NEMATELMINTHES. 379 



(389) H. A. Pagenstecher. Die Trichinen nach Versnchen dargestellt. Leip- 

 zig, 1865. 



(390) A.Schneider. Monographic d. Nematoden. Berlin, 1866. 



(391) A. Villot. "Monographic des Dragoneaux" (Gordioidea). Arrives de 

 Zool. expcr. et ghier., Vol. III. 1874. 



ACANTHOCEPHALA. 



The Acanthocephala appear to be always viviparous. At the time of 

 impregnation the ovum is a naked cell, and undergoes in this condition the 

 earlier phases of segmentation. 



The segmentation is unequal (Leuckart, No. 393), but whether there is an 

 epibolic gastrula has not clearly been made out. 



Before segmentation is completed there are formed round the ovum 

 thick protecting membranes, which are usually three in number, the middle 

 one being the strongest. After segmentation the central cells of the ovum 

 fuse together to give rise to a granular mass, while the peripheral cells at a 

 slightly later period form a more transparent syncytium. At the anterior 

 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 



1 Echin. proteus, which is parasitic in the 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. 



