ex. exopodite; en. endopodite.
Brachyura. All the Brachyura, with the exception of one or more species of land crabs[191], leave the egg in the Zoæa condition, and though there are slight variations of structure, yet on the whole the Crab Zoæa is a very well marked form. Immediately after leaving the egg ([fig. 210]) it has a somewhat oval shape with a long distinctly segmented abdomen bent underneath the thorax. The cephalo-thoracic shield covers over the front part of the body, and is prolonged into a long frontal spine pointing forwards, and springing from the region between the two eyes; a long dorsal spine pointing backwards; and two lateral spines.
To the under surface of the body are attached the anterior appendages up to the second maxilliped, while the six following pairs of thoracic appendages are either absent or represented only in a very rudimentary form. The abdomen is without appendages.
Fig. 224. Crab Zoæa after the third pair of maxillipeds and the thoracic and abdominal appendages have become developed.
at1. antenna of first pair; at2. antenna of second pair; mx1. first maxilla; mx2. second maxilla; mxp1. first maxilliped; mxp2. second maxilliped; mxp3. third maxilliped; oc. eye; ht. heart.
The anterior antennæ are single and unjointed, but provided at their extremity with a few olfactory hairs (only two in Carcinus Mœnas) and one or two bristles. The rudiment of the secondary flagellum appears in very young Zoææ on the inner side of the antennules ([fig. 223] At. I.). The posterior antennæ are without the flagellum, but are provided with a scale representing the exopodite ([fig. 223] At. II. ex) and usually a spinous process. The flagellum is very early developed and is represented in [fig. 223], At. II. en. The mandibles (md) are large but without a palp. The anterior maxillæ (mx 1) have a short two-jointed endopodite (palp) with a few hairs, and a basal portion with two blades, of which the distal is the largest, both armed with stiff bristles. The posterior maxillæ have a small respiratory plate (exopodite), an endopodite (palp) shaped like a double blade, and two basal joints each continued into a double blade. The two maxillipeds (mxp 1 and mxp 2) have the form and function of biramous swimming feet. The exopodite of both is two-jointed and bears long bristles at its extremity; the endopodite of the anterior is five-jointed and long, that of the second is three-jointed and comparatively short.
In the six-jointed tail the second segment has usually two dorsally directed spines, and the three succeeding segments each of them two posteriorly directed. The telson or swimming plate is not at first separated from the sixth segment; on each side it is prolonged into two well-marked prongs; and to each prong three bristles are usually attached ([fig. 224]). The heart ([fig. 224] ht) lies under the dorsal spine and is prolonged into an anterior, posterior, and dorsal aorta. It has only two pairs of venous ostia.
During the Zoæa stage the larva rapidly grows in size, and undergoes considerable changes in its appendages which reach the full Decapod number ([fig. 224]). On both pairs of antennæ a flagellum becomes developed and grows considerably in length. Before the close of the Zoæa condition a small and unjointed palp appears on the mandible. Behind the second maxilliped the third maxilliped (mxp3) early appears as a small biramous appendage, and the five ambulatory feet become distinctly formed as uniramous appendages—the exopodites not being present. The third pair of maxillipeds and three following ambulatory appendages develop gill pouches. The abdominal feet are formed on the second to the sixth segments of the tail as simple pouches.