308 



COMPARATIVE ANATOMY 



CHAP. 



appendages originate as biramose limbs ; the biramose character may, 

 however, be more or less indistinct, or may even be lost as the limbs 

 develop into the corresponding limbs of the adult. 



In attempting a comparative review of the limbs throughout the 

 class of the Crustacea, only the most important points can be touched 

 upon. The setae with which they are often provided, and whose form 

 and arrangement are extremely important for classification, cannot 

 here be taken into consideration. 



1. The Limbs of the Head. 



In all the Crustacea the head carries 5. pairs of limbs, which are 

 called, following the order from before backward, the anterior antennae, 

 posterior antennae, mandibles, anterior maxillae, and posterior maxillae. 

 The 3 anterior pairs correspond with the 3 pairs of limbs of the 



Nauplius. 



a. The Anterior Antennae (Antennules) (Fig. 209). 



These lie in front of the mouth, and consist typically of a single 

 row of joints. As a rule they function as organs of touch, but usually 

 also carry the olfactory organs, and occasionally the auditory organs. 



FIG. 209. Anterior antennae (antennules) of various Crustaceans. A, Of Astacus (after 

 Huxley) ; o, auditory sac. B, Of Munnopsis typica, Isopod <$ (after Sars). C,OfNebaliaGeo/royi 6 

 (after Claus), without the setae ; pi, plate. D, Of Cyclops serrulatus 6 (after Glaus) ; rf, olfactory 

 hairs. E, Of Daphnia pulex (after Leydig) ; rf, olfactory hairs ; g, ganglion. 



Entomostraca. In all Entomostraca the antennules consist typically of a single 

 row of joints. In the Phyllopoda (Fig. 209, E] they are small, carry numerous 

 olfactory hairs, and are called feelers or olfactory antennse. In the Ostracoda (Fig. 

 193) they are strongly developed and occasionally provided with olfactory hairs, but 

 chiefly function as locomotory organs for crawling and swimming. The anterior 

 antennse of the Copepoda are strongly developed as swimming feet in the free-swim- 

 ming forms, and are longer than any of the other limbs (Fig. 194). They carry 

 olfactory hairs, and serve in the males as organs for seizing and holding the female 

 during copulation (Fig. 209, D}. In the parasitic forms they are usually much 



