an1. antenna of first pair; an2. antenna of second pair; md. mandible; f. caudal fork.
Cladocera. The probable derivation of the Cladocera from a form similar to Estheria has already been mentioned, and it might have been anticipated that the development would be similar to that of the Phyllopods. The development of the majority of the Cladocera takes place however in the egg, and the young when hatched closely resembles their parents, though in the egg they pass through a Nauplius stage (Dohrn). An exception to the general rule is however offered by the case of the winter eggs of Leptodora, one of the most primitive of the Cladoceran families. The summer eggs develop without metamorphosis, but Sars (No. [461]) has discovered that the larva leaves the winter eggs in the form of a Nauplius ([fig. 209]). This Nauplius closely resembles that of the Phyllopods. The body is elongated and in addition to normal Nauplius appendages is marked by six pairs of ridges—the indications of the future feet. The anterior antennæ are as usual small; the second large and biramous, but the masticatory bristle characteristic of the Phyllopods is not present. The mandibles are without a cutting blade. A large upper lip and unpaired eye are present.
The adult form is attained in the same manner as amongst the Phyllopods after the third moult.
Malacostraca.
Owing to the size and importance of the various forms included in the Malacostraca, greater attention has been paid to their embryology than to that of any other division of the Crustacea; and the proper interpretation of their larval forms involves some of the most interesting problems in the whole range of Embryology.
The majority of Malacostraca pass through a more or less complicated metamorphosis, though in the Nebaliadæ, the Cumaceæ, some of the Schizopoda, a few Decapoda (Astacus, Gecarcinus, etc.), and in the Edriophthalmata, the larva on leaving the egg has nearly the form of the adult. In contradistinction to the lower groups of Crustacea the Nauplius form of larva is rare, though it occurs in the case of one of the Schizopods (Euphausia, [fig. 212]), in some of the lower forms of the Decapods (Penæus, [fig. 214]), and perhaps also, though this has not been made out, in some of the Stomatopoda.
Fig. 210. Zoæa of Thia polita. (After Claus.) mxp2. second maxillipede.
In the majority of the Decapoda the larva leaves the egg in a form known as the Zoæa ([fig. 210]). This larval form is characterised by the presence of a large cephalo-thoracic shield usually armed with lateral, anterior, and dorsal spines. The caudal segments are well developed, though without appendages, and the tail, which functions in swimming, is usually forked. The six posterior thoracic segments are, on the other hand, rudimentary or non-existent. There are seven anterior pairs of appendages shewn in detail in [fig. 211], viz. the two pairs of antennæ (At. I. and At. II.), neither of them used as swimming organs, the mandibles without a palp (md), well-developed maxillæ (two pairs, mx 1 and mx 2), and two or sometimes (Macrura) three pairs of biramous natatory maxillipeds (mxp 1 and mxp 2). Two lateral compound stalked eyes are present, together with a median Nauplius eye. The heart has in the majority of cases only one or two (Brachyura) pairs of ostia.