Fig. 248.—Lophopus crystallinus Pall., Cambridge, showing the rate of movement. The colony and the distances moved are × 2.
It is by no means certain what is the mechanism by which movement takes place in the above cases. The ectocyst of Cristatella is confined to the base of the colony, and there forms a thin slimy film, which lubricates the surface over which the animal moves. It has been stated[[557]] that progression is produced in the following way. The polypides are withdrawn by means of retractor muscles, which originate from the septa and inner surface of the sole. Thus at each retraction of any polypide, the muscle pulls on a portion of the sole. Should the expanded polypides place themselves in a suitable position, the movement will be in the direction of the resultant of the forces due to the separate retractor muscles; while it is probable that their cilia assist in the onward movement. It should be noted that it is definitely stated that a colony in which all the polypides are retracted can alter its position,[[558]] although even then the retractor muscles might still contract to some extent.
The movement probably depends on several causes. It must probably be conceded that the sole itself has some effect on this process. Its outer cells are contractile, and have the power of raising themselves from the underlying ectocyst. They may then again attach themselves, and this new attachment does not always take place in exactly the same place as the former one. Any movement of the muscles of the sole, or of the retractor muscles, will thus shift the skin to a new place.[[559]]
Protrusion of the Polypide.—While it is perfectly clear that retraction is principally performed by the great retractor muscles acting directly on the polypide, it is less easy to explain the converse movement. There can, however, be little doubt that protrusion is effected by the pressure of the fluid of the body-cavity, caused in large part by contractions of the common body-wall.
Now since, in Cristatella, the body-cavity is a continuous space, any pressure on the fluid must act uniformly on all its contents. The cause which determines the protrusion of a polypide is thus to a large extent the relaxation of the sphincter-muscle which surrounds its orifice, aided by special muscles which dilate the orifice. Any polypide which is retracted while the pressure of the fluid in the body-cavity is sufficient to keep other polypides protruded, must therefore keep either its retractor-muscles or its sphincter in a state of contraction in order to remain in that position. And as a matter of fact, Cristatella and Lophopus differ from most other Polyzoa in the readiness with which they expand their tentacles, after they have been induced to retract themselves by mechanical irritation.
Plumatella and other forms have a chitinous ectocyst, which, however, is sticky when it is first formed. By virtue of this property, the branches become attached to the leaf on which the colony is growing, and may have their natural transparency obscured by taking up foreign bodies. The stiffness of the ectocyst naturally involves some modification of the process by which the polypides are protruded. In some cases, this is effected by the separation of the endocyst from the ectocyst in the lower parts of the tube. The muscles of the body-wall can thus press on the fluid of the body-cavity without being restrained by the inflexible ectocyst. In other cases, the tube of ectocyst is rendered flexible by the presence of a thin line along one side where the chitin is deficient.
Fig. 249.—Plumatella repens L., R. Yare, × 30. a, Anus; b, polypide-bud; c, caecum of stomach; d, duplicature; e, epistome (see p. [476]); f, funiculus; g, ganglion; m, retractor muscle; p, parieto-vaginal muscles; ph, pharynx; s, statoblasts attached to f.
The upper end of the retracted tentacle-sheath is connected with the body-wall by bands known as the parieto-vaginal muscles (Fig. 249, p). These serve not only to dilate the orifice when protrusion is commencing, but also to prevent the polypide from being forced out too far. They are arranged in such a way that a circular fold, the duplicature (d), is never turned inside out, even in the state of complete protrusion of the polypide.
The mechanism of the protrusion of the polypide in the Gymnolaemata is in many cases obscure. The body-wall is not muscular in this group, in some forms of which, however, short strands known as the parietal muscles (Fig. 234, p) pass across the body-cavity from one point to another of the zooecium. As doubts have been thrown on the function of these muscles in causing protrusion, it will be worth while to refer to the detailed and convincing statements of Farre,[[560]] relating to this point.