/. CRUSTACEA. 



4:11 



may extend backwards as a shield, completely covering some or all 

 of the thoracic segments (fig. 412), or it extends right and left on 

 the sides of the body (fig. 426) and produces two valves strikingly 

 like those of a lamellibranch, the resemblance being strengthened 

 in the cirripeds and ostracodes by the extensive calcification. 



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 (better 



ttx 



FIG. 412. 



FIG. 413. 



PIG 412. Apus cancriformis. (After Ludwig-Leunis.) The segments mostly covered 



by the carapace. 

 FIG. 413. Antennal gland of Myste. (After Grobben.) Wr, blood lacunae; ea, external 



opening; /i, bladder; re, canal; s, internal vesicle. 



hepato-pancreas). The liver itself differs widely from the two 

 simple blind sacs of the Daphnidae (fig. 420) to the enormous 

 livers of the Decapoda (fig. 439, A). Excretory organs are repre- 

 sented by so-called green glands (antennal glands) and shell glands. 

 The latter, which received their name from the erroneous idea 

 that they produced the shell, open to the exterior on either side at 

 the bases of the fourth appendages, the maxillae (fig. 420, s). The 

 green gland opens similarly on the basis of the second antennas. 

 Both have essentially the same structure (fig. 413); they begin with 

 a terminal vesicle (in the case of the antennal gland in close rela- 

 tions with the reduced coelom), which passes into a slender, greatly 

 coiled tube. Their structure and development lead to the con- 

 clusion that they are modified segmental organs. Both occur 



