154 



PANTOPODA. 



During metamorphosis, the rudiments of the genital organs which 

 were not ohserved in the six-limbed larva become recognisable. 

 In the larva with four pairs of limbs (Fig. 71 B), a compact mass 

 of cells, the first rudiment of the genital gland, lies in the median 

 line dorsally to the enteron, somewhat near the fourth pair of 

 limbs (or first ambulatory limbs). The anterior end of this mass 

 splits later into two parts, which grow out towards the bases of the 

 limbs just mentioned. The posterior end of the germ-gland then 

 splits in the same way, the genital tubes which run into the limbs 

 thus arising. The wide tubular rudiment of the heart has formed 

 at the anterior part of the body, also from mesoderm cells, dorsally 

 to the genital rudiment, and thus directly beneath the integument. 



Fig. 72. — Larvae of Nymplum brevicollum soon after hatching. A, dorsal, 1), ventral aspect 

 (after Hoek). I-V, the five anterior limbs; bg, ventral chain of ganglia; d, yolk-mass; 

 di, diverticulum of the yolk-filled enteron in the limb; g, brain ; s, proboscis. 



The differences observed in Pallenc and Nymjihon give rise to the question 

 as to which method of development is to be considered the more primitive 

 among the Pantopoda ; in this respect the appearance of larval organs and the 

 casting of a larval integument, observed by Dohrn in Pallene, suggest that 

 the direct development of this form must be regarded merely as an abbreviation 

 of the indirect method of development, and that the latter is the more 

 primitive. In consequence of the more complete development of the embryos 

 in the egg the latter must have a richer supply of nutritive material. The 

 large amount of yolk in the eggs of Pallenc and Nymplwn would, under these 

 circumstances, appear as a later acquisition, and it then seems doubtful whether 

 we ought to ascribe to the first ontogenetic processes of these eggs a truly 

 primitive character, although we feel inclined to do so on account of the greater 

 resemblance of their development to that of other Arthropoda. 



The course of development in Phoxichilidium differs from that 

 of other Pantopoda in that the form of the larva undergoes 



