19. The mesolilast colls now retreat from the periphery of the egg into the cleavage cavity. The hypoblastic cells 



also sink in toward the centre, and the formation of a gastnila is thereby hronght about. The figure shows 

 the gastrula stage in horizontal optical section. 



20. Gastrula at a somewhat earlier stage, in longitudinal optical section. Owing to the small size of the cleavage 



cavity, and the large size of the hypoblast cells, the gastrulation is slow and difiicult. When the (now 

 eight) lateral and anterior hypoblast cells begin to sink, the four central liypoblast cells also sink a little, 

 but as the former sink deeper, they compress the latter in such a way that they present the appearance 

 shown in the figure, c c, epiblast. 



21. Gastrula seen from the side. Tlie epiblast is seen in optical section, g m, gastrula mouth. 



22. Later stage. The gastrula mouth is closing up. Later it disappears entirely. Tliat this side answers to the 



later ventral side is highly probable. 



23. Stage when the second pair of antennne, II (the first appendages to appear), have begun to appear. Optical 



cross-.section. The otlier two pairs of nauplius appendages soon follow. The cesophagus of the na\iplius 

 arises from an invagination of tlie epiblast. 



24. Nauplius just hatched, seen from below. The intestine (i) as yet has no anal opening. Under the bilaterally 



disposed ectoderm cells, at the posterior extremity of the body, are seen the primitive mesoblast cells (pms). 

 From these are developed the mesodermic structures of the later- formed somites. The duration of the 

 egg-development is about twenty-four hours. 



Note. — In tlie nauplius of Cyclops scrrulatus and EiyasHus Sicboldi, Grobbeu detected the presence of 

 a dorsal organ equivalent to that in Plujllopoda, &c. See Arbeitcn Zoolog. Inst. Wien, II, p. 262, Taf. 

 XVI. figs. 61, 62, 1879. 



25. Later stage, lateral view. In the posterior part of the body, the rudiment of the genital system {g e n) 



has already appeared. The intestine is now furnished with an anus (a n). 



26. A little older metanaupliiis larva, lateral view. The fourth pair of appendages (raaxillfe) have appeared. 



n, brain, n', secondary brain, a thickening of the ectoderm, interpreted by Grobben as a rudimentary 

 organ representing the compound eyes of Phyl/opoda and theii- ganglia, which do not develop further in the 

 Copepoda. 



27. Anterior portion of the same stage, seen from below, oc s, cesophagus. oc !, eye. 



28. A little older metanauplius than that represented by Fig. 26, from below. The anterior and posterior maxil- 



lipeds (V, VI) have now made their appearance, as well as the first pair of swimming-feet (VII). Under 

 the cuticle is also seen the second pair of swimming-feet (VIII), which become free at the next moult- 

 According to Glaus and Grobben the so-called two pairs of maxillipcds of Copepoda really represent but one 

 pair of appendages, the anterior pair being the outer branches, the posterior pair the inner branches. Both 

 togetlier will then represent the second pair of maxillae of other Crustacea, and the first pair of swimming- 

 feet will be the homologue of the first niaxillipeds of Decapoda. Assuming this view to be correct, the 

 reader must bear in mind that, in the figures of Copepoda on this plate, the Roman numerals higher than 

 V must be lessened by one. 



29. First Cetochilus stage, from ventral side. The third pair of swimming-feet (IX) has appeared, mt, meta- 



stoma or paragnathite. At subsequent moults, new thoracic segments and appendages are developed, until 

 the adnlt state is attained. 



