GEOWTH OF JULUS TEEEESTEIS. 283 



ducts, whereby the secretion of these caeca is conveyed out of the hody, 

 intercommunicate freely by means of short transverse canals, and, from 

 the sacculated appearance that they present towards their termination, 

 appear likewise to perform the office of reservoirs for the seminal fluid. 



(728.) In the female Julus, the organs of reproduction are as simple 

 in their structure as those of the male. They consist of a single elon- 

 gated bag or oviduct, covered on its exterior surface with a very great 

 number of ovisacs or cseca of various sizes, each of which secretes but a 

 single ovum. This oviduct extends backwards beneath the alimentary 

 canal from the vaginal outlet, which is double, and situated in the fourth 

 segment of the body, behind the second pair of legs. In the pregnant 

 female the oviduct appears smooth externally, being distended with the 

 ova that have passed into it from the ovisacs where they were formed, 

 and which are retained in readiness to be deposited immediately after 

 intercourse with the male. 



(729.) The ova, when fully developed, are found to present all the 

 structures belonging to a perfectly formed egg, the yelk, the germinal 

 vesicle with its macula, the membrana vitelli, the albumen, and likewise 

 the shell, lined by the membrana externa, or chorion, being all distinctly 

 recognizable. 



(730.) Another important distinction between these animals and 

 insects properly so called, is met with in the mode of their growth and 

 development. Insects (as we shall more fully explain hereafter) undergo 

 a more or less complete change in their outward form as they advance 

 through several preparatory stages to their mature state : during the 

 progress of these changes, that constitute what is usually called the 

 metamorphosis of insects, they are invariably unable to perpetuate 

 their species ; and it is only in their last or perfect condition, which is 

 ordinarily of very short duration, that the sexual organs attain their per- 

 fect development and are fit for reproduction. In this state all true 

 insects have six legs, which is one of the most important characters of 

 the class. The Myriapoda likewise undergo several changes of form as 

 they advance to maturity ; but these changes principally consist in the 

 repeated acquisition of additional legs ; so that in their perfect condition, 

 instead of the limited number of six legs met with in insects, these 

 organs have become extremely numerous. The progress of these trans- 

 itions from their immature to their fully-developed state has been well 

 observed by De Geer* and Savif ; and the result of their observations 

 is here given, in order that the reader may compare the different steps 

 of the process with what we shall afterwards meet with in the t more 

 highly organized Articulata. 



(731.) The eggs (fig. 140, A), which are very minute, are deposited 



* Memoires pour servir a 1'Histoire des Insectes. 7 vols. 4to. Stockholm, 1778. 

 f Osservazioni per servire alia storia di una specie di Julus comunissima. 

 Bologna, 1817. 



