XVI 



DEVELOPMENT 



435 



within themselves, they develop a temporary stomach and a 

 large sucking organ, and become for a time independent sucking 

 animals, imbibing the fluids in the common sac, and arranged 

 around its circumference with their mouths directed towards the 

 centre. Subsequently a second embryonic stage is entered upon, 

 the sucking organ being discarded, and the albuminous matter 

 which the larva has imbibed being treated anew like the original 

 volk of the egg. 



v OO 



It is an interesting fact that in this second embryonic stage a 

 well-marked " tail" or post-abdomen is formed, and the gaiiglionic 

 nerve-masses increase in number, a cerebral mass being followed 

 by eight pairs of ganglia in the body and eight in the tail. 



A B 



FIG. 224. Three stages in the development of Chdifer. 



A, Segmenting ovum ; B, embryo, with post-abdomen, maximum number of 

 and developing sucking apparatus ; C, larva. (After Barrois. ) 



Subsequently a great concentration takes place till, besides the 

 cerebral mass, only five closely-applied pairs of ganglia remain, 

 corresponding to the pedipalpi and the four p;iirs of legs. More- 

 over, the iirst pair iidvances, so as to lie on the sides of, and not 

 behind, the oesophagus. 



There are two ccdyses or moults during de\ elopmcnt . a partial 

 moult, concerning only the ventral surface of the " pro-embryo" 

 as it assumes the larval form, and a complete moult at the linal 

 stage, he l'o re emergence IVom the incuhating sac. 



At the end of \\inter the mother cuts a hole in the silken 

 ^eh, and the young brood issues forth. 1 



Km- tin- embryology of Chernetidea, see .1. I 'urn-Mis, " M> m. sur le de* veloppemenl 



des CluUifers," y,'< /-. X///**/- <!< Zixil. iii., ls;it;. Met -clmikull', '/.< /V.sr// ,-. /</>.<:. Znnl. 

 xxi., 1876, ji. 511 : and \Yjdov>k\, i'"tujres ~oof. ml' runt tnnul il< MIIMHI. 1892, 

 p. 120, may also In: ronsultrd. 



