The perivisceral fluid is of a dark brown colour in Thalassema, containing numerous spherical corpuscles deeply impregnated, according to Lankester, with haemoglobin, and also containing granules of a brown pigment. Haemoglobin is also found in certain of the muscles and in part of the epithelial lining of the body-cavity. Lankester also describes the presence of haemoglobin in the corpuscles of the perivisceral fluid in Hamingia.
The genital glands are, like those of the Sipunculoidea, formed by a special development of the cells lining the body-cavity. These cells are massed together along the wall of the ventral blood-vessel. In Echiurus and in Thalassema the cells break off and float in the body-cavity, developing into ova and spermatozoa. In Bonellia each cell does not become an egg, but a mass of cells breaks off, one of which increases in size at the expense of the others and forms the ovum. The mature sexual cells leave the body through the nephridia.
Fig. 222.—An adult male Bonellia viridis Rol. The original was 1.5 mm. long. The nervous system is not shown. (After Selenka.). a, Generative pore with spermatozoa coming out; b, anterior blind end of intestine attached to the parenchymatous tissue by muscular strands; c, green wandering cells containing chlorophyll; d, parenchymatous connective-tissue; e, epidermis; i, intestine; j, vas deferens; l, internal opening of vas deferens; m, the left anal vesicle; n, spermatozoa in the body-cavity.
Bonellia and Hamingia present very interesting cases of sexual dimorphism. In both genera the female is an animal of considerable size with the normal structure of the Echiuroidea, but the male (Fig. 222) is a microscopic Planarian-like animal, which lives in the mouth and in the nephridia of the female. Both in Bonellia[[486]] and in Hamingia the male is provided with a pair of hook-like ventral bristles; these are wanting in the female Hamingia. The surface of the male is ciliated, and the skin contains circular and longitudinal muscle-fibres. The body-cavity is developed, but does not reach to either end of the body. The alimentary canal is closed, neither mouth nor anus existing; it is supported by regularly arranged dorso-ventral muscle strands. A nerve-ring and a ventral cord exist. There are also two rudimentary organs corresponding with the anal vesicles of the female, and a single nephridium which acts as a duct for the spermatozoa; the latter arise from modified cells lining the body-cavity.
In both sexes the larvae develop to a certain stage without showing any trace of sexual differentiation, but after this stage, the development of the male is to a certain extent arrested; in some respects, indeed, it undergoes retrogressive changes. At this time it is found clinging to the proboscis of the female, thence it makes its way to the mouth, where it undergoes its final change; and then creeping out, finds its way into the nephridium of the female, and spends the rest of its life there in a special recess cut off by a fold from the excretory part of this organ. In Hamingia, however, Lankester, who first described the male, did not find any in the nephridia, but found five specimens, each 1⁄12 inch long, within the dilated pharynx of the female.
Development.—In Bonellia and Hamingia it seems probable that the ova are fertilised in the nephridium of the female; in the other genera they are fertilised in the water after leaving the body of the mother.
In Thalassema and Echiurus the growth of the embryo results in the formation of a typical Trochosphere larva, a type widely spread in the animal kingdom, being found in the Chaetopoda (Fig. 145, A), Polyzoa (p. [510]), and Mollusca. The large prae-oral lobe persists in the Echiuroidea as the proboscis; the mouth is ventral in position, with usually a ring of cilia encircling the body in front of and behind it; the anus is posterior and terminal. A pair of larval excretory organs are present, and a special nervous aggregation of cells at the apex of the prae-oral lobe is usually indicated by the presence of a bunch of long cilia.
The trunk of the Trochosphere is unsegmented, and in certain groups of animals it remains so, but in Chaetopods, and in Echiurus and Thalassema, it elongates and becomes divided up into a series of somites or segments. Of these there are fifteen in Echiurus, and apparently eleven in Th. mellita; in this stage the Gephyrean larvae have again so close a resemblance to the segmenting Chaetopod larvae as to be easily mistaken for them. The segmentation is shown in the following way: (i.) the middle layer of cells or mesoblast is typically segmented, and forms septa, which separate each segment from its neighbours; (ii.) the ventral nerve-cord arises as segmentally-arranged thickenings of the epiblast, which fuse together, but retain their segmented appearance for some time; (iii.) the skin shows the segmentation of the body both by the arrangement of the pigment and by bands of cilia. The latter are replaced in the adult by rows of spines, and on the fourteenth and fifteenth segments in Echiurus pallasii by the two peri-anal circles of bristles. Each bristle, like those of Chaetopods, originates from a single cell.
The anal vesicles arise quite late in the development; when they have acquired their openings into the body-cavity, they seem to take in water. In Thalassema, as described by Conn, this is accompanied by remarkable changes, amounting almost to a metamorphosis. The body increases in bulk fourfold, the cilia of the prae-oral ring disappear, and the animal now moves only by means of its proboscis; the pigment is absorbed, and all traces of segmentation disappear. A similar intaking of water is described by Spengel in Bonellia. In this genus the larva, which is coloured bright green, and has two brown eye-spots, is not such a typical Trochosphere as is that of Echiurus and Thalassema.