted. As indications that the conditions described in M embmnipora and Cribrilina are of special significance may be noted the fact that the ancestrula of many genera which have well-developed compensation-sacs in the rest of their zooecia is a illembranipora-like individual with a series of marginal calcareous spines, and the further fact that a considerable proportion of the Cretaceous Cheilostomes belong either to the Mernbraniporidae or to the Cribrilinidae. (ii.) In Scrupocellaria, Menipea and Caberea a single, greatly dilated marginal spine, the “ scutum ” or “ fornix, " may protect the frontal membrane. (iii.) In Umbonula the frontal membrane and parietal muscles of the young zooecium are like along the sutural lines

op.


»

4


|

a, '

~ .

¢

  • Q

Q

- Q


0"


u

¢

of Cfibfilina, showing

the entrance to the

compensation- sac on

the proximal side of the

A a on the fluid of the body-F IG. 8.-Diagrammatic Transverse CQVIW- Ill Ch€110St0maf2l Sectioni with a rigid frontal wall A, of Membranipora; B, of an

immature zooecium of Cribrilina; p.m., Parietal muscles,

Iullien showed that protrusion

and retraction were

rendered possible by the

existence of a “compensation-sac, " in communication with the external water. In its most fully developed condition (fi C the com ensatlon - s~ 9,) P 'sac

(c.s.) is a large cavity which lies beneath the calcified frontal wall and opens to the exterior at the proximal border of the operculum (fig. Io). It is joined to the rigid body-wall by numerous muscle-fibres, the contraction of which must exert a pressure on the fluid of the body-cavity, thereby protruding the polypide. The exchange of fluid in the sac may well have a respiratory significance, in addition to its object of facilitating the movements of the tentacles.

The evolution of the arrangements for protruding the polypide seems to have proceeded along several distinct lines: (i.) In certain those of Membranipora, but they become covered by the growth, from the proximal operculum (op).

and lateral sides, of a calcareous lamina covered externally by a soft membrane. The arrangement is perhaps derivable from a Cribrilina-like condition in which the outer layer of the spines has become membranous while the spines themselves are laterally united from the first. (iv.) In the Microporidae and Steganoporellidae the body-cavity becomes partially subdivided by a calcareous lamina (“ cryptocyst, " Jullien) which grows from the proximal and lateral sides in a plane parallel to the frontal membrane and not far below it. The parietal muscles are usually reduced to a single pair, which may pass through foramina (“opesiules ”) in the cryptocyst to reach their insertion. There is no compensation-sac in these families. (v.) Many of the Lepralioid forms offer special difficulties, but the calcareous layer of the 'frontal surface is probably a cryptocyst (as in fig. 9, C), the compensation sac being developed round its distal border. The “epitheca" noticed above is in this case the persistent frontal membrane. (vi.) In Microporella the opening of the compensation-sac has become separated from the operculum by calcareous matter, and is known as the “ medianipore." Jullien believed that this pore opens into the tentacle-sheath, but it appears probable that it really communicates with the compensation-sac and not with the tentacle sheath. The mechanism of protrusion in the>Cyclostomata is a subject which requires further examination. The most singular of the external appendages found in the Polyzoa are the avicularia and vibracula of the Cheilostomata. The avicularium is so called from its resemblance, in its most highly differentiated condition, to the head of a bird. In Bugula, for instance, a calcareous avicularium of this type is attached by a narrow neck to each zooecium. The avicularium can move as a whole by .means of special muscles, and its chitinous lower iaw