The respiratory plate of the second maxilla attains its full development and that on the first maxilla disappears[189]. The Nauplius, so far as is known, does not occur in any other Decapod form except Penæus.
Fig. 217. Latest Protozoæa stage of Sergestes larva (Elaphocaris). (After Claus.)
mxp´´´. third pair of maxillipeds.
The next most primitive larval history known is that which appears in the Sergestidæ. The larval history, which has been fully elucidated by Claus, commences with a Protozoæa form ([fig. 217]), which develops into a remarkable Zoæa first described by Dohrn as Elaphocaris. This develops into a form originally described by Claus as Acanthosoma, and this into a form known as Mastigopus ([fig. 218]) from which it is easy to pass to the adult.
The remarkable Protozoæa ([fig. 217]) is characterised by the presence on the dorsal shield of a frontal, dorsal and two lateral spikes, each richly armed with long side spines. The normal Zoæa appendages are present, and in addition to them a small third pair of maxillipeds. The thoracic region is divided into five short rings, but the abdomen is unsegmented. The tail is forked and provided with long spines. The antennæ, like those of Penæus, are long—the second pair biramous; the mandibles unpalped. Both pairs of maxillæ are provided with respiratory plates; the second pair is footlike, and has at its base a glandular mass believed by Claus to be the equivalent of the Entomostracan shell-gland. The maxillipeds have the usual biramous characters. A helmet-shaped upper lip like that of a typical Nauplius is present, and the eyes are situated on very long stalks.
Fig. 218. Mastigopus Stage Of Sergestes.
(From Claus.)