De CRUSTACEA—BRANCHIOPODA CHAP. 
these points the eyes of the Limnadiidae are intermediate between 
those of Apus and those of the Cladocera. 
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, suppled 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,’ 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, limed by an apparently 
glandular ectoderm, and this is its condition in many Cladocera ; 
in Limnadia lenticularis it is a pateh of glandular epithelium on 
a raised papilla. Zimnadia has been observed to anchor itself 
to foreign objects by pressing its dorsal organ against them, and 
many Cladocera do the same thing; Sida erystallina, for example, 
will remain for hours attached by its dorsal organ to a water- 
weed 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” the anus opens directly backwards; and 
1 Arb. zool. Inst. Wien, vi., 1886, p. 267. 
2 T do not understand Packard’s account of the telson in Thamnocephalus. 
