Heterocope.—The furcal processes are short, at most twice as long as broad; endopodites of all swimming legs 1–jointed.
Eurytemora.—The furcal processes are long, at least three and a half times as long as broad; the endopodite of the first pair of legs 1–jointed, those of the other pairs 2–jointed.
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.