CRUSTACEANS 403 



Appendages. — The letter Y may be taken as a diagram of 

 the type on which the appendages are constructed, the stem 

 of the letter corresponding to a basal stalk by which the appen- 

 dage is attached to the body, while its two forks represent 

 outer and inner branches. The same thing may be expressed 

 in another way, by saying that the typical limb in a higher 

 Crustacean is cleft or forked. We will now see how far the 

 appendages of the Lobster conform to this Y diagram, and in 

 doing so it will be convenient to start with the abdomen, as 

 this is the least modified region. Of the seven segments here 

 present only the first six bear appendages, while the last, usually 

 known as the telson, forms the middle portion of the tail-fin. 

 An average abdominal segment, say the fourth, bears two small 

 forked appendages somewhat inappropriately termed swimmerets, 

 and it will be seen from the diagram that these conform to the 



Y type. The appendages of the sixth abdominal segment are 

 relatively large, and form the side-parts of the great tail-fin. 

 Each of them, however, may be compared to a Y in which the 

 main stem is shortened and broadened, while the two branches 

 are flattened out into oval plates. It may further be noted 

 that the first two pairs of abdominal appendages in the male 

 are curiously modified, while in the female the appendages 

 of the first abdominal segment are either absent or very much 

 reduced. 



Thoracic appendages. — The four hindermost segments of the 

 thorax are stout -walking-legs, of which the two first pairs end 

 in small pincers. At first sight they deviate entirely from the 



Y type, for they are obviously not forked. This is a case 

 where the facts of development are of use in throwing light on 

 a problem, for we find that in a very young Lobster these 

 walking-legs are forked, but their outer branches are compara- 

 tively feeble and ultimately disappear altogether. There is a 

 further peculiarity about the three first pairs of these legs to 

 which attention must be called, but before doing so it is 

 necessary to speak of the nature and position of the breathing 

 organs or gills. These are delicate plume -like outgrowths of 

 the body, limited to the thoracic region, and sheltered in a gill- 

 cavity on either side, the outer wall of which is formed by a 

 firm gill-cover that constitutes the side of the carapace or hard 

 shield covering the greater part of head and thorax. When 



