KING CRABS. 



213 



35 -- THE SPECTRE, OR SKELETON 



SHRIMP (Caprella). 



" Their usual mode of progression is compared by Fabricius, Goodsir, and Gosse, to that of the 

 larvae of the Geometric Moths. They sometimes walk in this way for a considerable time, and then 

 suddenly stop, remaining perfectly motionless, not even moving their antennae. They seldom 

 attempt to swim, and will, when placed in the water, independently 

 of anything to rest upon, generally drop to the bottom." Like all the 

 lower Crustacea, the Caprellce cast their skins often. Before the 

 process commences, the animal lies for a time to all appearance dead. 

 The Skeleton Shrimp carries its ova in an incubatory pouch, 

 which is developed when required. "It consists of four plates, 

 two attached to the third and two to the fourth segment of the 

 body, arising upon the under surface and the inside of the bran- 

 chiae. As soon as the young are old enough to enjoy a separate 

 state of existence they quit the protection of the pouch in which 

 they have been nurtured, and, passing out, climb, gipsy-like, to the back of their mother, where 

 they are seen holding on in every conceivable attitude. In the British Museum is preserved a speci- 

 men of an exotic species in which death has not separated the parent from her offspring. They 

 may be seen attached, as if climbing from the incubatory pouch to the back of the parent." 



THIRD LEGION. MEROSTOMATA. 



ORDER V. XIPHOSURA (KING CRABS). 



The Merostomata, or " thigh-mouthed Crustacea," are represented to-day by the Horseshoe Crabs 

 of America (Fig. 36) and the King Crabs of the China Seas (Fig. 37). There is only one living genus 

 (Liimdus), but it is found as far back in time as the Lower Secondary rocks, whilst forms, differing 



but little from those 



now existing, occur 



pretty numerously in 



the Coal Measures of 



England and America, 



and one is found 



so far back as the 



Upper Silurian for- 

 mation. The soft parts 



are encased within a 



double shield - shaped 



shell divided into two 



parts, the first repre- 

 senting the head and 



the second the thorax 



and abdomen. The 



eyes are fixed on the 



anterior surface of the 



head - shield, beneath 



which are the walking 



limbs. The abdomen, however, is quite rudimentary, being 

 partly represented by the posterior portion of the hind or thoracic 

 shield, and partly by the long ensiform tail-spine. Under the 

 hinder shield the leaf-like gills are placed. But in the larvae 

 we find the body-segments free and unanchylosed, and the tail- 

 spine undeveloped, thus bearing out the characters common to the 

 class, and connecting the living Limulus of to-day with its Fig. 37. THE KIXG CRAB (Limulus 



J moluccanus). 



far-off ancestors in the Coal and Silurian periods. The limbs are 



all attached to the head, and correspond to the antennae and the jaw-feet of the Crab and Lobster. 



They are, however, called upon to fulfil the double oifice of jaws and legs, which they do most 



Fig. 36. THE HORSESHOE CRAB (Limulits poly- 

 , UNDER SURFACE SHOWING JAWS. 



