I. CRUSTACEA 



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usually the abdominal feet and the mouth-parts retain the two-branched condi- 

 tion. Embryology further shows that even in the crabs all the feet are at first 

 schizopodal and that the walking legs lose the exopoditc during growth. There 

 is evidence to show that the schizopodal foot is not the primitive type. This 

 is furnished by the phyllopod foot (fig. 375, //), which consists of a median axis, 

 b, bearing on the inside six cnditcs, i, and on the outer side two exiles, a 

 flaliclln-m or epipodite, a, and a gill, k. This furnishes the schizopodal form by a 

 loss of the four basal endites (those nearest i) and the development of the two 

 terminal endites into exopodite and endopodite. Still the schizopodal condition 

 is so nearly universal among Crustacea that it must be accorded great weight in 

 classification. 



The appendages furnish a further diagnostic character in that two 

 pairs of antennae are present in the Crustacea (see, however, Trilobitze). 

 Antenna?, it will be remembered, are preoral appendages innervated from 

 the brain. In some cases, as many Entomostraca, the second pair may 

 lose their sensory functions and become mere swimming organs. 



llr 



FIG. 376. 



FIG. 377. 



FlG. 376. A pus equal-is* (after Packard). 

 FIG. 377. Antennal gland of Mysis (after Grobben). 

 external opening; h, bladder; re, canal; s, internal vesicle. 



FIG. 378. Otocyst of crayfish, as, crista acustica; n, nerve. 



FIG. 378. 

 blr, blood lacuna-; ea, 



Concerning the internal organs but few general remarks can be made. 

 Salivary glands are wholly absent; on the other hand, the stomodeum is 

 usually widened into a strong chewing 'stomach,' and behind this empty 

 the ducts of the so-called liver. This varies between the two simple blind 

 sacs of the Daphnidse (fig. 383) and the enormous livers of the Decapoda 

 (fig. 400, .1). It is not exclusively glandular (hepato-pancreas), but is an 

 expansion of the digestive tract, and like it partakes in the resorption of 

 nourishment. Excretory organs are represented by so-called green glands 



