Fig. 221.—View of a female Bonellia viridis Rol., opened along the left side, × 2. a, Proboscis cut short; b, a bristle passed through the mouth into the pharynx; c, convoluted intestine; d, anal tufts or vesicles; e, ventral nerve-cord; f, ovary borne on ventral vessel running parallel with e; g, position of anus; h, points to position of external opening of nephridium; i, nephridium. This line is on a level with the internal funnel-shaped opening.
The green colour of B. viridis is due to a special pigment, "Bonellein," which at one time was thought to be identical with chlorophyll. A similar green colour is found in Hamingia arctica, Thalassema baronii, and the larvae of many forms.
A short distance behind the mouth, on the ventral surface, the female Bonellia and both sexes of Thalassema and Echiurus bear two incurved stout chitinous hooks; these gave the name Gephyrea Armata to the above-mentioned genera. In addition to these, Echiurus has a row of chitinous bristles surrounding the posterior end of the body; the row is single in E. unicinctus, double in E. pallasii. These bristles are formed, like the hooks on the introvert of the Sipunculoidea, by epidermal cells; those of B. minor and of the posterior rings in Echiurus are said to arise each from a single cell, just as the bristles do in Chaetopods.
The skin consists of very much the same layers as does that of Sipunculus; the cuticle is thin, the epidermis is modified into numerous glandular cells, papillae, and pits, from which the bristles arise. A third layer of oblique or circular fibres is usually found inside the longitudinal muscle-layer. The proboscis is solid, and contains much connective-tissue and numerous muscle-fibres running in all directions; the ventral groove is ciliated.
The alimentary canal in the Echiuroidea consists of a long thin-walled tube with numerous convolutions; it is not coiled as in Sipunculids, but the loops are irregularly arranged, and are supported by numerous fine muscular strands which run from the skin. There is a ciliated groove running along one side of the intestine, as in the Sipunculids. The anus is terminal. The most striking peculiarity of the alimentary canal of the Echiurids is the existence of a collateral intestine or "siphon." This is a narrow tube which arises from the main canal not very far from the mouth, and re-enters it again lower down. A similar structure occurs in some Echinids, and in the Capitelliformia (pp. [272], [305]). Its function is not certainly known.
Another characteristic feature of the Echiurids is the presence of "anal vesicles," branching structures which unite into a common stem opening into the intestine close to the anus. The free end of each of the branches terminates in a ciliated funnel-shaped opening. The function of these structures may be excretory, or they may control the amount of fluid in the body-cavity.
A closed vascular system exists in Echiurids, consisting of a contractile dorsal vessel running along the dorsal surface of the anterior end of the alimentary canal, and continued along the axis of the proboscis. At the tip of the proboscis it bifurcates, and each branch descends along the edge until it reaches the base where, having encircled the oesophagus, the two unite, and are continued as the ventral vessel which runs along the dorsal surface of the nerve-cord, and eventually ends blindly. There is also a vessel which passes from the ventral vessel and encircles the intestine, opening into the posterior end of the dorsal vessel. In Echiurus the same vessel encircles a stout muscle which runs from the base of one of the ventral bristles to the other. In Thalassema Lankester states that the fluid within the vessels is colourless, and does not contain corpuscles similar to those in the body-cavity fluid.
The "brown tubes" or nephridia vary in number in the Echiurids. In the female Bonellia there is but one; in B. viridis the right, in B. minor the left usually persists. In shape, colour, contractility, and minute structure they closely resemble those of Sipunculus. Hamingia is said to have a pair of brown tubes; Echiurus has two pairs, except E. chilensis, which has three; their internal openings are produced into long coiled slits in some genera. Thalassema gigas has one pair; Th. neptuni, Th. baronii, Th. formosulum, and Th. exilii, two; whilst Th. vegrande, Th. moebii, Th. erythrogrammon, Th. caudex, and Th. sorbillans have three pairs.
The nervous system consists of a ventral cord lying in the body-cavity, as in the Sipunculoidea, but attached to the skin, and of a circumoesophageal ring. With the growth of the proboscis this ring is drawn out, and the two branches run along the sides of the proboscis and unite at the tip. There is no specialisation of brain, nor are any special sense organs present, but the ventral cord gives off paired nerves at regular intervals, which, uniting dorsally, form rings in the skin in some and probably in all species.