FIG. 27.—Dorsal view of _Anomalocera pattersoni_, ♂, × 20. (After Sars.) ] It has been known for a long time that some of the marine Copepods are phosphorescent, and, indeed, owing to their numbers in the plankton, contribute very largely to bring about that liquid illumination which will always excite the admiration of seafarers. In northern seas the chief phosphorescent Copepods belong to _Metridia_, a genus of the Centropagidae; but in the Bay of Naples Giesbrecht[42] states that the phosphorescent species are the following Centropagids: _Pleuromma abdominale_ and _P. gracile_, _Leuckartia flavicornis_ and _Heterochaeta papilligera_; _Oncaea conifera_ is also phosphorescent. It is often stated that _Sapphirina_ (p. 69) is phosphorescent, but its wonderful iridescent blue colour is purely due to interference colours, and has nothing to do with phosphorescence. Giesbrecht has observed that the phosphorescence is due to a substance secreted in special skin-glands, which is jerked into the water, and on coming into contact with it emits a phosphorescent glow. This substance can be dried up completely in a desiccated specimen and yet preserve its phosphorescent properties, the essential condition for the actual emission of light being contact with water. Similarly, specimens preserved in glycerine for a long period will phosphoresce when compressed in distilled water. From this last experiment Giesbrecht concludes that the phosphorescence can hardly be due to an oxidation process, but the nature of the chemical reaction remains obscure. =Fam. 2. Candacidae.=—This family comprises the single genus _Candace_, with numerous species distributed in the plankton of all seas. Some species, e.g. _C. pectinata_, Brady, have a practically world-wide distribution, this species being recorded from the Shetlands and from the Philippines. =Fam. 3. Pontellidae.=—This is a larger family also comprising widely distributed species found in the marine plankton. _Anomalocera pattersoni_ (Fig. 27) is one of the commonest elements in the plankton of the North Sea. =Sub-Order 2. Podoplea.= The boundary between the fore and hind part of the body falls in front of the fifth thoracic segment. The appendages of the fifth thoracic pair in the male are never modified as copulatory organs. TRIBE I. AMPHARTHRANDRIA. The first antennae in the male differ greatly from those in the female, being often geniculated and acting as prehensile organs. [Illustration: FIG. 28.—_Euterpe acutifrons_, ♀, × 70. _Abd.1_, 1st abdominal segment; _Th.5_, 5th thoracic segment. (After Giesbrecht.) ] [Illustration: FIG. 29.—First antenna of _Euterpe acutifrons_, ♂. (After Giesbrecht.) ]