Here it bifurcates; a branch lies on either side of the prostomium, surrounds the pharynx, and then the two fuse and form a ventral supra-neural vessel which ends blindly at the posterior end of the nerve-cord. A neuro-intestinal vessel connects the ventral vessel to the peri-intestinal ring. The origin of the dorsal vessel is sometimes dilated, and its walls muscular. Contractions were observed in it by Greeff2.
The coelomic fluid is sometimes colourless, but milky, as in Priapulus and Halicryptus, sometimes coloured pink or red. Amoeboid corpuscles are always to be found in it, and appear to be the sole kind possessed by Echiurus. Spherical or biconcave cells coloured, or, as in Priapulus and Halicryptus, colourless, may be present as well. There appear to be no less than three blood pigments occurring in Gephyrea; a brownish pigment in small clumps in the amoeboid cells of Echiurus, and the spherical corpuscles of Thalassema Neptuni; haemoglobin in the spherical corpuscles of the last named, and in Hamingia arctica; a pink or red pigment not yielding any absorption-bands, but the colour of which is intensified by contact with air or oxygen in the biconcave discs of Sipunculus nudus; it is termed haemerythrin by Krukenberg. The pink pigment contained in similar corpuscles in other species of Sipunculus, in some species of Phas-colosoma, and of Bonellia minor, has not been investigated spectrosco-pically 1. The coelomic fluid of Sipunculus and Phascolosoma also contains remarkable structures moving freely in it - the 'Topfchen' - which are shaped like a bell, with a ciliated mouth. They appear to be coelomic cells detached from some portion of the alimentary canal, etc.
The liquid in the blood-vessels is corpusculated, and the corpuscles resemble those of the coelome in Sipunculus nudus and Echiurus. The 'Topfchen' are said to occur in it in the former (Brandt). In Thalassema Neptuni, however, it is colourless, and does not contain 'corpuscles similar to those of the coelome' (Lankester).
1 The cilia of the funnels are said to cause a current solely outwards from the coelome in Bonellia viridis (de Lacaze Duthiers). The asserted absence of funnels in some instances is probably due to the examination of specimens preserved in spirit without previous preparation. The anal caeca in question probably acquire their connection with the rectum. See account of male Bonellia and the development of Echiurus, p. 623, and p. 624. In this case they are to be regarded as homologous serially with the anterior nephridia described further on.
A Danielssen and Koren ascribe to Hamingia arctica a vascular system differing in some respects from the account given in the text; but Horst finds in H. glacialis (probably the same species) a system wholly conformable to it.
With the exception of the Priapulidae all Gephyreans possess organs known as genital or uterine pouches, segmental organs, or anterior ne-phridia, which open outwards anteriorly on the ventral surface. The genus Bonellia possesses but a single organ, as is sometimes the case in Hamingia arctica; so too Saccosoma, Epithetosoma, and tubicolous Sipuncididae2. Other Sipunculidae, and H. arctica as a rule, possess a single pair, Echiurus two, the various species of Thalassema one, two, three, or four pairs, one behind the other. The external apertures of these organs lie one on either side of the ventral nerve-cord, and behind the ventral hooks in Echiurids; but when three pairs are present, the first usually opens in front of and a little laterally to those structures (Lampert). The walls of the organs are muscular, and except at the period of sexual activity, when they are much distended with sperm or ova, they are small in size. Internal openings into the coelome have not been found in all Sipunculids. In others, and in Echiurids, they are present.
In Sipuncididae they may be situated on the wall of the sac, close to the outer opening (Aspidosiphon fuscus, several species of Sipunculus including S. nudus, Phascolion Strombi); or near its apex (Sipunculus edulis, Phascolosoma nigritorquatus, Ph. falcidentatus), and are either funnel-shaped or bilobed. They are always anterior, near the external aperture in Echiuridae, and are either simple funnels (Bonellia, Hamingia ?), bilabiate funnels (Echiurus Pallasii, Thalassemia Neptuni, &c), or they are drawn out on each side into a long gutter, which is twisted into a spiral (Echiurus unicinctus, many species of Thalassemd). The funnels are ciliated in the Echiuridae, and there can be little doubt that such is the case in Sipuncididae also. The homology of these organs with nephridia is undoubted, but it is by no means a settled fact that they are excretory in function.
1 The brown pigment of Echiurus is probably identical with that occurring in other parts of the body, e. g. in the walls of the anal caeca. The pink pigment of Sipunculus nudus tinges, according to Ray Lankester, the sheath of the nerve-cord and a band underlying the alimentary canal. The same authority states that in Th. Neptuni haemoglobin is present in the muscles of the middle region of the body, in the thick coelomic epithelium covering the mesenterial bands of the alimentary canal and of the anterior nephridia. Orange-red granules are plentiful in the same animal in the coelomic epithelium of the vessels, of the sheath of the nerve-cord, of the median ventral line of the intestine, of the four anterior nephridia, and the anal caeca. So too in Echiurus Pallasii. Sluiter states that the alcohol in which a specimen of Th. erythrogrammon was preserved gave the spectrum of haematin, indicating the existence of haemoglobin in the living animal.
2 De Lacaze Duthiers found a pair of organs in a single specimen of B. viridis. See p. 74.
A. Sc. "N. (4), x. 1858. The single organ is usually that of the right side in B. viridis, of the left in
The sexes are said to be separate, and are undoubtedly so in most cases, the possible exceptions being among Sipuncididae 1. The genital organs of the Priapulidae consist of two ducts, one right, the other left, opening posteriorly near the anus, and attached by a ventral mesentery to the body-walls. The lobulated genital gland lies on either side this mesentery in the female, but nearly surrounds the duct in the male. The main duct gives off lateral branches which divide repeatedly, and in the female anastomose. The ova take their origin from isolated cells of the epithelium, which grow at the cost of their neighbours; the sperm from the epithelium at the base of short caeca appended to the canals. In all other Gephyreans the genital products are found floating in the coelome. In the Sipuncididae little is really ascertained as to the genital glands. In Sipunculus edulis and Phascolosoma nigritorquatus the ovaries are small sausage-shaped bodies, placed in a row between the dorsal retractor muscles of the pharynx, attached to them and to the alimentary canal. They consist of fibrous sacs, which are eventually burst by the growing cells they contain. The testes are not known.