"Kept in aquaria, the dependence of the animal upon the nature of the sand and its method of locomotion may be readily observed. A vigorous individual buries itself in a few moments in the following manner: Running out the introvert to nearly its full extent, and applying it to the surface of the sand till some spot of less resistance is found, the animal still further expands the introvert so that it penetrates the sand, provided this is not too dense and firm, for then the body is merely shoved backward. When the introvert is inserted, the contraction of the longitudinal muscles of the body-wall brings the whole body forward somewhat, in case the introvert is fixed in the sand. In case soft ooze was present, this fixation did not take place, and the introvert was merely pulled out again, but when the sand was of the right consistency the introvert was fixed by becoming much swollen at the tip, and then constricted just posterior to this swollen area. This bulb-like area exerts lateral pressure on the sand, as could be seen by movements of the grains. The swelling of the anterior end of the introvert is brought about by the body-wall contracting elsewhere, and forcing in liquid to distend that end. Owing to the curved form assumed by the body in the normal contracted state when first removed from its burrow, the entrance of the introvert may often be nearly vertical, and hence the entire body is soon raised nearly upright in the water above the sand. If the body has thus been warped forward sufficiently to become somewhat fixed in the sand, the introvert is rolled in and again thrust forward from this new point of resistance, and so on till the animal is entirely buried. This locomotion increases in speed as the creature becomes more completely surrounded by sand, and is the only means of moving from place to place.
"On a smooth surface, or on one not presenting the right degree of resistance, the Sipunculus does not change its position, but remains till death finally occurs, rolling its introvert in and out and contracting its body-wall to no purpose.
"The essential factors in the mechanism bringing about this hydrostatic locomotion are an elongated contractile sac filled with liquid, and some means of definitely co-ordinating the contractions of the sac.
"In natural environment the animals are found with sometimes one, sometimes the other end nearer the surface of the sand: in the aquaria the same was observed, but when the water became stagnant and impure the anterior end with expanded branchiae was often protruded somewhat above the surface of the sand."
The genus Phascolosoma contains at least twenty-five species, for the most part small. Ph. margaritaceum, however, measures[[483]] 10 cm. in length, and Ph. flagriferum, 13 cm. The latter is produced at the hinder end of its trunk into a long whip-like process, which recalls the horny spike of Golfingia. Most species live free, but a few inhabit the shells of dead Gasteropods or of Dentalium, or the abandoned tubes of worms. They occur in practically all seas.
Fig. 216.—Specimens of the Coral Heteropsammia cochlea, with Aspidosiphon heteropsammiarum or A. michelini living in a state of commensalism with them. (From Bouvier.)
Dendrostoma contains but five species, which are all found within the tropics in the Pacific or in the West Atlantic. They are shallow-water forms, and some are found between tide-marks.
Phascolion is a smaller genus, containing but ten species, which may have been derived independently from different species of Phascolosoma, and in this case the genus should be broken up. The members of this genus live in Mollusc shells, such as Dentalium, Turritella, Buccinum, Chenopus (Aporrhais), Nassa, Strombus, and generally acquire the coiled shape of their host. They are usually attached to the shell by means of certain adhesive papillae found on their posterior end. Ph. strombi fills its shell with mud, which must be kept together by some secretion of the animal. The body lies in a tube in this mud, and the introvert projects from the small round opening at the end of the tube, and explores the ground in every direction. They are found in all seas, but more especially in the colder waters.
