160 MORPHOLOGY OF INVERTEBRATE TYPES 



running from it obliquely forward marks the limit between the 

 head or cephalon and thorax. This groove is an infolding of the 

 integument, serving for the attachment of the gastric muscles 

 and corresponds evidently to the cervical groove of the lobster. 

 The thoracic sulci running toward the lateral margins of the 

 carapace still represent the last traces of the external segmenta- 

 tion. At the anterior end of the carapace are eight beadlike 

 eyes. The portion of the carapace between the eyes and chelicera 

 is called clypeus. On the ventral surface of the cephalothorax 

 between the coxae of the limbs there is a plate called sternum. 

 It represents the fused sternites of the five thoracic somites. 

 Attached to the anterior edge of the sternum is a movable lower 

 lip. The latter protects the mouth from below as the upper Up 

 or rostrum does from above. Between the rostrum and the 

 clypeus edge are two powerful chelicera. 



The pedicel which binds the abdomen to the cephalothorax 

 has a dorsal sclerite of peculiar shape. It is called lor um and 

 represents the tergite of the first abdominal somite. 



The abdomen has the shape of an elongated egg. It terminates 

 in an anal tubercle in front of w r hich and ventral in position are 

 six spinnerets. Immediately in front of the anterior spinnerets 

 is a small spiracle or opening leading into the tracheal tubes. 

 Not far from the anterior edge of the ventral surface is a trans- 

 verse genital fold. At the ends of the fold are two lung-slits 

 leading into the book-lungs. The integuments of the abdomen 

 are soft and do not show any traces of segmentation. 



It is clear that in order to understand segmentation in Agelena, 

 one must seek other evidence than that afforded by the structure 

 of the exoskeleton. Here the anatomy of internal organs, com- 

 parative anatomy and embryology come to our aid. Some 

 Arachnida, such as scorpions and Solifugae, possess a segmented 

 abdomen. The nervous system retains even in adult spiders its 

 composition of distinct neuromeres, and the embryo shows a 

 transitory external segmentation of the abdomen which is even 

 supplied with appendages. With all that the opinion as to the 



