Dorsal Organ.—A structure very characteristic of adult Phyllopods is the “dorsal organ” (Figs. [2], 5, D.O), whose function is in many cases obscure. It is always a patch of modified cephalic ectoderm, supplied by a nerve from the anterior ventral lobe of the brain on each side; but its characters, and apparent function, differ in different forms. In the Branchipodidae the dorsal organ is a circular patch, far forward on the surface of the head (Figs. [2], 5, D.O). Its cells are arranged in groups, which remind one of the retinulae in a compound eye; each cell contains a solid concretion, and the concretions of a group may be so placed as to look like a badly-formed rhabdom. Claus,[[16]] who first called attention to this structure in the Branchipodidae, regarded it as a sense-organ. In Apodidae the dorsal organ is an oval patch of columnar ectoderm, immediately behind the eyes; it is slightly raised above the surrounding skin, and is covered by a very delicate cuticle (with an opening to the exterior?), and below it is a mass of connective tissue permeated by blood; Bernard has suggested that it is an excretory organ.
Most Limnadiidae resemble the Cladocera in the possession of a “dorsal organ” quite distinct from the above; in Limnetis and Estheria it has the form of a small pit, lined by an apparently glandular ectoderm, and this is its condition in many Cladocera; in Limnadia lenticularis it is a patch of glandular epithelium on a raised papilla. Limnadia has been observed to anchor itself to foreign objects by pressing its dorsal organ against them, and many Cladocera do the same thing; Sida crystallina, for example, will remain for hours attached by its dorsal organ to a waterweed or to the side of an aquarium. Structures resembling a dorsal organ occur in the larvae of many other Crustacea, but the presence of this organ in the adult is confined to Branchiopods, and indeed in many Cladocera it disappears before maturity. It is certain that the sensory and adhesive types of dorsal organ are not homologous, especially as rudimentary sense-organs may exist on the head of Cladocera together with the adhesive organ.
The telson differs considerably in the different genera. In the Branchipodidae[[17]] the anus opens directly backwards; and the telson carries two flattened backwardly directed plates, one on each side of the anus, the margins of each plate being fringed with plumose setae. In Artemia the anal plates are rarely as large as in Branchipus, and never have their margins completely fringed with setae; in A. salina from Western Europe, and in A. fertilis (Fig. [4], A) from the Great Salt Lake of Utah, there is a variable number of setae round the apical half of each lobe, but in specimens of A. salina from Western Siberia the number of setae may be very small, or they may be absent; in the closely allied A. urmiana from Persia the anal lobes are well developed in the male, each lobe bearing a single terminal hair, but they are altogether absent in the female. Schmankewitch and Bateson have shown that there is a certain relation between the salinity of the water in which Artemia salina occurs and the condition of the anal lobes, specimens from denser waters having on the whole fewer setae; the relation is, however, evidently very complex, and further evidence is wanted before any more definite statements can be made.
Fig. [4].—A, Ventral view of the anal region in Artemia fertilis, from the Great Salt Lake; B, ventral view of the telson and neighbouring parts of Lepidurus productus; C, side view of the telson and left anal lobe of Estheria (sp.?).
In the Apodidae the anal lobes have the form of two-jointed cirri, often of considerable length; in Apus the anus is terminal, but in Lepidurus (Fig. [4], B) the dorsal part of the telson is prolonged backwards, so as to form a plate, on the ventral face of which the anus opens, much as in the Malacostraca.
In the Limnadiidae (Fig. [4], C) the telson is laterally compressed and produced, on each side of the anus, into a flattened, upwardly curved process, sharply pointed posteriorly, and often serrate; the anal lobes are represented by two stout curved spines, while in place of the dorsal prolongation of Lepidurus we find two long plumose setae above the anus. In the characters of the telson and anal lobes, as in those of the head, the Limnadiidae approximate to the Cladocera. In Limnetis brachyura the ventral face of the telson is produced into a plate projecting backwards below the anus, in a manner which has no exact parallel among other Crustacea.
The appendages of the Phyllopoda are fairly uniform in character, except those affected by the sexual dimorphism, which is usually great.
Fig. [5].—Chirocephalus diaphanus, male. Side view of head, showing the large second antenna, A2, with its appendage Ap, above which is seen the filiform first antenna; D.O, dorsal organ; E1, median eye.