380 ACANTHOCEPHALA. 



called by Leuckart the embryonic nucleus. The embryonic nucleus becomes 

 divided into four linearly arranged groups of cells, of which the hindermost 

 but one is the largest, and very early differentiates itself into (i) a peripheral 

 layer, and (2) a central mass formed of two distinct bodies. The peripheral 

 layer of this segment grows forwards and backwards, and embraces the 

 other segments, with the exception of the front end of the first one which 

 is left uncovered. The envelope so formed gives rise to the splanchnic and 

 somatic mesoblast of the adult worm. Of the four groups of cells within it 

 the anterior gives rise to the proboscis, the next to the nerve ganglion, the 

 third, formed of two bodies, to the paired generatives, and the fourth to the 

 generative ducts. The whole of the above complex rapidly elongates, and 

 as it does so the enveloping membrane becomes split into two layers ; of 

 which the outer forms the muscular wall of the body (somatic mesoblast), 

 and the inner the muscular sheath of the proboscis and the so-called gene- 

 rative ligament enveloping the generative organs. The inner layer may be 

 called the splanchnic mesoblast in spite of the absence of an intestine. 

 The cavity between the two mesoblastic layers forms the body cavity. 



The various parts of the adult continue to differentiate themselves as 

 the whole increases in size. The generative masses very early shew traces 

 of becoming differentiated into testes or ovaries. In the male the two 

 generative masses remain spherical, but in the female become elongated : 

 the rudiment of the generative ducts becomes divided into three sections 

 in both sexes. The most remarkable changes are, however, those undergone 

 by the rudiment of the proboscis. 



In its interior there is formed a cavity, but the wall bounding the front 

 end of the cavity soon disappears. By the time that this has taken place 

 the body of the adult completely fills up the larval skin, to which it very 

 soon attaches itself. The hollow rudiment of the proboscis then becomes 

 everted, and forms a papilla at the end of the body, immediately ad- 

 joining the larval skin. This papilla, with the larval skin covering it, 

 constitutes the permanent proboscis. The original larval cuticle is either 

 now or at an earlier period thrown off and a fresh cuticle developed. The 

 hooks of the proboscis are formed from cells of the above papilla, which 

 grow through the larval skin as conical prominences, on the apex of which 

 a chitinous hook is modelled. The remainder of the larval skin forms the 

 skin of the adult, and at a later period develops in its deeper layer the 

 peculiar plexus of vessels so characteristic of the Acanthocephala. The 

 anterior oval appendages of the adult cutis, known as the lemnisci, are 

 outgrowths from the larval skin. 



The Echinorhyncus has with the completion of these changes practically 

 acquired its adult structure ; but in the female the ovaries undergo at this 

 period remarkable changes, in that they break up into a number of spherical 

 masses, which lie in the lumen of the generative ligaments, and also make 

 their way into the body cavity. 



The young Echinorhyncus requires to be transported to its permanent 

 host, which feeds on its larval host, before attaining to sexual maturity. 



