TIDDLER CRAB — HYMAN. 449 



shell it at once swims to the surface of the water. There it finds con- 

 ditions that give it a chance to survive. It is swimming among count- 

 less larvae of other Crustacea and mingling with the young of 

 worms and mollusks and starfishes and jellyfishes. not to mention 

 hosts of minute forms such as lower Crustacea and small jellyfishes 

 and protozoa. Besides these animals there are millions upon millions 

 of microscopic plants swimming and floating along. In this world 

 of tiny forms, young fishes swim about as devouring giants. They 

 take toll of all the smaller forms alike and many of the zoeas are 

 eaten. It is well that they are hatched at dusk and have all night to 

 be scattered by the tide before they enter upon the adventures of 

 their first day. 



The body of the zoea is composed of a head and abdomen, between 

 which lies a small region representing the thorax. (Fig. 1, f.) The 

 seven pairs of appendages of the zoea are attached to the ventral 

 surface of the cephalothorax. It is from their attachments that we 

 know that most of the cephalothorax is composed of the head region 

 and only a small part of it is the thorax. Moreover the region of the 

 legs or pereiopods which forms the largest part of the adult body is 

 represented in the zoea by a minute area in the region where the 

 abdomen is joined to the cephalothorax. 



The first two pairs of appendages, the antennules (fig. 2) and 

 antennae (fig. 3), are sense organs just as in the adult. They are 

 quite different from those of the adult, however, in both structure 

 and function. The antennules are little more than knobs on the 

 surface of the head, each bearing a few sensory hairs at its tip. They 

 show no sign of a statocyst, or organ of equilibration. Some of their 

 hairs are tactile and others are sensitive to chemical changes in the 

 water, whether this be called taste or smell. The antennae are queer, 

 spikelike appendages extending down parallel with the frontal spine 

 of the carapace. Each bears a movable process, the exopodite, on its 

 lateral surface. The exopodite bears one or two minute tactile hairs 

 at its tip. 



The other three pairs of head appendages form the mouth parts of 

 the zoea. They resemble very closely the same parts in the adult in 

 their essential structure. The mandibles (fig. 6) are simple blades 

 with their median borders produced into denticles for cutting and 

 mashing the food. They do not have sensory palps. The maxillules 

 (fig. 4) are flattened plates. The basal segment of each is produced 

 into two median lobes, each of which has stout spines at its tip. These 

 lie just laterally to the mouth and posteriorly to the mandible. Each 

 maxillule bears a small palp with tactile hairs. The maxillae (fig. 5) 

 also are flattened appendages. Their basal segments are produced 

 into median lobes with spiny tips. There is also a palp with tactile 

 hairs and laterally the scaphognathite is already developed. These 

 42803°— 22 29 



