Fig. 2.—Structure of branchial region.
dn, dorsal nerve.
vn, ventral nerve.
Gill-slits.—The possession of gill-slits is as interesting a feature in the organization of Balanoglossus as is the presence of tracheae in Peripatus. These gill-slits occupy a variable extent of the anterior portion of the trunk, commencing immediately behind the collar-trunk septum. The branchial bars which constitute the borders of the clefts are of two kinds:—(1) Septal bars between two contiguous clefts, corresponding to the primary bars in Amphioxus; (2) Tongue-bars. The chief resemblances between Balanoglossus and Amphioxus in respect of the gill-slits may be stated briefly as follows:—(α) the presence of two kinds of branchial bars in all species and also of small crossbars (synapticula) in many species; (β) numerous gill-slits, from forty to more than a hundred pairs; (γ) the addition of new gill-slits by fresh perforation at the posterior end of the pharynx throughout life. The chief differences are, that (a) the tongue-bar is the essential organ of the gill-slit in Balanoglossus, and exceeds the septal bars in bulk, while in Amphioxus the reverse is the case; (b) the tongue-bar contains a large coelomic space in Balanoglossus, but is solid in Amphioxus; (c) the skeletal rods in the tongue-bars of Balanoglossus are double; (d) the tongue-bar in Balanoglossus does not fuse with the ventral border of the cleft, but ends freely below, thus producing a continuous U-shaped cleft. The meaning of this singular contrast between the two animals may be that we have here an instance of an interesting gradation in evolution. From serving primitively as the essential organ of the cleft the tongue-bar may have undergone reduction and modification, becoming a secondary bar in Amphioxus, subordinate to the primary bars in size, vascularity and development; finally, in the craniate vertebrates it would then have completed its involution, the suggestion having been made that the tongue-bars are represented by the thymus-primordia.
Gill-pouches and Gill-pores.—Only rarely do the gill-slits open freely and directly to the exterior (fig. 1). In most species of Balanoglossus each gill-slit may be said to open into its own atrial chamber or gill-pouch; this in its turn opens to the exterior by a minute gill-pore. There are, therefore, as many gill-pouches as there are gill-slits and as many gill-pores as pouches. The gill-pores occur on each side of the dorsal aspect of the worm in a longitudinal series at the base of a shallow groove, the branchial groove. The respiratory current of water is therefore conducted to the exterior by different means from that adopted by Amphioxus, and this difference is so great that the theory which seeks to explain it has to postulate radical changes of structure, function and topography.
Excretory and Vascular Systems.—It seems likely that the coelomic pore-canals were originally excretory organs, but in the existing Enteropneusta the pore-canals (especially the collar canals) have, as we have seen, acquired new functions or become vestigial, and the function of excretion is now mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom. The vascular system itself is quite peculiar, consisting of lacunae and channels destitute of endothelium, situated within the thickness of the basement-membrane of the body-wall, of the gut-wall and of the mesenteries. The blood, which is a non-corpuscular fluid, is propelled forwards by the contractile dorsal vessel and collected into the central blood-sinus; this lies over the stomochord, and is surrounded on three sides by a closed vesicle, with contractile walls, called the pericardium (Herzblase). By the pulsation of the pericardial vesicle (best observed in the larva) the blood is driven into the glomerulus, from which it issues by efferent vessels which effect a junction with the ventral (sub-intestinal) vessel in the trunk. The vascular system does not readily lend itself to morphological comparison between such widely different animals as Balanoglossus and Amphioxus, and the reader is therefore referred to the memoirs cited at the end of this article for further details.
Reproductive System.—The sexes are separate, and when mature are sometimes distinguished by small differences of colour in the genital region. Both male and female gonads consist of more or less lobulated hollow sacs connected with the epidermis by short ducts. In their disposition they are either uniserial, biserial or multiserial. They occur in the branchial region, and also extend to a variable distance behind it. In exceptional cases they are either confined to the branchial region or excluded from it. When they are arranged in uniserial or biserial rows the genital ducts open into or near the branchial grooves in the region of the pharynx and in a corresponding position in the post-branchial region. An important feature is the occurrence in some species (Ptychoderidae) of paired longitudinal pleural or lateral folds of the body which are mobile, and can be approximated at their free edges so as to close in the dorsal surface, embracing both the median dorsal nerve-tract and the branchial grooves with the gill-pores, so as to form a temporary peri-branchial and medullary tube, open behind where the folds cease. On the other hand, they can be spread out horizontally so as to expose their own upper side as well as the dorsal surface of the body (fig. 1). These folds are called the genital pleurae because they contain the bulk of the gonads. Correlated with the presence of the genital pleurae there is a pair of vascular folds of the basement membrane proceeding from the dorsal wall of the gut in the post-branchial portion of the branchio-genital region, and from the dorsal angles made by the pleural folds with the body-wall in the pharyngeal region; they pass, in their most fully developed condition, to the free border of the genital pleurae. These vascular membranes are called the lateral septa. Since there are many species which do not possess these genital pleurae, the question arises as to whether their presence or their absence is the more primitive condition. Without attempting to answer this question categorically, it may be pointed out that within the limits of the family (Ptychoderidae) which is especially characterized by their presence there are some species in