waite: antennal glands in homakus americanus. 197 



Tlie shell gland of the second maxillary segment has long been con- 

 sidered as representing an Annelid nephridium, both from its resemblance 

 in function to that organ and from the fact that it is derived from both 

 ectoderm and mesoderm. This gland is widely distributed in Crustacea. 

 It is known as an adult organ, the shell gland, in many Eutomostraca 

 and some Malacostraca ; moreover, an organ similar in development aud 

 appearance is found during embryonic and larval life in the basal seg- 

 ment of the second maxilla of certain Malacostraca, although it disap- 

 pears before the adult stage is reached. As a larval organ the shell gland 

 has been described in the fifth segment of Palsemonetes (Allen, '93 a ) and 

 in some Schizopods and Decapods (Claus). 



Thus in many Eutomostraca and Malacostraca glands closely re- 

 sembling one another are developed in both the second (antennal) aud 

 fifth (second maxillary) segments ; but in some cases — chiefly Euto- 

 mostraca — that of the fifth segment remains as an adult organ (shell 

 gland), while that of the second segment atrophies. On the other hand, 

 in most of the higher Malacostraca, it is the organ of the second seg- 

 ment (antennal gland) which persists in the adult, while the organ of 

 the fifth segment, if it appears at all, degenerates before adult charac- 

 ters are assumed. 



According to the comparative anatomical work of A. Dohrn, Claus, and 

 Grobben, the shell gland is composed of the same parts as is the anten- 

 nal gland. 



The development of the shell gland in the different families of Crus- 

 tacea has been treated of by several authors. It appears that the gland 

 arises from two sources ; the mesodermic part forms the' endsac, the 

 ectodermic portion the " canal." Hence it agrees in the main with the 

 development of the antennal glands. We may therefore conclude that 

 the antennal and shell glands in Crustacea are serially homologous 

 organs, being similar in structure and development. 



Lebedinski ('89, p. 197, Tab. III. Figs. 79-81, '90, p. 184) describes in 

 Eriphia spinofrons the development of a " Segmentalorgan " as an evagi- 

 nation of the somatopleure, which becomes tubular and grows forward 

 until it ends blindly within the base of the first maxilliped. There arises 

 concurrently an ectodermic invagination in the wall of the coxopodite of 

 this appendage, which meets the evagination from the somatopleure, and 

 their lumina become continuous. We have, then, in this Crustacean a 

 gland in the sixth segment, which is apparently of the same series as the 

 antennal and shell glands. 



The branchial glands of Crustacea were first noted in Crayfish, Crabs, 

 vol. xxxv. — no. 7. 4 



