Notes on the Anatomy of Onchidella nigricans

Abstract

(O. nigricans ) is a small euthyneurous pulmonate of the sub-order Stylommatophora, class Gastropoda. It is an inhabitant of the littoral zone and is found clinging to the rocks between tide marks. The animals are usually found in damp shaded places such as in deep dark cracks and under projecting ledges. They are found in large numbers on the surface of the plant Coduim a member if the Chlorophyceae which grows in dense masses on the rocks in some parts. I have collected this species of Onchidella from various bays in Cook St., including Island Bay, Lyall Bay and Titahi Bay. Onchidella however never comes within the limits of Wellington Harbour. Plentiful along the south coast of the North Island, as far as Wellington Heads its distribution then ceases quite suddenly despite the fact that the line of rocks is continued through the Heads into the harbour itself. Whether this is the result of the nature of the water, - temperature, salinity or aeration, or whether it is controlled by the distribution of food, I have not discovered, but I think it is due to one of the factors of the former set. This distribution is rendered more unusual by the fact that the animal lives quite happily for long periods in the laboratory provided that it has a small rock covered with lime- encrusted seaweed, on which to live, and fresh, well aerated sea-water renewed every second day. Stale water, four or five days old, always results in a high percentage of mortality due to suffocation. The largest specimens I have seen were those collected from Fishermen's Island which lies about 200 yards out from the mainland, a deep channel running between the two. Here O. nigricans was found intermingled with O. flavescens both species occurring in large numbers. I found them to vary in size up to 2.5 cms long and 1.75 cms wide whereas those on the mainland seldom exceeded 1.25 - 1.5 cms in length. Mr. A.W.B. Powell, Conchologist and Palaeontologist of the Auckland Institute and Museum reports having found O. nigricans together with O. patelloides on the West coast in the neighbourhood of Auckland. He states that O. nigricans is usually associated with rocks in clean-water situations while O. patelloides seems to prefer the mud-flats.