ioo COMPARATIVE ANATOMY 



shell gland passes through it, and opens at its extremity 



(fig. 22, V). 



The two pairs of pre-oral appendages are known as the first 

 and second antennae. The first antennae, though small, can 

 be recognised without difficulty. They lie within the thickened 

 anterior edge of the cephalic shield, a little to the outside 

 of the angle formed between it and the mandible. Each 

 antenna (fig. 22, /) consists of a proximal cylindrical segment 

 and a somewhat larger distal blade-like segment bearing two 

 or more setae at its extremity. The second antenna? (fig. 22, //) 

 are so minute and rudimentary that their existence has often 

 been denied. As a matter of fact they seem to be always 

 present at any rate, the author has never failed to discover 

 them in every specimen of A, cancriformis or the allied 

 Lepidurus productus which he has examined carefully. Being 

 evanescent organs, the second antennae vary considerably. In 

 most cases they are simple, unjointed filiform appendages, 

 the extremities of which may or may not be bent like a hook. 

 Occasionally, however, one finds a more perfect jointed 

 appendage consisting of two segments. Such a one is shown 

 in fig. 22. Though the two pairs of antennae are pre-oral 

 in position there is every reason to believe that they belong 

 to a post-oral series of appendages, as will be shown when 

 the nervous system and development are described. 



The internal anatomy is difficult to make out in the spirit-pre- 

 served specimens which alone are available for study in England. 



The mouth opens into a short oesophagus, which ascends 

 vertically towards the region of the eyes. The oesophagus 

 is lined by a chitinous reflection of the outer integument, 

 and is therefore a stomodaeum. In the upper part of the 

 head the oesophagus opens into a widish triangular sac or 

 stomach, which receives right and left the ducts of a pair of 

 digestive glands. These ducts curve round parallel to the 

 anterior border of the cephalic shield, and each gives off six 

 external branches, which in turn give off a number of short 

 ductules ending in glandular acini. The lobes of the digestive 

 gland occupy the greater part of the cephalic region. Another 

 pair of glands, described as salivary glands, open by small 

 ducts into the ventral side of the stomach. The last-named 

 organ is placed at right angles to the oesophagus, and narrows 

 posteriorly to form a straight intestine running backwards 



