DIPLOPODA — THE EMBRYONIC ENVELOPES. 



235 



second cuticular envelope is said to rise from the body. This 

 envelope is secreted after the ventral flexure has taken place 

 (Heathcote). Since the limbs were then developed, it no doubt 

 shows outgrowths corresponding to the latter. 



The fact that two cuticular envelopes (besides the later cuticular 

 covering of the body) form in Julus seems to be evident from 

 Metschnikoff's description, since confirmed by Heathcote, but 

 he does not throw much light on their nature. 



The larva of Julus passes through a resting stage within the pupal integument, 

 and finally reaches the stage at which other Diplopoda leave the egg. The 

 embryonic or pupal envelope, which has for some time become cpiite separated 

 from the body, splits, and the larva now for the first time becomes capable of 

 free movement. 



A B 



tfS^ 



Fig. 121. — Two larval stages of StrongyloS'ima Guerinii (after Metschnikoff, from Balfour's 

 Text-book). In A the larva is surrounded by the cuticular envelope which is provided with 

 the egg-tooth ; in IS it lias shed this envelope, and has entered upon free life, at, 

 antenna, above and posterior to which, in Fig. A, the egg-tooth can be seen; 3,i, -5, the 

 three pairs of legs of the embryo. 



"While, in Julus, the secretion of the cuticular embryonic envelope 

 takes place specially early, in other forms it occurs later, when the 

 limbs have already appeared as rudiments, so that these become 

 separately ensheathed by it (Fig. 118, ch). Its character as a larval 

 envelope is thus more evident. In Stronriylosoma a special larval 

 organ appears as a cephalic thickening of this envelope in the form 

 of a chitinous cone (Fig. 121 A). According to Metschnikoff, this 

 serves for splitting the egg-shell, and may thus be described as an 

 egg-tooth, like the corresponding organ in Geophilus. This latter, 

 indeed, belongs to a pair of limbs (Figs. 114 and 115, ez), and is 

 thus not homologous with the unpaired egg-tooth of the Diplopoda, 



