80 THE APODID^: PART I 



said ; the Annelidan character of the ventral cord of 

 Apus has long been acknowledged as an unmistak- 

 able sign of relationship between it and the Annelida. 

 Lankester has also called attention to the fact that 

 the ventral cord resembles more nearly that of a 

 Chaetopod than that of a Crustacean. He sees its 

 archaic character in the fact that the longitudinal 

 strands are separated by a considerable interval. 

 This reasoning is however doubtful, because in the 

 rudimentary segments of Apus the ganglia in each 

 segment are close together. While it is true that 

 a great interval between the longitudinal halves of the 

 ventral cord of an Annelid is generally supposed to 

 denote an archaic condition, this state in Apus has 

 clearly been secondarily acquired. Further, the pre- 

 sence of well-developed parapodia, which were 

 gradually transformed into Crustacean limbs, is con- 

 clusive evidence that the Annelid from which Apus 

 was derived was not a primitive form. The drawing- 

 out of the longitudinal commissures of the anterior 

 ganglia of the ventral corcl which, in the bent 

 Annelid, were massed together (see Fig. 18), is clearly 

 a secondary modification, due to the travelling 

 forwards of the brain. It will be referred to again 

 in the next section in connection with the migration 

 of the eyes. 



Till now, it has never been quite understood why 

 the ventral cord should suddenly cease with the 

 limbs, so that no ventral ganglia are developed in 

 the limbless segments. The explanation of this we 

 have already seen, viz., that the posterior end of the 



