124 MORPHOLOGY OF IXVKRTKBRATE TYPES 



or segments. Externally it may be divided into an anterior 

 fused portion called the cephalothorax and a posterior clearly 

 segmented portion called the abdomen or pleon. The cephalo- 

 thorax is composed of fourteen segments, the anterior six of 

 which belong to the head or cephalon, the posterior eight to the 

 thorax. The somites of one region of the body have little in 

 common with the somites of the other two regions, since every 

 component of a somite has been modified to meet the require- 

 ments of position and function. Thus the somites of the thorax 

 became fused together to give more rigidity to this region, rigid- 

 ity needed on account of the powerful development of the 

 thoracic legs or pereiopods, especially those of the first pair. 



Skeleton. The skeleton of the lobster is of two kinds. 

 One is external, produced as a chitinous secretion by the hypo- 

 dermis of the integument, and is called the exoskeleton. The 

 other is internal, produced by infoldings of the ectoderm called 

 apodcmes. This skeleton is known under the term of endophrag- 

 mal skeleton or simply endoskeleton. The exoskeleton is thick 

 and calicified. Its color is derived from pigment produced by 

 special cells or chromatoblasts situated below the hypodermis. 

 The exoskeleton of every segment being the product of the 

 integument, naturally has more or less the shape of a ring or 

 cylinder, as, for example, in the case of the abdominal somites. 

 The dorsal surface of such an exoskeletal ring is called the 

 tergum or tergite, the ventral the sternum or sternite, and the 

 sides the pleura. The calcification of the pleurae in this case 

 makes the ring thoroughly rigid and the pleurae themselves 

 little distinct. In many arthropods, however, the pleurae re- 

 main as soft as the intersegmental membrane, and only the 

 tergite and sternite are hard. Such is the case with many 

 arachnids and insects. Where the somites are separated from 

 each other by a thin articulating membrane, there the corre- 

 sponding sternites and tergites are easily recognized. But 

 when adjoining somites fuse together, it becomes often difficult 

 to recognize them and may even require special comparative 



