Fig. 211. The Appendages of a Crab Zoæa.
At. I. first antenna; At. II. second antenna; md. mandible (without a palp); mx. 1. first maxilla; mx. 2. second maxilla; mxp. 1. first maxilliped; mxp. 2. second maxilliped.
ex. exopodite; en. endopodite.
The Zoæa larva, though typically developed in the Decapoda, is not always present (e.g. Astacus and Homarus), and sometimes occurs in a very modified form. It makes its appearance in an altered garb in the ontogeny of some of the other groups.
The two Malacostracan forms, amongst those so far studied, in which the phylogenetic record is most fully preserved in the ontogeny, are Euphausia amongst the Schizopods and Penæus amongst the Decapods.
Schizopoda. Euphausia leaves the egg (Metschnikoff, No. [468]‑[9]) as a true Nauplius with only three pairs of appendages, the two hinder biramous, and an unsegmented body. The second pair of antennæ has not however the colossal dimensions so common in the lower types. A mouth is present, but the anus is undeveloped.
