314 ACANTHOCEPHALA. 



end of the embryo there appears a superficial cuticle bearing in front a ring 

 of hooks. 



The embryo is now carried out with the excreta from the intestine of 

 the vertebrate host in which its parent lives. It is then swallowed by some 

 invertebrate host 1 . 



In the intestine of the invertebrate host the larva is freed from its 

 membranes, and is found to have a somewhat elongated conical form, ter- 

 minating anteriorly in an obliquely placed disc, turned slightly towards the 

 ventral surface and armed with hooks. Between this disc and the granular 

 mass, already described as formed from the central cells of the embryo, is a 

 rather conspicuous solid body. Leuckart supposes that this body may re- 

 present a rudimentary functionless pharynx, while the granular mass in 

 his opinion is an equally rudimentary and functionless intestine. The body 

 wall is formed of a semifluid internal layer surrounding the rudimentary 

 intestine, if such it be, and of a firmer outer wall immediately within the 

 cuticle. 



The adult Echinorhyncus is formed by a remarkable process of develop- 

 ment within the body of the larva, and the skin is the only part of the 

 larva which is carried over to the adult. 



In Echinorhyncus proteus the larva remains mobile during the forma- 

 tion of the adult, but in other forms the metamorphosis takes place during 

 a quiescent condition of the larva. 



The organs of the adult are differentiated from a mass of cells which 

 appears to be a product of the central embryonic granular mass, and is called 

 by Leuckart the embryonic nucleus. The embryonic nucleus becomes divided 

 into four linearly arranged groups of cells, of which the hinderniost but 

 one is the largest, and very early differentiates itself into (1) 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 



1 Echin. proteus, which is parasitic in trie adult state in many freshwater fish, passes 

 through its larval condition in the body cavity of Gammarus pulex. Ech. angustatus, 

 parasitic in the Perch, is found in the larval condition in the body cavity of Asellus 

 aquaticus. Ech. gigas, parasitic in swine, is stated by Schneider (No. 394) to pass 

 through its larval stages in maggots. 



