XIPHOSURA. 



789 



exopodite and helps to separate and hold apart the gill-book 

 appendages. 



The seventh pair of appendages are the chilaria of Owen (VII). 

 They are unjointed flattened processes, projecting vertically 

 downwards, and limiting the extra-oral space posteriorly. 



The significance of these processes has been much debated, and their 

 appendicular nature denied, but Kishinouye's discovery of a mesoblastic 

 somite and a pair of ganglia in the ventral chain corresponding to them 

 in the embryo, and Brauer's demonstration of a transient pair of appen- 

 dages in their position in the embryo of the Scorpion, remove all doubt 

 as to their nature. 



The number of the abdominal appendages is six pairs, corre- 

 sponding to that of the depressions in the upper surface and of 

 the movable spines at the sides. They are lamellar in form and 

 resemble the limbs of many Crustacea in so far as they are 

 partially divided into a slender internal process and a broad 

 external plate. The appendages of the anterior pair (Fig. 511, 

 10} are united in the middle line for the greater part of their 

 length to form the operculum (VIII) which bears the paired 

 generative apertures near the base on the posterior aspect. 

 The opercular endopodites are more prominent in the female 

 than in the male. The operculum is inserted, in the adult 

 on the hinder part of the cephalothorax, but in the embryo 

 its halves are clearly seen to 

 be the appendages of the an-, 

 terior abdominal segment. 



In the remaining five pairs 

 (IX-XIII) the appendages of 

 opposite sides are free from 

 one another (the sterna of the 

 segments from which they 

 spring being each produced into 

 a pointed process between 

 them) and their exopodites bear 

 on their posterior surfaces the 

 delicate branchial lamellae, 

 superposed like the leaves of a 

 book, to the number of 150 to 200 for each limb (Fig. 512). 

 The animal is able to propel itself in the water by the flapping 

 action of the abdominal appendages. 



A remarkable skeletal structure, the entosternite, lies in the 



FIG. 512. Posterior aspect of one of the 

 branchiate abdominal limbs of Limulus. 

 1 sternite of the segment ; 2 branchial 

 lamellae ; 3 outer, and 4 inner plate of 

 the appendage (after Van der Heyden). 



