The study of the conditions under which hybrids between various species can be produced is of importance in relation to several problems of evolution, whilst the examination of the morphological relations between the parent forms and those of their hybrid progeny is able in many cases to throw additional light on questions of the transmission of parental characteristics, and of prepotency. Within the limits of the animal kingdom no more suitable species can be found than certain of the Echinoidea, and hence a number of observations have already been made upon members of this group. In the present research it is sought to extend these obseivations, more especially by determining systematically, over a period of several months duration, the exact relationships of structure and size between certain hybrid and parent larval forms. The plutei made use of in these observations were in all cases obtained from artificial fertilisations, carried out in the manner already fully described in a former paper. Suffice it to mention here that the method consists in shaking pieces of the ovaries and testes in small jars of water, and then mixing portions of the two liquids, and at the same time carefully noting the temperature. After standing an hour, the now fertilised ova are poured into a large jar of water, holding as a rule from 2 to 3-g litres. These large jars, covered with glass plates, were allowed to stand in a tank of running water, whereby the temperature of their contents was kept nearly constant, it varying less than a degree during twenty-four hours, and not more than about two degrees during the whole course of the experiment. As a rule the fertilised ova were allowed to proceed on their normal development for eight days, as it has been shown that the arm lengths of the artificially produced Strongylocentrotus larvae reach their maximum length at the period. The plutei were then killed by adding sufficient saturated corrosive sublimate solution to the water to form a 2 per cent, solution. After allowing them to settle, the supernatant water was gradually poured off, and the larvae transferred first to 50 per cent, alcohol, and then to 80 per cent., in which latter fluid they were preserved until required. For microscopical examination, they were mounted in glycerin. No staining is necessary, as the calcareous skeleton, which was almost entirely relied on for diagnostic differences, is very obvious and sharply defined. These larvae were then measured, in sets of fifty, by means of a micrometer eye-piece. Occasionally, in the case of certain hybrids which were obtained in but small numbers, less than fifty individuals were measured. The actual numbers are given in the Table at the end of the paper. In every instance, the position of each pluteus measured was read off on the mechanical stage and noted down, so that the same individual should not be measured twice.