I. CRUSTACEA. 



411 



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

 of the thoracic segments (fig. 413), or it extends right and left on 

 the sides of the body (fig. 420) and produces two valves strikingly 

 like tliose of a lamellibranch, tlie resemblance being strengthened 

 in tlie cirripeds and ostracodes by the extensive calcification. 



Concerning tlie internal organs bnt few general remarks can 

 be made. Salivary glands are wholly absent; on tlie other liand 

 the stomodeiim is usually widened into a strong chewing ' stomach,' 

 and behind this empty the ducts of the so-called liver (better 



Fig. 413. 



Fig. 413. 



Fig. 412.— ■4;7»s cnncriformis, (After Ludwig-Leiinis.) The segments mostly covered 



bv the carapace. 

 Fig. 413.— Antennal gland of Mysix. (After Grotaben.) Clr, blood lacunas; ea, external 



opening; /(, bladder; ?t, canal; s, internal vesicle. 



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

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

 livers of tlie Decapoda (fig. 4.39, A). E.xcretory organs are repre- 

 sented by so-called (/ree)i glands (antennal glands) and slicll glands. 

 The hitter, which received their name from the erroneous idea 

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

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

 green gland opens similarly on the basis of the second antenna?. 

 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 



