. The Tanganyika problem; an account of the researches undertaken concerning the existence of marine animals in Central Africa . e outer and inner lateralteeth. Perhaps, however, the most notable feature whichthe radula presents is the peculiar structure of the mediantooth. The outer surface of this tooth is concave, like themedian tooth in Anchylotus, T/iiara, Melania brevis (Dorb.),and Melanopsis. But it differs from all these forms inhaving no predominant median denticle, there being insteadtwo lateral predominant denticles and a median only forms which appear to possess this
. The Tanganyika problem; an account of the researches undertaken concerning the existence of marine animals in Central Africa . e outer and inner lateralteeth. Perhaps, however, the most notable feature whichthe radula presents is the peculiar structure of the mediantooth. The outer surface of this tooth is concave, like themedian tooth in Anchylotus, T/iiara, Melania brevis (Dorb.),and Melanopsis. But it differs from all these forms inhaving no predominant median denticle, there being insteadtwo lateral predominant denticles and a median only forms which appear to possess this peculiarity of THE TANGANYIKA PROBLEM. 259 the median tooth are the different species of the genusSigaretus. In Spekia zonata the oesophagus leads into a doublestomach, of which the anterior chamber contains a crystal-line style ; but it is so small and so little elongated that atfirst sight it does not seem to have the characters whichthis structure usually presents. The intestine takes the course represented in figure42, and communicates with a dilated rectum, openingin the usual way just within the border of the Fig. 42.—Semi diagram of the anatomy of Spekia zonata. The heart has the regular tsenioglossate characters. InS. zonata, owing to the naticoid shape of the body, thereis a forward displacement of the internal viscera, whichresults in the pulmonary vein not going directly forward tothe base ot the stenidium, as in Tanganyicia rnfofilosa andmost other Prosobranchs, but in its being bent slightlybackwards in a more or less acute curve before it reachesthe base of the gill. This relative displacement of theheart and gill is fully described by Haller in the NaticasTrochita radians, Ergcea plana, Crepidula peruviana(Lam.), Tanaais ungiiiforniis (Lam.), and Crucibuhun(sp. ?), and it is curious to find that this displacement has 17* 26o THE TANGANYIKA PROBLEM. proceeded less in Spekia than in any of the fornis whichHaller figured The gill in S. zonata is very much
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