SECT. IV THE MUSCULATURE 57 



themselves have clearly disappeared ; their sinewy 

 connections, however, being more resistant, and still 

 functional as attachments for the mandibular, maxillar, 

 and oesophageal muscles, &c., have been retained, 

 massed together as described in our imaginary 

 Annelid. This sinewy mass is known as the sternal 

 plate or entosternite. It is clear that, if the origin we 

 have ascribed to it is correct, it becomes a morpho- 

 logical characteristic of great value, and, whenever 

 met with among the Crustacea in the same position, 

 i.e., within the angle of the bent intestine, must be 

 referred back to the ventral muscle bands of a bent 

 Annelid. We shall see in the second part how im- 

 portant this point is in establishing the relationship 

 between Apus and Limulus. The Arachnida are, 

 we believe, the only other animals with an entoster- 

 nite. The origin and significance of it in this case 

 will be discussed in another section. 



Again, turning to the posterior end of the body, 

 we find, as we expected, that as we pass from front 

 to back the longitudinal bands gradually widen out, 

 as the limbs are less and less developed, until, in 

 the limbless segments, they unite to form a simple 

 dermo-muscular tube. 



Thus, in the arrangement of its longitudinal 

 musculature, Apus is a typical carnivorous Annelid 

 with its five anterior segments bent round in adapta- 

 tion to the browsing manner of life. 



On turningnow to the circular musculature, we find 

 a more complete differentiation. The circular muscle 

 layer in the carnivorous Annelids is, as a rule. 



