14 THE APODIDy PART i 



Excepting in the number of limbs, all external traces 

 of its having been composed of five segments are 

 obliterated. Internally, however, there are abundant 

 indications of its origin from the bending of the 

 Annelidan segments. 



The mouth lies ventrally and faces posteriorly, a 

 fact which, taken in connection with the sharp bend 

 in the cesophagus, to be described later, is enough of 

 itself to suggest the original doubling of the segments. 

 This ventral position of the mouth is an important 

 characteristic of the whole class of the Crustacea, 

 which has not received the attention it deserves. 



Projecting ventrally and posteriorly over the mouth 

 is a large upper lip, corresponding with the prostomium 

 of the Annelid ancestor. This upper lip is thus a 

 primitive feature among the Crustacea ; it occurs in a 

 more or less pronounced form in most Nauplii, and 

 persisted as a very highly developed organ in the 

 Trilobites, but in the modern Crustacea it is 

 generally more or less rudimentary 



The under edge of the Annelidan mouth would also 

 naturally project backwards as a sharp fold (see Fig. I,/}. 

 Such a fold in Apus, however, would form an obstacle 

 to the pushing of food forward towards and into the 

 mouth by means of the maxillae and ventral parapodia 

 of the anterior trunk segments ; hence we find it 

 modified into two lateral projections, the middle part 

 of the fold being merely indicated by a low ridge, 

 which is not sufficient to form a barrier across the 

 ventral surface. These two projections have been 

 handed on to the higher Crustacea as the paragnatha. 



