The Anatomy and the Histology of Bud-formation in the Serpulid Filograna implexa, together with some Cytological Observations on the Nuclei of the Neoblasts.*

Data are given which show that in Filograna there is much variation in many of the external features, including those which are supposed, according to some authors, to distinguish Filograna from Salmacina. Hence it is concluded that these two forms are not separate genera, but should be both included in the genus Filograna. The course of the ciliary currents in the tube is as follows:—Water enters the posterior end of the tube; when it reaches the thorax it is all directed dorsally by the thoracic membrane and passes out of the tube by the dorsal portion of the anterior aperture. The branchial (feeding) current collects from a more ventral field. (Text-fig. 2.) The longitudinal muscles are well developed. Their anterior subdivisions form the muscles of the collar and branchiæ. Circular muscles are visible in the achætous (i. e., post-thoracic) zone only. It is suggested that these act as a compression-pump on the peri-enteric sinus and drive the blood in this forwards into the branchial filaments to expand these—or at least that they maintain the blood there under pressure. The gut-musculature consists of a sheath of circular fibres surrounding all the pre-abdominal region of the canal. In connection with the uncini there is an interesting musculature mechanism—a layer of turgid vacuolated cells in the parapodial wall is arranged to act as a passive spring-like opposition to the retractor muscles of the uncini. (Text-fig. 8.) The central nervous system is of a primitive Annelid type. There are two ventral cords widely separate, in close contact with the epidermis. Each branchial filament is supplied with two nerves, one internal or mesial, and the other external or lateral: the latter bifurcates in each filament. (Text-fig. 5) The blood-spaces are of the nature of open sinuses, except anteriorly, where true tubular vessels are present. The main peri-enteric sinus surrounds the gut from the hind end of the proboscis to the anus, and is a continuous uninterrupted cavity. The blood in it spreads into the mesenteries and dissepiments, giving rise to dorsal, ventral, and inter-segmental channels: these are open canals with no intrinsic epithelium. The blood in them spreads in an extremely thin film over the basement-membrane of the epidermal cells. Anteriorly there is an annular vessel round the proboscis, from which branchial and collar vessels arise. The walls of the vessels and sinuses are formed by the membranous peritoneum only. Movement of the blood is effected by anti-peristaltic contractions of the thoracic gut-muscles and by movements of the body : the circular muscles in the body-wall probably aid directly. The form of the blood-system is considered to support the hypothesis that the cavity of the blood-system in Annelids represents (or has been derived from) the blastoc'l of the embryo. One pair only of excretory organs is present, lying in the collar-somite, and having the form of simple S-shaped tubes. The mesial ends of these unite to form a common terminal duct which passes dorsally through the prostomium, and opens terminally. Internally there is a granular enlargement on the tubes—the glandular sac; a pore places the cavity of the “sac” in communication with the cœlom of the somite anterior to the collar, and hence is a nephrostome. A large flame (200 μ in length) arises near the nephrostome and beats into the anterior cœlom; the cilia in the excretory duct are also long and flame-like. Buds are abundant during spring and early summer. Sexual individuals are mature and buds rarer during summer and autumn. A stock bearing a bud is generally asexual, and the bud itself is always asexual. The eggs develop in the calcareous tubes of the parent until the embryo is almost ready to settle. Embryos developed during early summer enter a sexual phase in the same season, those produced later reach their first reproductive phase (asexual) next spring. Individuals last more than one year, hence after a sexual phase there is a return to asexual reproduction in the next spring. In the formation of a bud, the anterior region, including branchiæ, prostomium, collar-somite, and the next succeeding somite is formed by an outgrowth of new tissues at the plane of fission. The original stock-somites posterior to this are transformed into bud-somites, the anterior six or seven being transformed from abdominal into thoracic segments. The interior of a bud is filled with opaque segmental blocks, composed of active phagocytes and embryonic cells. These persist for some time after the bud is freed, gradually diminishing in size and in density. The stock regenerates the posterior end after the bud has been freed, and occasionally the regeneration begins while the bud is still attached. The position of the plane of fission on the abdomen of the stock is not constant. There is a tendency for it to divide the abdomen, whatever the length of this, into two more or less equal halves. After a sexual phase (in which the abdomen is very long) the next bud produced is correspondingly long. At other times it frequently happens that the abdomen divides when it contains twice as many segments as the thorax. There are many variations of these types of configuration, and many irregular and exceptional cases. Two abnormal buds are described in each of which a bud-head is being produced terminally and posteriorly. It is further peculiar that in both the bud is attached to one half only of the stock. The production of a bud entails a complete histological disintegration and regeneration of bud-tissues, and is accompanied by a similar but much less complete process of rejuvenation of stock-tissues. The cells concerned in the phagocytosis and replacement of tissues are derived from neoblasts in the ventral body-wall. The cells produced by these neoblasts give rise to ectodermal, endodermal, and mesodermal structures indifferently, and are probably identical with the cells which are responsible for the growth in length of all the tissues of the animal at posterior growing-point, independently of budding phenomena. Neoblast, s are identical wit, h the sex-cells (i. e., oögonia and spermatogonia). Nuclei of neoblasts in the resting-stages possess definite chromosomes (pro-chromosomes) arranged in pairs in an otherwise perfectly clear vesicle. In every division the neoblasts pass through a contraction phase which strongly resembles the “synesesis” phase seen in gametogenesis, and is either preceded by or followed by a synaptic union, during which the individuals of a pair are so intimately associated that no line of division can be seen between them.