ON THE DEVELOPMENT OF PARASITIC COPEPODS. 6/ 



ectoderm (Fig. 45) but the forward movement of the appendages 

 carries their peripheral ends forward (Figs. 4548) until they 

 assume a longitudinal direction. Contractile fibrilLne begin to 

 form in the muscle cells in stage E (Fig. 47) and the nucleus 

 and undifferentiated cytoplasm is pushed to one side. In the 

 liberated nauplius the muscle fibers run almost the whole length 

 of the animal and show cross striations (Fig. 5 i). Each append- 

 age then has at least one muscle fiber attached' to the anterior 

 and one to the posterior border of its base. Muscle cells that 

 go into the hollow appendages as they grow out, form muscles 

 attached to the bifurcated ends of the appendages (Fig. 5 1 , left 

 side). 



The same description in general holds good for the dichelestid 

 and Lceniargns. In these the mesoderm of the appendages is 

 clearly derived only from marginal cells. In Lamargus the 

 ectoderm massed in the middle of the dorsal side by growth of the 

 dorsal muscle fibers forms a more conspicuous "dorsal organ" 

 than in the other species and the elements arising from its dis- 

 integration are more numerous. 



In relation to the formation of the appendages might be men- 

 tioned the segmentation of the nauplius of L^margns. Soon 

 after the closure of the blastopore the embryo is divided by bands 

 of thinner ectoderm into three segments corresponding to the 

 three pairs of nauplius appendages. This segmentation slowly 

 disappears with the development of the nauplius. Other species 

 show it but to a less degree than Lceinargus. This segmentation 

 might be used as evidence that the nauplius of ancestors of crus- 

 tacea was segmented or it might be considered as coenogenetic 

 and associated with the development of the appendages and 

 neuromeres of the nauplius. 



2. Post nan pi ins mesoblast (Pandarus si nit at us). 



At the closure of the blastopore some of the marginal cells are 

 turned in (Fig. 49, Mp) and become the mesoblast of the post 

 naupliar segments. These cells are much larger than the sur- 

 rounding cells (Fig. 41) and form a mass at the posterior end of 

 the animal that is destined to develop mesoblastic somites by 

 teloblastic growth. By rapid division the cells become small and 



