KIDNEYS 



two pairs — the antennaiy gland, opening at the base of the 

 second antenna, and the maxillary gland, opening on the second 

 maxilla. These two pairs of glands rarely subsist together in 

 the adult condition, though this is said to be the case in Nehalia 

 and possibly Mysis ; the antennary glands are characteristic of 

 adult Malacostraca^ and the larvae of the Entomostraca, while the 

 maxillary glands (" shell-glands ") are present in adult Entomo- 

 straca and larval Malacostraca, that is to say, the one pair replaces 

 the other in the two great subdivisions of the Crustacea. The shell- 

 gland of the Entomostraca is a simple' structure consisting of a 

 coiled tube opening to the exterior on the external branch of the 

 second maxilla, and ending blindly in a dilated vesicle, the end- 

 sac. The antennary gland of the Malacostraca is usually more 

 complicated : these complications have been studied especially by 

 Weldon,- Allen, and Marchal ^ in the Decapoda. In a number 

 of forms we have a tube opening to the exterior at the base of 

 the second antenna, and expanding within to form a spacious 

 bladder into which the coiled tubular part of the kidney opens, 

 while at the extremity of this coiled portion is the vesicle called 

 the end-sac. This arrangement may be modified ; thus in 

 Palaemon Weldon described the two glands as fusing together 

 above and below the oesophagus, the dorsal commissure expand- 

 ing into a huge sac stretching dorsally down tlie length of the 

 body. This closed sac with excretory functions thus comes to 

 resemble a coelomic cavity, and the view that it is really coelomic 

 has indeed been upheld. 



A modified form of this view is that of Vejdovsky, who 

 describes a funnel-apparatus leading from the coiled tube into 

 the end-sac of the antennary gland of Amphipods ; he regards 

 the end-sac alone as representing the coelom, while the funnel 

 and coiled tube represent the kidney opening into it. 



Not very much is known of the development of these various 

 structures. Some authors have considered that both antennary 

 and maxillary glands are developed in the embryo from ecto- 

 dermal inpushings, but the more recent observations of Waite ^ 

 on Homarus americanus indicate that the antennary gland at 



^ The Cumacea, Anaspidacea, and certain Isopods possess a maxillary gland 

 only. 



2 Qicart. J. Micr. Sci. xxxii., 1891, p. 27.9. 



* Arch. Zool. Exp. (2) x., 1892, p. 57. 



■* Bull. 3fus. Covip. Zool. Harvard, xxxv., 1899, p. 1,52. 



