432 



CRUSTACEA. 



more clumsily ; they carry the eggs about with them in sacs, placed 

 to the right and left on the abdomen. Many of them possess a 

 cement gland at the end of the oviduct : the secretion of this gland 

 passes out with the eggs and gives rise to the stiff covering of the 

 ovisacs. During copulation, which is only an external approximation 

 of the two sexes, the male fastens one or 'more spermatophores on to 

 the genital segment of the female, and, indeed, on to special openings 

 through which the spermatozoa pass into the receptaculum seminis, 



and fertilize the ova either within 

 the body of the mother, or as they 

 pass out into the developing ovisacs. 

 Development takes place by means 

 of a complicated metamorphosis, 

 which, in many parasitic forms, is 

 a retrograde one. The larva?, when 

 hatched, have the Nauplius form, 

 with an unpaired frontal eye and 

 three pairs of appendages. Hooked 

 seta? on the second and third pairs 

 of appendages serve to conduct the 

 food into the mouth, which is 

 covered by a large upper lip (fig. 

 342, ). The posterior region of the 

 body is destitute of appendages, 

 and terminates with two seta? at the 

 sides of t lie anus; it corresponds to 

 the thorax and abdomen, which are 

 as yet undifterentiated. 



The alterations undergone by the 

 young larva? in the course of their 

 further growth are connected with 

 a number of successive moults, and 

 consist principally in an elongation 

 of the body and the appearance of 

 fresh appendages. Even in the next larval stage (fig 342, b), 

 a fourth pair of appendages, the future maxilla?, makes its ap- ' 

 pearance behind the three original pairs, which develop into the 

 antenna? and mandibles. In a later stage three fresh pairs of 

 appendages are formed. Of these the first corresponds to the 

 niaxillipeds, while the two last pairs represent the first rudiments 

 of the anterior swimming feet. In this stage (Metanauplius) (fig. 



FIG. 343. Metanauplius of Cycloysine. 

 O, eye ; G, rudimentary genital 

 organs ; SD, antennal gland ; A\ A 1 ', 

 anteunse ; MJ, mandible ; A/a-, max- 

 illa ; Mf, rnaxilliped. 



