218 



INVERTEBRATE ZOOLOGY 



Great Lakes at considerable depths and is identical with speci- 

 mens found in similar lakes of northern Europe. 



III. Order Stomatopoda 



In the Stomatopoda, the posterior three thoracic segments bear 

 complete biramous appendages which are used in swimming, 

 while of the five anterior to these all but the first bear appendages 

 modified as prehensile maxillipeds. The terminal segment of 

 these prehensile appendages folds into a groove of the preceding 

 segment as a knife blade folds into its handle. 



The carapace is short, for at least the posterior four segments 

 of the thorax are not included within it. Antennules and stalked 

 eyes are borne on two movable, inde- 

 pendent somites at the anterior extremity 

 of the head. Gills are borne on the 

 abdominal appendages. These are chiefly 

 burrowing forms living in mud. Squilla 

 (Fig. 102), Gonodactylus, Chloridella are 

 characteristic genera. 



IV. Order Decapoda 



The lobsters, crayfishes, shrimps, and 

 crabs agree with the Schizopoda in hav- 

 ing a carapace covering the entire cepha- 

 lothorax. The head bears stalked eyes, 

 antennules, antennae, mandibles, and two 

 pairs of maxillae. Of the thoracic append- 

 ages, the anterior three pairs are modified 

 as maxillipeds while the remaining five 

 pairs are locomotor and lack an exopodite. 

 All of the walking legs bear either pinch- 

 ers or claws. The first pair of walking 

 legs, which are known as the chelipeds, 

 bear extremely strong pinchers termed 

 the chelae. One jaw of the pinchers is 

 the movable distal segment of the endopodite called the finger, 

 while the other jaw is an immovable outgrowth from the next 

 to the last segment forming the thumb. 



■ The Decapoda have marked powers of regeneration. If an 

 appendage is removed, a new one begins to develop beneath the 

 old shell but does not become evident externally until the shell 



Fig. 



102. —Squilla 

 {From Bigclow). 



alba. 



