. Text-book of embryology. Embryology. VIII AETHEOPODA 203 Nauplii, showing, however, the ridge-like rudiments of two or three pairs of postoral appendages. In the case of both Ewphausia and Fenaeus a succession of moults leads, as in other Nauplii, to the development of the thoracico- abdominal rudiment, and to the appearance in it of ring-like segments which first appear in the anterior or thoracic portion. The appendages belonging to these segments, which are the maxilli- pedes, are strongly developed, especially the first, which has a long exopodite used in swimming. A cephalic shield or c
. Text-book of embryology. Embryology. VIII AETHEOPODA 203 Nauplii, showing, however, the ridge-like rudiments of two or three pairs of postoral appendages. In the case of both Ewphausia and Fenaeus a succession of moults leads, as in other Nauplii, to the development of the thoracico- abdominal rudiment, and to the appearance in it of ring-like segments which first appear in the anterior or thoracic portion. The appendages belonging to these segments, which are the maxilli- pedes, are strongly developed, especially the first, which has a long exopodite used in swimming. A cephalic shield or carapace makes its appearance as a frill or fold round the head region, under- neath which the future paired eyes appear as dark areas. In Penaeus the anterior part of this shield is produced into a sharp rostral spine. The larva is now in the stage known as the Protozoaea (Fig. 150). Its further development into what is known as a Zoaea larva involves the growth and segmentation of the abdominal portion of the body, the thoracic segments remaining extremely narrow, especially the posterior ones, and the paired eyes become stalked. In Euphausiadacea the stalks are so short that the paired eyes do not project beyond the edge of the carapace, and the larva is consequently known as a Calyptopis. The majority of Decapoda pass through the Nauplius stage during their embryonic life and only enter on their larval life as Pig. 160. — The Protozoaea larva of Nyctiphanes ausiralis. (After Spence- Bate.) Letters as in preceding iigure. oc^, simple anterior eye; oc2, compound eye; , thoracic segments. ANCESTRAL CKUSTACEAN We may now pause to consider how far the Nauplius larva may be regarded as representing an ancestral Crustacean form. Since it occurs in all the lower groups of Crustacea with no greater modifications than are found, for instance, in the different types of larva amongst Echinoidea; and since it also occurs in isolated cases amongst the higher Crustacea; and
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