There are two types of ground at Port Erin, differing widely in the general environmental conditions, and each with a characteristic type of urchin on it. The difference is sufficiently well marked for the local fishermen, who collect the urchins for sale to summer visitors, to distinguish them as different types. The littoral urchins from the Breakwater are larger and thicker shelled than those from the Breast. Their gonads reach a greater maximum size, and, in fact, never fall as low even as the maximum volume for the Breast urchin gonads. This difference is probably correlated with the much greater food supply on the Breakwater, since the gonad is the one organ of the body in which the animal can store reserve food material. Spawning commences in March or April and lasts for about two months, but is slightly earlier on the Breakwater than on the Breast. Ripening also takes place considerably earlier in both sexes on the littoral ground. Spawning seems to set in at a temperature of about 7° C., and the colder winter of 1931, compared with 1932, was associated with a later ripening and spawning, i n the former year. Similarly it is suggested that the slightly later spawning on the Breast than on the Breakwater is correlated with the slightly later rise in the temperature of the water on the deeper water ground in the spring.