368 COMPARATIVE ANATOMY CHAP. 



The fact that in many Crustacea, e.g. the Branchiopoda, which go 

 through a long series of metamorphoses from the Nauplius larva, the 

 pairs of ostia are formed with the heart in order from before backward, 

 cannot be brought up as an objection to the above. For this method 

 of development corresponds in general with the manner of ontogenetic 

 differentiation of the Arthropodan and Annelid body which takes place 

 in that order. The whole question is closely connected with the view 

 to be treated of later as to the phylogenetic significance of the Crus- 

 tacean larval forms. 



VIII. The Excretory Organs (Antennal Glands, Shell Glands). 



Although comprehensive comparative investigations as to the 

 methods of excretion are still wanting, we at least know that this 

 function is performed in very various ways and by very various organs. 

 We shall here take into consideration only two of these organs, the 

 shell and the antennal glands. Certain intestinal appendages and 

 dermal glands which also seem to serve for excretion, are mentioned 

 in the sections on the intestine and the integument. It must not, 

 however, be thought that this exhausts the number of parts of the 

 body which have some share in excretion. 



Confining ourselves to the antennal and shell glands, we note : 



(1) Number and Position. Each of these glands occurs as a single 

 pair. The former emerge at the basal joint of the posterior antennae. 

 The gland itself lies either entirely in this basal joint or more or less in 

 the adjoining cavity of the head. The shell gland lies in the shell 

 fold or in the cephalothoracic carapace in a region which originally 

 corresponds with the 2d maxillar segment. Its aperture lies on or 

 near the posterior maxillae. 



(2) Occurrence. The antennal gland is widely distributed in the 

 Crustacea. It seems wanting only in the Isopoda. ' While in the 

 Malacostraca it is, as a rule, best developed in the adult, in the Ento- 

 mostraca it only appears in the larval stages, and it but rarely per- 

 sists in the adult 6ven as a rudiment. In the Decapoda the antennal 

 gland has been called the green . gland. The shell gland is widely 

 distributed among adult Entomostraca. Among the Malacostraca it has 

 been observed in Nebalia, and further, in the Isopoda, Anisopoda, 

 Cumacea, and in the larvae of some forms (Sergestes, Euphausia) in whose 

 adult condition it is wanting. In Nebalia it is found in a reduced 

 condition. 



(3) Structure and Development. The structure of the antennal 

 gland (Fig. 246) is everywhere essentially the same. We distinguish 

 in it (1) a terminal saccule, (2) a coiled urinary canal which emerges 

 through (3) a urinary bladder on the basal segment of the posterior 

 antennae. The constitution of the epithelial wall is different in the 

 terminal sac and in the urinary canal. The wall of the latter is often 

 shown in transverse section to consist of one single cell, its lumen thus 



