16 THE APODIDyE PART i 



ning round the front of the head, makes the latter 

 broad and flat, obliterates all external marks of seg- 

 mentation, and effectually disguises its origin out of 

 five Annelidan segments. 



The coils of the shell glands form one of the most 

 notable marks of the shield (see Frontispiece). Their 

 origin, position, and structure will be discussed in 

 the section on the excretory and other glands. 



THE BODY PROPER. 



On removing the shield we find a long vermiform 

 annulate body, Fig. 3. In the anterior part of the trunk 

 region the rings correspond in number with the limbs 

 or parapodia ; as we approach the posterior region* 

 however, the limbs are much more numerous than 

 the rings. We find two, three, four, or as many as 

 six rudimentary limbs on one ring. The last five 

 rings have no limbs at all. This whole phenomenon, 

 which has hitherto puzzled morphologists, may be 

 explained as follows. 



The great length of the original Annelid being 

 of no use to the Crustacean-Annelid, the hinder 

 part of the body remains in the latter at an un- 

 developed or larval stage. The rule in the develop- 

 ment of Annelid larvae is that the successive 

 segments form in front of the anal segment, and 

 differentiate from before backward, those furthest 

 from the anal segment being the most developed. 

 In Apus, we find in front of the anal segment 



