CONCERNING THE COURSE OF THE LATERO- SENSORY CANALS IN RECENT FISHES, PRE- FISHES AND NECTURUS By EDWARD PHELPS ALLIS, JR. Menton, France IN connection with work I have under way it seemed desirable to establish, in so far as the literature at my disposal would permit, the homologies of the latero-sensory lines on the cheeks of Fishes. It, however, soon became apparent that their homologies could not be definitely established until their develop- ment was much better known than it is at present. Certain tentative conclusions regarding their homologies and also those of certain parts of the other sensory lines were, however, arrived at, and they seemed sufficiently important to warrant publication. The descriptions of the sensory lines in a certain number of recent Fishes, and also those in Necturus and the Petromyzontidae, will therefore here be first reviewed as briefly as the purposes of this paper permit, and the canals in certain fossil Fishes and prefishes will then be considered. Nearly all of the figures given in illustration are purely diagrammatic, the main lines or canals alone being given without the related organs, pores and tubules. When a line or canal anastomoses in the median line with its fellow of the opposite side it is shown in contact with the outline of the figure. The lines on the dorsal and ventral surfaces of the head are shown as if visible in lateral view. The distinctive shadings of the several lines and the index-lettering employed are explained on page 415. PETROMYZONTIDAE The snout of these Cyclostomata is of viscero-trabeculo-cranial origin (Allis, 1932a) and there is no spiracle. The main-infraorbital latero-sensory line in the Ammocoetes of Petromyzon planeri is said by Alcock (1898) to be a straight and nearly horizontal line which forms a direct anterior prolongation of the lateral line of the trunk. It lies between the dorsal and ventral divisions of the musculus capitis lateralis, and running forward first crosses the external surface of the otic capsule and then passes superficial to the eye which still lies deep beneath the external surface, and beyond that continues onward toward the end of the snout. In later stages of development, as the eye pushes its way outward toward the external surface, that part of the main-infraorbital line that in earlier stages lay directly superficial to it shifts ventrally and ultimately acquires a suborbital 362 Edward Phelps Allis, Jr position, the parts of the line that lie posterior and anterior to the eye retaining approximately their primitive positions. The suborbital portion of the line, as thus described by Alcock, now appears to form a somewhat detached and independent portion of the entire line, and I thus find it in small adults of Entosphenus tridentatus (fig. 1). There is accordingly in these specimens no part of the main-infraorbital line that has the topographical position of the postorbital section of the infraorbital canal of the gnathostome Fishes. In Petromyzon dorsatus, however, the anterior end of that part of the line that lies posterior to the eye is shown by Johnston (1905) bending downward behind the eye and so acquiring a postorbital position. In Petromyzon planeri and Entosphenus tridentatus the suborbital and antorbital portions of the line are both innervated by branches of the ramus buccalis lateralis (Allis, 1931 b) and the postorbital portion by branches of what Johnston calls the ramus com- municans VII-X (VII-X anastomosis), the proximal portion of this nerve 10> '0). 10). Fit. _ t ~~~~.e , )--' Fig. 1. Entosphenus trzdentatus. Young adult. being considered by him to correspond to the ramus oticus of the gnathostome Fishes. The supraorbital line in both Petromyzon planeri and Entosphenus tridentatus is represented by one or two organs that lie on the dorsal surface of the head posterior to the single median nasal aperture and are innervated by the ramus ophthalmicus superficialis lateralis. Johnston, however, considers what corre- sponds in Petromyzon dorsatus to the anterior portion of the main-infraorbital of Petromyzon planeri and Entosphenus tridentatus to form an anterior division of the supraorbital and to be innervated by branches ofthe ramus ophthalmicus profundus, but this would seem to be incorrect, for the line lies lateral and hence morphologically ventral to the nasal capsule. No supratemporal line is described by either Alcock or Johnston, and I did not find it in Entosphenus tridentatus. According to both Alcock and Johnston there is, in the specimens examined by them, a ventral line of pit organs which begins at the ventral edge of the buccal opening and from there extends posteriorly to the hind end of the The Latero-Sensory Canals in Fishes 363 bronchial basket. The prebranchial portion of this line is said by both authors to be innervated by a branch of the ramus hyomandibularis facialis, but they differ as to the innervation of the bronchial portion. According to Alcock there is a latero-sensory nerve descending in each of the bronchial arches and innervating the related portion of the ventral line, while according to Johnston such a nerve descends in the glossopharyngeus arch to innervate the related portion of the ventral line, the more posterior portion of the latter line being innervated by a nerve that runs downward posterior to the bronchial basket and then forward ventral to it. Alcock says that there is in the diaphragm of each bronchial arch of Petromyzon planeri a number of sense organs that are innervated by a branch of the lateralis nerve that she finds running downward in each of these arches. Johnston says that in Petromyzon dorsatus these organs have the size, shape and general appearance of neuromasts, but that the sense cells are not furnished with terminal hairs. They are said by him to be innervated by communis fibres, and he accordingly considers them to be taste organs notwithstanding that they differ so markedly in size and shape from ordinary terminal buds. No line corresponding to the preoperculo-mandibular line of Fishes is described either by Alcock or Johnston in Petromyzon, and I did not find it in Entosphenus tridentatus. I, however, found in the latter cyclostome a line of these organs, not described by either Alcock or Johnston, that begins slightly ventral to the anterior end of the suborbital section of the main- infraorbital line and from there extends ventrally and slightly posteriorly and nearly reaches the anterior end of the ventral line. A similar line is described by Razzauti (1916) in Petromyzon planeri as the circumbuccal line, and I found it innervated in Entosphenus by what is actually a branch of the ramus buccalis. This line would thus seem both in innervation and general position to belong to the mandibular arch, this suggesting that it may be the serial homologue in that arch of the so-called hyomandibular line of Platt's descriptions of early embryos of Necturus, to be considered immediately below. NECTURUS In early embryos of Necturus three longitudinal lines of thickened ectoderm are said by Platt (1896) to develop on each side of the head. These lines give rise to ridges which she calls the dorso-lateral, epibranchial and ventral, the epibranchial ridge lying between the other two along the dorsal edges of the gill clefts. Later, a series of transverse so-called intersegmental ridges develop, each ridge extending primarily from the dorso-lateral to the ventral longitudinal ridge. Dorsal to the epibranchial ridge these intersegmental ridges are said to lie between successive somites, and two of these somites are said to lie dorsal to the hyomandibular cleft. Ventral to the epibranchial ridge, the intersegmental ridges lie superficial to interbranchial expansions of the gut that later acquire contact with the internal surfaces of the overlying intersegmental ridges. The bronchial clefts then break through along these 364 Edward Phelps Allis, Jr lines of contact and the related intersegmental ridges are broken down and dispersed, no latero-sensory organs developing in relation to them. The hyomandibular cleft does not break through, the related expansion of the gut losing its primarily acquired contact with the overlying intersegmental ridge and leaving the latter unbroken throughout its entire length. This ridge is called by Platt the hyomandibular, which because of the relation of the ridge to the hyomandibular cleft is eminently appropriate, but this same term, hyomandibular, is currently employed to designate, in the Elasmobranchii, certain totally different latero-sensory lines. The ventral end of the ridge anastomoses in Necturus with the ventral longitudinal ridge, and as the organs that later develop in relation to that part of the ventral ridge that lies anterior to this point of anastomosis are innervated by a nerve called by Platt the ramus hyomandibularis facialis, as are also the organs developed in relation to the so-called hyomandibular ridge, Platt includes both lines under the term hyomandibular. The so-formed line has the topographical position of the preoperculo-mandibular line of the Holostei and Teleostei, and also that of the spiracular-gular line of Garman's (1888) descriptions of Chlamydoselachus, which latter line I have considered to be the homologue of the preoperculo- mandibular line of the Holostei and Teleostei (Allis, 1928). There is, however, nothing in existing descriptions of Chlamydoselachus and the Holostei and Teleostei to indicate that the mandibular (gular) part of this line has been developed in relation to a pre-existing prebranchial portion of a ventral longitudinal line. Furthermore, the development of this line in Ceratodus, explained in the following section of this paper, would seem to establish that the ventral end of a line that corresponds to the so-called hyomandibular of early embryos of Necturus has simply swung forward and been prolonged into the mandible.