The morphology of the chondrocranium of Gymnarchus niloticus.

Abstract

SUMMARY. The chondrocranium of two larval stages of Gymnarchus niloticus—one 15-day old, the other of an unknown but of slightly younger stage—have been studied. The morphological relations of the arteries, veins and nerves of the head have been described in detail, and the results have necessitated a rectification of some of the statements made by Assheton. The cartilage of the chondrocranium is greatly developed but conform to the general Teleostean type with certain peculiarities of its own. The larval skull is platytrabic. The auditory capsule is greatly developed and contains a modified ear and a sac-like extension of the air-bladder. The internal cavity of the capsule is divided by a partition which is partly ossified. In the lateral wall of the capsule is a vacuity covered by a membrane lateral to which extends the expanded portion of the temporal and supratemporal latero-sensory canals. There is an extensively developed orbital cartilage with pre-optic and metoptic roots and a pila antotica secundaria. The anterior and posterior myodomes are absent. All the eye muscles are collected in a fibrous orbital capsule. The internal carotid foramen is separated from the hypophyseal fenestra. There is no pro-otic bridge, There is a well-developed trigemino-facialis chamber. A lateral commissure is present. Nasal capsules are practically absent except for a broad and massive internasal septum and a rudimentary lamina orbitonasalis. The pterygoquadrate is in a pro-cartilagenous fusion with the hyomandibula. Neither otic, nor basal, nor basi-trabecular articulations are present. The larval jaw suspension is methyostylic. Unique features in Gymnarchus not present in any known living teleost are: (1) presence of a pila antotica secundaria, (2) splitting of the common truncus hyomandi-bularis of other fishes into more or less three separately originating nerves—n. mandibularis, n. opercularis superficialis, and n. hyoideus, (3) formation of an extra nerve, the dorsalis cutaneus facialis, in the facialis system.