The whole subject of the modification of the pigment and structure of Crustacean eyes is an interesting one, because it presents us with one of those cases in which the direct response to a stimulus acting within the lifetime of the individual seems to run parallel to the fixed adaptations of a whole species, which have become hereditary and apparently independent of the external stimulus of light or of the absence of light. As far as is known, however, the direct response of the individual to the absence of light is limited to the reduction or disappearance of the pigment, and does not extend to those structural changes in the ommatidia which are characteristic of so many deep-sea forms. =Order II. Decapoda.=[123] The Decapoda, together with the Euphausiidae, make up the Division Eucarida, the members of which differ from the Orders hitherto described in a number of characters, _e.g._ the presence of a carapace covering the whole of the thorax, the absence of a brood-pouch formed of oostegites, the presence of a short heart, of spermatozoa with radiating pseudopodia, and of a complicated larval metamorphosis, of which the Zoaea stages are most prominent. The Decapoda differ from the Euphausiidae chiefly in the anterior three thoracic limbs being turned forwards towards the mouth to act as maxillipedes, and in the five succeeding thoracic limbs being nearly always uniramous and ambulatory or chelate; there are typically present three serial rows of gills attached to the thoracic segments, an upper series (“pleurobranchiae”) attached to the body-wall above the articulation of the limbs, a middle series (“arthrobranchiae”) attached at the articulation of the limbs, and a lower series (“podobranchiae”) attached to the basal joints of the limbs. These gills are enclosed in a special branchial chamber on each side of the thorax, formed by lateral wings of the carapace known as “branchiostegites.” The gills of each series are never all present in the same animal, the anterior and posterior members showing a special tendency to be reduced and to disappear. In this manner “branchial formulae” can be constructed for the various kinds of Decapods, which differ from the ideal formula in a manner distinctive of each kind. The second maxilla is always provided with an oar-like appendage on its outer margin (exopodite), known as the “scaphognathite,” which, by its rhythmical movement, keeps up a constant current of water through the gill-chamber. A complicated auditory organ is present on the basal joint of the first antennae; this is a sac communicating with the exterior and lined internally with sensory hairs. The animal is said to place small pieces of sand, etc., in its ears to act as otoliths. _Anaspides_ (see p. 116) is the only other Crustacean which has an auditory organ in this position.