22 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, 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. Glaus, 1 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 Limuadiidae resemble the Cladocera in the possession 

 of a " dorsal organ " quite distinct from the above ; in Limnetis 

 and Estli.eria it has the form of a small pit, lined by an apparently 

 glandular ectoderm, and this is its condition in many Cladocera ; 

 in J/niinndia 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 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 ' 2 the anus opens directly backwards ; and 



1 Arb. zool. /"//>/. H'ii'H., vi., 1886, j>. 267. 

 2 I do not understand Packard's account of the telson in Thamnocephalus. 



