The integumental spines of the Priapididae are cuticular elevations lined with elongated hypodermis cells, and in Priapulus caudatus, according to Scharff, some or all of the cells end in projecting sensory hairs2. The papillae of Echiuridae consist of thickenings of the cutis, into which a nerve penetrates and branches; its branches have interpolated ganglion cells, and end in the ordinary hypodermis cells (Greeff). Two or more pigment specks, representing eyes, lie on the supra-oesophageal ganglion in Phascolosoma. Eye-specks are also present in Phymosoma.
The mouth is anterior and terminal in the Sipunculidae and Priapulidae, at the base of the prostomium and surrounded by it in Echiuridae. The anus is dorsal, and more or less anterior in the Sipunculidae, terminal in other Gephyrea. The digestive tract itself takes a straight course from mouth to anus in Priapulus (except in P. glandifer) and Halicryptus; it is convoluted in Echiuridae, principally in side to side loops, whereas in the Sipunczdidae it is coiled in a close dextral spiral, except in certain tubi-colous forms, in which the coils are loose. The axis of the spiral is generally occupied by a muscle, usually attached in front close to the anus, and frequently behind as well, at the apex of the body. From this muscle fibres pass to the walls of the tract. Muscle-fibres, especially well-developed in Echiuridae, also attach the tract to the body-walls. The pharynx and the rectum are always provided in Gephyrea achaeta with specially developed bands, which attain great prominence as the retractor muscles in connection with the former.
The retractors in question are derived from the longitudinal muscle-layer of the body; are short in the Priapididae, of considerable but variable length in the Sipunculidae. In the last-named there is either a single ventral retractor, or two ventral, or two ventral and two dorsal. The tract itself is divisible in Sipunculidae into a muscular pharynx, intestine, and extremely short rectum. In Echiuridae it consists of an anterior ('buccal') region, which ends at the spot where the dorsal blood-vessel forms a peri-intestinal ring, of a middle region of great extent, in which the longitudinal muscle-layer is internal to the circular instead of the reverse as is usual, and of a short rectum. In the Echiurids, Bonellia, Thalassema, Echiurus, and possibly Hamingia, a siphon, or collateral intestine, comparable to the structure seen in some Echinoids, extends for the greater part of the length of the middle region (supra) on its ventral aspect. A band of longitudinal muscle-fibres underlies the siphon. .The inner aspect of the pharynx is armed in Priaptdus and Halicryptus with numerous teeth, analogous in structure with the spines of the body; the cuticula lining it is fairly thick, as is that of the rectum, and both alike are shed when the outer cuticle is cast off.
The tract is ciliated throughout in Echiuridae, and there is a ventral furrow with especially long cilia in its middle region. So> too, in Sipunculus nudus, according to Keferstein. A delicate cuticle lines the tract in Priapididae. Thalassema has a ventral caecum at the end of the middle region, which has been found filled with a gelatinous substance in Th. erythrogramnion. The rectum in the same genus is dilated terminally, and a layer of peculiar cells underlies its peritoneal epithelium. A caecum is sometimes present at the commencement of the rectum in Sipunculids, and tufts of branched caeca close to the anus. But in Echiuridae with some exceptions (Saccosoma, Epi'thetosoma), there is always a pair of anal caeca, the branchiae of Greeff, or posterior nephridia appended to the rectum and opening into it. They have muscular walls, and communicate with the coelome by means of ciliated funnels with short stalks. In Bonellia viridis and Hamingia, they bear a number of branched stems, each branch ending in a funnel. The branched stems are replaced by simple stems in B. minor. Scattered ciliated cells occur in the epithelium of the caeca in Echinrus Pallasii and Thallasema erythrogrammon.
It is possible that these structures serve two functions, (i) the removal of an excess of water from the coelome, (2) the formation of excretory products by the epithelium of the caeca themselves1. Large granules are found in their lining cells in B. minor, and coloured drops or granules are massed in the groups of larger cells observable in Echiurus Pallasii.
1 There is a space between the two sheaths of the ventral nerve-cord in Sipunculus nudus, filled in prepared specimens by a coagulum with nuclei and pigment clumps in suspension, but no vascular elements. Hence it is not a blood-vessel (Andreae). A vessel or blood-space is said by Greeff to inclose the nerve-cord of Echiurus, but Spengel, who investigated the same animal, did not find anything of the kind, and professes himself unable to explain Greeffs figures. In addition to the facts given in the text, Spengel states that in Echiurus transverse nerves pass across the proboscis from one side of the nerve-ring to the other, and that in the young animal the cord has a ganglionated aspect, but the interganglionic gaps are irregular. See p. 624 post.
The wart-like papillae which cover the dorsal aspect at the posterior extremity of the body in Priapulus caudatus are glandular, according to both Apel and Scharff.
A vascular system is absent in Priapulidae and a few genera of Sipunculidae and Echiuridae. When present it is a closed system. In the Sipunculidae it usually consists of a dorsal contractile vessel closely applied to the fore-part of the digestive tract; but in some species of Sipunculus, e. g. S. nudus, a second similar vessel is applied to the ventral aspect of the same region. There is in many Sipunculids a peripharyngeal vascular ring or plexus. Vessels from the ring or plexus enter the tentacles, but in the lobed tentacles of Sipnnculus they are replaced by a plexus. Branches have also been observed passing from the dorsal vessel to the rectum in some cases. Caeca, simple or slightly branched, may be appended to the dorsal vessel in a single or double row. Ciliated cells are found here and there in the epithelial lining of the vessels, or a band of ciliated cells in the dorsal vessel. In the Echiuridae, starting from a peri-intestinal ring or sinus (supra, p. 619), a vessel courses along the dorsal surface of the oesophagus and pharynx to the apex of the prostomial lobe.