A Review of Professor Spengel's Monograph on Balanoglossus.1 by E

REVIEW OF SPENGEL'S MONOGRAPH ON BALANOGLOSSDS. 385 A Review of Professor Spengel's Monograph on Balanoglossus.1 By E. W. MacBridc, B.A., Fellow of St. John's College; Demonstrator in Animal Morphology to the University of Cambridge. With Plates 29 and 30. THE eighteenth monograph of the Naples series lies at length before us. As a monument of patient industry and a mine of anatomical facts it stands in the foremost rank of zoological treatises. It contains minute descriptions of all the known species of Balanoglossus, as well as complete dis- cussions, not only as to the relationship of this interesting form to various phyla of the animal kingdom, but also as to the special morphology of every organ in the body of Balanoglossus. As the conclusions at which Professor Spengel has arrived are at variance with the views as to the relationship of the Enteropneusta which have been widely held in England for the last ten years, it seemed to me that a short account of the principal additions to our knowledge contained in the monograph might be useful to zoological students who lack the time necessary to peruse so huge a tome as the one before us. I shall append to the account of the new facts brought to light a short discussion on Professor Spengel's views as to the phylogenetic position of the Enteropneusta, as it seems to me that he has adduced nothing which militates in the slightest 1 "Die Bnteropneusten des Golfes vou Neapel und der angrenzenden Meeresabscbnitte," J. W. Spengel, 'Series of Naples Monographs,' No. 18. 386 E. W. HAOBRIDE. against the view now so widely accepted as to the Vertebrate affinities of the group. Before entering on the new points contained in the monograph it will be necessary to give a rapid sketch of the history and the extent of our present knowledge of the anatomy and development of Balanoglossus. Kowalevsky1 was the first who gave a tolerably correct account of the anatomy. Our knowledge on this point was almost completed by Professor Spengel's2 preliminary account in 1884. Between the years 1884 and 1886 Bateson3 contributed a series of papers on the development and morphology of Balanoglossus, which laid a solid foundation for the interpretation of the anatomical facts as indicative of Vertebrate affinities. Quite recently Morgan* has published a paper on the growth and development of Tornaria, which entirely confirms Bateson's account, making allowance for the difference between direct and free larval development; and Marion,5 Schimkewitsch,6 and Ktihler7 have given during the last few years descriptions of different species. Fig. 1, PI. 29, is a diagram representing a horizontal section through an adult. We notice that there are three regions to be distinguished in the body, viz. the praoral lobe or proboscis, the thick " collar" region, and the trunk. The proboscis region is largely filled with muscle, but it also contains a small coelomic space opening on the left by a ciliated pore. The collar and trunk both contain a pair of such spaces, and the I "Anatomie des Balanoglossus delle Chiaje," par A. Kowalevsky, in 'Mem. Acad. Imp. Sc. St. P&ersbourg,' 1866. 3 J. W. Spengel, "Zur Anatomie des Balanoglossus," 'Mitt d. Zool. Station z. Neapel,' Bd. v, 1884. 3 Bateson, " Development of Balanoglossus," various papers, ' Quart. Journ. Micr. Sci.,' 1884—1886. 4 T. H. Morgan, " Growth and Development of Tornaria," ' Journal of Morph.,' vol. v, 1891. » A. P. Marion, " Sur deux especes de Balanoglossus," ' Arch. Zool. Exp.' (2), Tome iv. II Sohimkewitsch," Ueber Balanoglossus Meresclikovski,"' Zool. Anz.,11888. 7 B.. Kohler, " Contributions a l'Etude des Ent6ropneustes," ' Inter. Monatsschrift Anat. Hist.,' Bd. iii, Heft 4. REVIEW OF SPENGEL's MONOGRAPH ON BALANOGLOSSUS. 387 collar coelom on each side opens by a short ciliated canal into the outer part of the first gill-slit. Projecting into the proboscis coelom from behind we observe a spongy mass of tissue with blood-vessels, Bateson's proboscis gland, which is without doubt an excretory organ. Posterior to this again, is a vesicle (Pc), the sac of the proboscis gland, immediately behind which we see the heart. This last communicates by a narrow slit with the great dorsal vessel. The alimentary canal commences just •posterior to the proboscis with a ventral mouth ; its first portion in the collar region constitutes a simple buccal cavity; the next part in the anterior part of the trunk bears a series of pairs of gill-sacs opening to the exterior by pores situated dorso- laterally. Immediately ventral to these openings we see the openings of the gonads, the sexual glands in both sexes being represented by a series of simple or branched sacs opening directly to the exterior. Fig. 2 on the same plate represents a diagrammatic sagittal section of the animal. We notice that the alimentary canal in the trunk region is differentiated into two tubes—a lower one called the oesophagus (sensu stricto), and an upper one, the branchial tube, into which the gill-sacs open. We notice also that the inner openings of these are elongated vertically, and almost completely divided by a projection from their dorsal edge, the tongue-bar \ the outer gill-pores (Brex.), on the con- trary, are simple circular openings. In the collar region, dorsal to the blood-vessel, we see a longitudinal band of tissue connected before and behind with an invagination of the ectoderm. This is the central nervous system, and it is connected with the dorsal ectoderm by three cellular strands, the dorsal roots. In its interior are a series of small isolated cavities lined by columnar cells. In the front part of the collar region a dorsal divei ticulum of the pharynx is seen reaching far forward into the base of the proboscis, and underlying the heart and sac of the proboscis gland. This is the notochord; it has a cuticular sheath, which is largely thickened, ventrally giving rise to a chitinous skeletal rod. A portion of this rod is secreted by the inner ends of the ectoderm cells on the base of the proboscis; be- VOL. 36, PART 3. NEW SEE. D D 388 E. W. MACBBIDB. tween the ectodermic and notochordal portions a wedge of tissue projects—really the modified tip of the ventral pocket of the proboscis coelom (Pr. cce. v.) ; Kdhler, however, mistook it for a blood-vessel, and described the skeletal rod as completely divided into two by vascular tissue. In fig. 2 the great ventral vessel is seen as well as the dorsal. Pig. 3 is a transverse section through the base of the proboscis, showing the mutual relations of heart, notochord, proboscis gland, and proboscis coelom. Fig. 4 is a transverse section through the collar region, the- main point to notice being that the dorsal vessel has on each side of it a space half filled with muscular tissue—the peri- hsemal cavity {PH.). This, as can be seen in Fig. 1, is a forward diverticulum of the trunk coelom. Fig. 5 is a transverse section through the branchial region of the trunk. The figure-of-8 form of the alimentary canal is well seen, the upper part being the branchial tube, the lower the oesophagus. Where the one passes into the other there is on each side a ridge of thickened epithelium, and in life these two ridges are closely apposed, entirely separating the two tubes. The tongue-bar is seen on each side of the branchial tube in longitudinal section, and is seen to contain a portion of the trunk coelom. On each side of the dorsal surface is seen a slight projection—the genital wing—in the proximal part of which the gonad opens on each side. Sections of the dorsal and ventral vessels are seen, and also of the dorsal and ventral nerve-cords. The whole ectoderm is ciliated, and has a well- developed plexus of nerve-fibrils amongst the bases of its cells. The dorsal and ventral nerve-cords are mere local thickenings of this plexus: the latter is confined to the trunk region; the former, however, passes into the posterior end of the central nervous system through the posterior ectodermic pit. Just behind the collar a ring of nervous matter puts the dorsal and ventral nerve-cords in communication with each other. The nerve-plexus is well developed in the proboscis region, and is especially thick round the narrow neck by means of which the proboscis joins the collar, constituting a kind of anterior nerve-ring. REVIEW OF SPENGJSL'S MONOGRAPH ON BALANOGLOSSUS. 389 The development has been worked out by Bateson as men- tioned above, the main points observed being the following:— The egg undergoes total and regular segmentation, and forms a one-layered blastophere. Regular embolic invagination fol- lows, giving rise to a gastrula, and the blastopore is situated posteriorly and slightly dorsally. It closes in the position about where the future anus is formed. The archenteron gives rise to an anterior and to two lateral pairs of pouches, as shown in fig. 6; the former becomes proboscis coelom, the latter the collar and trunk coelomic cavities. The notochord arises partly as an anterior diverticulum of the gut; its posterior part, however, is constricted off as a dorsal groove of the buccal cavity. The central nervous system is separated from the ectoderm by delamination, but is added to anteriorly and posteriorly by fore and aft invaginations from the anterior and posterior surfaces of the collar. The species on which Bateson worked develops up to a comparatively advanced stage within the egg-membrane, and hence its external larval cha- racteristics have been very much modified.

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