ACE RAT A 



231 



in the ocean itself. None of them have any economic 

 importance aside from their acting as scavengers and serv- 

 ing as food for fishes. 



CLASS II. ACERATA. 



In these arthropods the body is divided into two regions 

 a cephalothorax in front and an abdomen behind. The 

 cephalothorax bears the eyes (of which there may be sev- 

 eral pairs) and six pairs of appendages, none of which 

 can be considered as antennae. The abdomen may have 

 or may be without apparent appendages. The respira- 

 tory organs are confined to the abdomen, and in their 

 development are always connected with the abdominal 

 limbs. They may be of three kinds: (1) External gills 

 borne on the abdominal legs; (2) internal sacs (lungs) 

 with numerous leaf -like folds; (3) air-tubes or tracheae, 

 strikingly like those of the Insecta, 

 but with a different history. The 

 reproductive organs open near 

 the middle of the body. 



SUBCLASS I. MEROSTOMATA. 



Here belong the horseshoe 

 crabs (fig. 59) of our east 

 coast (and a number of fossil 

 forms), which breathe by means 

 of leaf-like gills; which have 

 both simple and compound eyes, 

 and which have the bases of 

 the walking-feet of the cephalo- 

 thorax modified to serve as jaws. 

 Recent investigations show that 

 the horseshoe crabs are not related to the true crabs, but 

 are to be rather closely associated with the scorpions. 



Fio.59. Horseshoe crab (Limu- 

 lus polyphemus). 



