Studies on the Head in Fishes Embryological, Morphological, and Phylogenetical Researches

The lateral-line system of gars consists of superficial neuromasts, which are arranged in lines termed pit lines, canal neuromasts and spiracular organs, which are located within diverticula of the hyoid gill pouch. Both canal and superficial neuromasts possess polarized hair cells whose directional sensitivity parallels the long axis of their respective lines. However, the apical surfaces of canal neuromasts are larger and possess far more hair cells than do those of superificial neuromasts, but superficial neuromasts have longer kino-cilia and, presumably, longer cupulae. The receptors of the lateral-line system are innervated by three pairs of cranial nerves: anterior, middle and posterior lateral-line nerves. The anterior lateral-line nerves innervate neuromasts of the supraorbital, infraorbital and preoperculo-mandibular canals as well as dorsally located anterior pit lines, cheek (horizontal, vertical and mandibular) and gular pit lines of superficial neuromasts and the spiracular organ. The middle lateral-line nerves innervate dorsally located middle pit lines and a single neuromast in each temporal canal. The posterior lateral-line nerves innervate dorsally located posterior pit lines, neuromasts of the supratemporal commissures and all remaining postotic and trunk neuromasts. The ganglion of the anterior lateral-line nerve is divided into dorsal and ventral subganglia; the single ganglion of the middle lateral line nerve has no recognizable subdivisions, and the ganglion of the posterior lateral-line nerve consists of rostral and caudal subganglia. Analysis of the roots of these nerves and review of the embryonic origin of their ganglia as well as comparisons with cranial nerves in other anamniotes suggest that the anterior and posterior lateral-line nerves of gars may represent the fusion of four to five separate lateral-line nerves at some stage in vertebrate phylogeny. Thus, with the addition of the middle lateral-line nerve, and the possible existence of a ventral lateral-line nerve of the trunk, it is possible that the earliest jawed vertebrates possessed six or even seven pairs of lateral-line nerves.

Section: The Identity and Phylogeny O F Lateral-line Nervesmentioning

confidence: 99%

Section: The Identity and Phylogeny O F Lateral-line Nervesmentioning

confidence: 99%

Morphology, Distribution and Innervation of the Lateral-Line Receptors of the Florida Gar, <i>Lepisosteus platyrhincus</i>; pp. 24–37

Song¹,

Northcutt²

1991

“…Lateral line placodes thus constitute the basic ontogenetic unit responsi ble for the formation of the lateral line system [Northcutt, 1992a;Northcutt et al, 1994[. There is sufficient infor mation on the development of these placodes in elasmobranchs [Holmgren, 1940;Landarcre, 1916: Disler. 1977], bony fishes [Landacre and Conger.…”

Section: Introductionmentioning

confidence: 99%

Evolution of Gnathostome Lateral Line Ontogenies

Rg¹

1997

An outgroup analysis of multiple ontogenies provides the most robust approach to understanding phylogeny. Such an analysis of the lateral line system among extinct and extant gnathostomes reveals that lateral line placodes constitute the basic ontogenetic unit responsible for the development of this system. Six pairs of lateral line placodes appear to have existed in the earliest gnathostomes, and eight stages (stages A–H) can be recognized in their differentiation. Terminal truncation (heterochronic changes) in the primitive sequence of placodal development has occurred in one or more placodes in each gnathostome radiation, with the most extensive truncations occurring in arthrodire placoderms, lepidosirenid lungfishes and extant amphibians. The most extensive nonterminal changes in the primitive sequence of placodal development involve the failure of electroreceptors to form within the lateral zones of the elongatiang sensory ridges of the placodes. This nonterminal change appears to have occurred independently in ancestral neopterygian bony fishes, in many amphibians and, possibly, in the extinct acanthodians. At least two teleost radiations, osteoglossomorphs and ostariophysines, have re-evolved electroreceptors which may represent additional nonterminal changes in placodal patterning or, possibly, a change in the embryonic source of these receptors.

“…Bjerring [ 1977,1984] and Jarvik [ 1980] accepted Platt's vesicle as phylctic evidence that two prcmandibular head segments exist, and they accepted Holmgren's [1940] observation that this vesicle gives rise to the inferior ob lique muscle (fig.4). In addition, both Bjerringand Jarvik adhered to a strict interpretation that every dorsal cranial nerve primitively contained all components presently seen in dorsal spinal nerves as well as gustatory and lateral line components.…”

Section: G O O Dric Hmentioning

confidence: 99%

“…Although Platt and subsequently Lamb [1902] believed that her anterior somite, subsequently referred to in the literature as Platt's vesicle, initially formed myoblasts which would subse quently degenerate. Holmgren [ 1940] argued that Platt's vesicle gives rise to the inferior oblique muscle. Following Platt's discovery of this anterior somite.…”

Section: Craniate Head Organizationmentioning

confidence: 99%

Ontogeny and Phylogeny: A Re-Evaluation of Conceptual Relationships and Some Applications

1990

116

The history of biology reveals numerous interpretations of the relationship between ontogeny and phylogeny, among them Garstang's critical realization that phylogeny can only be the history of changes in an ancestral ontogeny. One extrapolation of Garstang's conclusion is that the study of phylogeny should involve comparisons of ontogenies, in order to allow a reconstruction of the sequence of changes that led to the present variation in these ontogenies as well as the mechanisms underlying these changes. It is suggested that this can be done by recognizing homologous developmental stages among various ontogenies and subjecting these stages to a cladistic analysis. Some of the problems involving such analysis are explored in the course of examining the phylogeny of pharyngeal pouch ontogenies as a model system. It is concluded that sequences of ontogenetic stages are conserved (von Baerian recapitulation) but that terminal (Haeckelian recapitulation) and nonterminal alterations in the ancestral ontogeny are frequent. Both terminal and nonterminal alterations occur with approximately equal frequency in the case of pharyngeal pouches, but non-terminal deletion is rare to nonexistent. Examination of two additional morphological cases in which ontogeny has been utilized to infer phylogenetic changes – ontogenetic parcellation of neural systems and craniate head organization – reveals a number of problems from the point of view of a cladistical analysis of ontogenesis. Ontogenetic parcellation is essentially a recasting of Haeckel's biogenetic law and is rejected on the grounds that ontogenies do not parallel phylogenies, and only primitive and derived characters, not organisms, can be recognized. It is argued that the phylogenetic significance of transient neural characters can be determined only within a cladistical context and that insufficient ontogenetic data presently exist to evaluate most characters. Finally, the history of current models of craniate head organization is traced, and this information, along with that from more recent developmental studies, necessitates rejection of the Goodrich and Jarvik-Bjerring models. A new model is presented that purports to incorporate current ideas of somitomere and neuromere distribution, but it is deemed inadequate because, like previous models, it is based on information gleaned primarily from the ontogeny of a single species, whereas the model sought is a model of the ontogeny of ancestral craniates. To arrive at such a model, we must generate a morphotype, based on shared primitive features of cephalic ontogenies of all craniate radiations, which involves a cladistic analysis of these ontogenies.