This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Fig. 104. Young Holothuria.
(541). The special instruments of touch, the only sense allotted to these animals, are the branched tentacula around the mouth, which seem by far the most irritable parts of the body. The nervous system is so obscurely developed, that even Delle Chiaje was unable to detect any traces of its existence; nevertheless there is little doubt of the presence of nervous threads in the muscular envelope of the animal, although, from the dense tissues wherein they are imbedded, it is next to impossible to display their 6ourse; most probably, as in the Echinus and Asterias, these communicate with a circular cord that embraces the oesophagus. No ganglia have as yet been discovered even in the Holothuridae; and consequently, although the muscular actions of the body are no doubt associated by nervous filaments, the movements of these creatures appear rather to be due to the inherent irritability of the muscular tissues themselves, than to be under the guidance and control of the animal. In many species, the slightest mechanical irritation causes such powerful and uncontrollable contractions of the integument, that the thin membranes of the cloaca, unable to withstand the pressure, become lacerated, and large portions of the intestine and other viscera are forced from the anal aperture.
So common, indeed, is the occurrence of this circumstance, that the older anatomists were induced to suppose that, by a natural instinct, the animals, when seized, vomited their own bowels. It is, in fact, extremely difficult to obtain perfect specimens of the Holothuridae, from the constant occurrence of this accident: but, although annoying to the naturalist, such a phenomenon affords the physiologist an important lesson, teaching that here, as in the lower Zoophytes, the muscular system possesses an innate contractile power, which would seem only to be destroyed by incipient putrefaction; but so little is this contractility under command, that, once excited to an inordinate extent, it becomes totally unmanageable, even though its continuance inevitably causes the evisceration of the creature.
In order to complete our account of the organization of the Echinodermata, we have still to investigate the structure of the Fistularidoe - a group that, from the external appearance of the individuals composing it, and the total absence of the tubular feet met with in other families, has been improperly separated by some modern writers from the class under consideration. Nevertheless we shall find the position assigned to these animals by Cuvier to be in strict accordance with the character both of their outward form and internal structure; only, instead of placing them with the lowest of the Echinoderms, they would have been more properly situated at the head of the class, as most nearly approximating the Annelida in all the details of their economy. We have already given a description of the outward form of a Fistularia (§ 443), and seen the completely annulose condition of its body, although the radiating tentacula around the mouth are evidently analogous to those of the Holothuria, already described.
(543). The Sipunculus inhabits shallow seas, concealing itself at the bottom in holes that it excavates in the sand. Having once located itself, it is seldom found to quit its concealment, but, retaining its hold upon the sides of the retreat which it inhabits, by dilating the posterior part of its body it occasionally protrudes its head from the orifice, either for the purpose of procuring food, or of respiring more freely the water • of the ocean.
(544). These animals are much sought after by fishermen, who employ them as baits for their hooks; and one species, Sipunculus edulis, is used in China as an article of food.
(545). The body is covered externally with a delicate cuticle, easily separable by maceration or simple immersion in spirit; and when thus detached it forms so loose a covering, that Linnaeus, deceived by the appearance of an animal thus preserved, applied to it the name of Sipunculus saccatus.
(546). The muscular investment, placed beneath the skin, is composed of strong fasciculi arranged in three distinct layers. The external stratum is disposed in circular rings, beneath which spiral fibres may be observed crossing each other at various angles; and lastly, the inner coat is made up of about thirty powerful longitudinal bands, extending from one extremity of the body to the other. Such an arrangement is evidently sufficient for the general movements of progression; but in order to facilitate the retraction of the tentacular apparatus around the mouth, eight additional muscles surround the oesophagus; and by their action the whole of the oral apparatus is completely inverted and drawn inwards.
(547). The tentacula around the oral orifice are the principal agents employed in seizing and swallowing food, an office to which they are peculiarly adapted by their great sensitiveness and power of contraction; but, as we have found to be generally the case among the Echinoder-mata, sand and fragments of shell form the great bulk of the contents of the intestine, so that it is by no means easy to state precisely the nature of the food upon which the Sipunculi are nourished.
(548). The structure of the alimentary canal and of the nutrient apparatus conforms too accurately with what we have already seen in Holothuria to permit of a moment's hesitation concerning the relationship that exists between the apodous Echinodermata and the Holo-thuridae. The oesophagus (fig. 105, h) is narrow, and soon dilates into a kind of stomachal receptacle (c); but, although the diameter of the intestinal tube is at this point perceptibly larger than in any other part of its course, there is no other peculiarity to distinguish it from the rest of the intestine. In the Annelida, the digestive apparatus is invariably straight, traversing the body from one extremity to the other, a circumstance that distinguishes them remarkably from the Echino-derms we are now considering; for in Sipunculus we find a digestive canal six or seven times the length of the animal, within which it is folded upon itself in various distinct convolutions. Leaving the stomach, if we may so call the dilatation above alluded to, it passes down (d d d) nearly to the tail, where it is reflected upon itself, and mounts up again as far as the point where it commenced; here it again turns back, and, once more reaching the bottom of the tegumentary sac, becomes a second time directed upwards, and re-ascends as far as the point e, where the anus is situated.