TYPE ORUaTACEA. 



387 



which may form a dorsal carapace, as in Apus, or a bivalved 

 shell, as in Limnadia, Zimnetis, and EstJieria (Kg. 174), an 

 adductor muscle being developed for the closure of the shell 

 within which the entire body may be withdrawn. The anten- 

 nules are as a rule small and are provided with olfactory 

 hairs ; the antennae, on the other hand, are well developed ex- 

 cept in Apus, in which they are in some species quite small 

 and in others entirely wanting. In the shelled forms they are 

 biramous, consisting of a several-jointed protopodite termi- 

 nated by two many-jointed tlagella, and serve as oarlike loco- 

 motor organs, but in Branchipus they are short strong struc- 



FiG. 174. — Esiheria compleximanus (after Packard). 

 afi = antennule. afi = antenna. m = shell-muscle. 



tures without any locomotor function, serving in the males 

 as clasping organs of use in copulation. The mandibles are 

 reduced to toothed plates, lacking a palp, and the first max- 

 illae show an almost similar reduction, while the second are 

 entirely wanting in some genera, such as Zifnnetis. The suc- 

 ceeding appendages are not limited to the thoracic region of 

 the body, taking the genital opening as the limit between the 

 two regions. Thus in Apus cancriformis there are eleven 

 thoracic appendages, while behind the genital ring there are 

 no less than over fifty locomotor limbs, and in such forms as 

 Limnetis and Estheria (Fig. 174) it is difiicult to distinguish 

 between the thorax and the anterior abdominal segments. 



The heart of the Branchiopoda is a more or less elongated 

 organ with several ostia. and is usually limited to the anterior 

 portion of the thoracic cavity, though in JBrancTiipus it extends 



