1 64 THE APODID^: PART i 



larva. So far from its being comparable with the 

 Trochophoran larva of its original Annelidan an- 

 cestors; it is essentially a Crustacean larva, con- 

 taining from six to ten of the original Annelidan 

 segments, five of which are bent round to form the 

 head. This method of development by the appear- 

 ance at first of the head and the gradual addition of 

 the new segments has been clearly inherited from the 

 Annelids, and is, in this connection, very important. 

 This free-swimming larval Crustacean head develops 

 chiefly those organs which are necessary to it, those 

 not especially needful remaining rudimentary. These 

 useful organs are the anterior pair of sensory antennae 

 and the second pair of rowing antennae, which are also, 

 as we have seen, sensory organs. The use of the 

 third pair of limbs is not clear, unless they serve for 

 holding on to stationary objects ; it seems necessary 

 to attribute some function to them, since, if they had 

 no such larval function, they would probably appear 

 more in their definitive form. The two pairs of 

 maxillae have no function to perform in the larva and 

 are only developed later. It is generally said that 

 the second pair of limbs degenerates ; this is not 

 strictly true, only relatively so. They are precociously 

 developed in the larva, and, according to Brauer's 

 measurements for L. productus, continue to grow, 

 not, however, in proportion to the growth of the larva. 

 The slight change they undergo is due to a change of 

 function. The shield which belongs to the fifth seg- 

 ment, being a useful organ, is visible from the first 

 The unpaired "eye" is developed before the paired eye, 



