THE TANGANYIKA PROBLEM. 
259 
the median tooth are the different species of the genus 
Sigaretus. 
In Spekia zonata the oesophagus leads into a double 
stomach, of which the anterior chamber contains a crystal- 
line style ; but it is so small and so little elongated that at 
first sight it does not seem to have the characters which 
this structure usually presents. 
The intestine takes the course represented in figure 
42, and communicates with a dilated rectum, opening 
in the usual way just within the border of the mantle. 
Fig. 42. — Semi diagram of the anatomy of Spekia zonata. 
The heart has the regular tsenioglossate characters. In 
S. zonata, owing to the naticoid shape of the body, there 
is a forward displacement of the internal viscera, which 
results in the pulmonary vein not going directly forward to 
the base of the stenidium, as in Tanganyicia rufofilosa and 
most other Prosobranchs, but in its being bent slightly 
backwards in a more or less acute curve before it reaches 
the base of the gill. This relative displacement of the 
heart and gill is fully described by Haller in the Naticas 
Trochita radians , Ergcea plana, Crepidula peruviana 
(Lam.), Tanacus unguiformis (Lam.), and Crucibulum 
(sp. ?), and it is curious to find that this displacement has 
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