Modifications in Size, Form, and Function of Homologous Crustacean Appendages,

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1905. Excerpt: ... CEPHALIC APPENDAGES. Antennules--Plate I. The Antennules of the Malacostraca differ from all the other appendages in having a clearly defined three-jointed basal portion, at the distal end of which, is usually found one or two ratni which vary greatly in length and motility. Frequently, as tnSquillaempusa (Fig. 17), an accessory ramus is developed on one of these rami. The antennules are often wanting in the Entomostraca and, when present, vary in size from the short few-jointed forms found among Cladocera to the well developed, many-jointed structures of the Copepoda. It has been asserted repeatedly by writers upon the Crustacea, that the antennules have been derived from a uniramosc type and that the flagella are never true exopodite and endopodite. This statement is based upon the fact that the flagella are borne upon a three-jointed basal stalk and upon the supposed fact that none of the Crustacea during development pass through a stage in which the antennules are biramose. Certain of the Brachyuran zoea, however, are found to have the antennules consisting of an unjointed, biramose structure. The development of the inner ramus is somewhat arrested in both the antennules and the antennae for each appears, at first, in an unjointed, uniramose condition. After a few moults, a short endopodite develops upon the inner side of the antenna and just a little later a similar branch appears on the inner margin of the antennulc. At this time the basal portion is unjointed but during the succeeding moulting periods the basal part becomes jointed, the rami are elongated and the appendage takes on the form of the adult structure. In the zoea of the Lobster, Homarus americanus, at a period when the exopodites are still present on the pereiopods and the squame of th...