(549). It is easy to account for this extreme length of the intestine when we consider the nature of the materials used as food, and the small proportion of nutriment contained among the sand and broken shells found in the digestive canal: but the remarkable position of the anal aperture is only explicable by a reference to the peculiar habits of the creature; for (living, as it does, in a narrow excavation bored in the sand, from which it seldom issues), had the excrements been discharged, as in Holoihuria, through a terminal orifice, their constant accumulation at the bottom would soon expel the animal from its retreat; but, by the arrangement adopted, it is only necessary that the anterior part of the body should be protruded from its concealment, and the excrementitious matter may be cast out without inconvenience. The intestine is retained in situ and supported at all points by innumerable tendinous bands, that arise from the interior of the muscular walls of the body and form a kind of mesentery.

(550). In Sipunculus, the character of the circulating system is in all essential points strictly analogous to that of the other Echinodermata; and moreover, from the superior concentration visible in every part, we have the multiplied organs of the other families exhibiting so much simplicity of arrangement, that whatever may have appeared obscure or complicated in our description of Echinus and Holoihuria will receive elucidation from the diagrammatic form in which all the vessels connected with the circulation of the blood are represented in fig. 105. The intestinal vein (m) may be readily traced along the entire length of the alimentary canal: commencing near the anal extremity of the bowel, it follows all its convolutions, and receives from every part the minute vessels which ramify over the intestinal walls. These venous ramifications undoubtedly perform the office assigned to the lacteals of higher animals, and imbibe the nutritive particles furnished by digestion, which of course are conveyed into the great venous trunk (m.) Arrived opposite to the termination of the oesophagus, the intestinal vein divides into two vessels: one performing the office of a branchial artery, by conveying a part of the blood to the respiratory organs in the neighbourhood of the mouth; the other, which we may call the aorta, distributing the remainder to all parts of the tegumentary system. The branchial vessel (n) runs from the bifurcation of the intestinal vein to the base of the oral tentacles, where it forms a vascular circle around the commencement of the oesophagus, analogous to that which we have seen in Holoihuria; and in connexion with this circular vessel we find the "ampulla Poliana" (h), which Delle Chiaje conceives to be here, as in other cases, a receptacle for the circulating fluid.

From the vascular circle around the mouth, vessels are given off to ramify minutely through the substance of the tentacula (a); so that these appendages may be considered as respiratory organs like those of Holoihuria. The other vessels derived from the oral circle have not been traced; but we may conclude from analogy that arteries supplying the mouth and alimentary canal are furnished from this source.

Anatomy of Sipunculus: a, oral tentacles; b, oesophagus; c, stomach.

Fig. 105. Anatomy of Sipunculus: a, oral tentacles; b, oesophagus; c, stomach; d d d, intestinal canal; e, position of anal orifice; f,p, ovaria; g, external orifice of ovary; h, ampulla Poli-ana; i, cerebral ganglia; I, heart; m, intestinal vein; n, branchial vessel; o, aortic trunk.

(551). The aorta (o) is the other large vessel derived from the intestinal vein, and is seen to pass in a flexuous course from its origin to the posterior extremity of the body, following the median line, and giving off transverse branches on both sides opposite to every ring of the muscular integument. At the commencement of the aorta is a dilated vesicle (I), which may be looked upon as a heart (auricle, Delle Chiaje.) The vesicle alluded to is of a conical form, the apex of the cone being directed towards the tail of the animal; and, from the impossibility of making mercury pass from the aorta through this organ in the direction of the intestinal vein, it is probable that it contains an apparatus of valves so disposed as to prevent any retrograde motion of the blood. At the termination of the aorta there appears to be a second enlargement, to which the name of ventricle has been given, and which is perhaps also capable of contraction, so as to assist in the propulsion of the circulating fluid.

The blood of these animals is of a purple colour in the veins, but red in the arterial vessels.

(552). We have seen that the tentacula are, from their vascularity, well adapted to fulfil the office of a respiratory apparatus; but it may be presumed that they are not the only agents by which respiration is accomplished. Upon the outer surface of the body, in the neighbourhood of the anal opening, two apertures are visible, which lead into two long sacculi (f, p), the entrance being guarded by muscular fibres (g): their texture presents transverse and longitudinal striae; and they contract spontaneously, even after the animal is dead; internally they are lined with a mucous membrane. The use of these organs is not precisely known; Cuvier regarded them as belonging to the generative system, while Delle Chiaje looks upon them as respiratory organs.

(553). In this elevated form of the Echinodermata, so nearly allied to the Homogangliate type, we may naturally expect a more complete development of nervous ganglia than we have yet met with in the class; and accordingly we find, upon the anterior part of the oesophagus, two little nervous tubercles (i) from which nervous filaments issue to be distributed to different parts of the body; one of these, in particular, may be traced along the whole length of the intestine, from the mouth to the anus.

(554). "We are entirely ignorant concerning the mode of reproduction in these creatures. Nevertheless, at certain seasons of the year, on opening the visceral cavity it is found to be filled with a fluid of a reddish tint, in which thousands of minute white bodies resembling millet-seeds are seen to float: should these be ova, they are probably expelled through an orifice that exists in the vicinity of the tail.