TRACHEATA 301 
produced into a process which helps to surround the mouth 
and form a biting organ. In the female of L. polyphemus, the 
first five of these appendages are chelate, but in the male the 
first is enlarged but is not chelate; in both sexes the sixth 
appendage terminates in a number of elongated flattened 
plates, and this limb is used in the burrowing or digging 
operations in which the animal delights. All these append- 
ages are borne on the prosoma; the seventh appendage, or the 
first mesosomatic, consists of a semicircular plate-like structure 
hinged on to the body and bearing on its posterior face the 
two genital pores. ‘This genital plate or operculum folds over 
and almost covers the five succeeding appendages, which are also 
plate-like, but, hke the former, exhibit traces of a double origin. 
Each of these, from the eighth to the twelfth, carries a pair of 
respiratory organs, in the form of branchiae composed of a great 
number of thin plates like the leaves of a book. Behind the 
twelfth is the unsegmented metasoma, which bearsno appendages. 
An internal skeleton or endosternite, in the form of a plate 
of fibro-cartilage, lies between the alimentary canal and the 
elongated nerve collar. It is not connected in any way with 
the exoskeleton, but gives origin to a number of muscles. <A 
somewhat similar structure is found in Apus and in some 
other Crustacea. 
The alimentary canal, as is usual with the Arthropoda, 
excepting the Insects, runs in a median line from mouth to 
anus without convolutions. The mouth is situated some dis- 
tance behind the anterior end of the body; it leads into a 
suctorial pharynx, which is lined by chitinous ridges. The 
pharynx runs forward and widens into a stomach, which turns 
back, curves posteriorly, and is separated by a valve from the 
mid-gut ; the latter is the absorbent part of the alimentary tract. 
It extends through the body, and terminates in a short procto- 
daeum with folded walls. The only gland which opens into the 
alimentary canal is a large yellow organ, the so-called liver, 
which communicates with the mid-gut by two pairs of ducts. 
The vascular system is very complete, and, unlike many 
Arthropods, the arteries do not end in irregular lacunar spaces, 
but are connected with the veins by a definite capillary system. 
The heart consists of an elongated tube, partially divided into 
