314 SYNOPTIC COLLECTION. 



ACANTHOCEPHALA. 



The development of Echinorhynchus (No. 780), a 

 member of the Acanthocephala, is instructive as showing 

 what changes a parasitic mode of life may bring about in 

 the embryo. The egg, in cleaving, forms four protective 

 embryonal membranes. At a very early stage the embryo 

 develops a disc with hooks, and the anterior end of the 

 body on which they are situated can be retracted. In 

 this condition the embryo still enclosed in its envelopes 

 leaves the parent while in the intestine of the hog (E. 

 gigas) or the fish (E. angustatus) and from the intestine 

 is ejected with the faeces. The embryos of E. gigas are 

 swallowed by the larvae of Cetonia aurata while eating. 

 In the stomach of this beetle the envelopes soften and 

 the enclosed embryo becomes free. It now pierces the 

 intestine of this intermediate host and undergoes a most 

 complicated change of structure resulting in a stage simi- 

 lar to the adult. This adult, however, does not become 

 sexually mature until its intermediate host, the beetle, is 

 devoured by its permanent host, the hog: in the intestine 

 of the latter animal its final development is reached. 



This worm is an excellent example of specialization by 

 the suppression of parts. The process has gone so far 

 that not only is the adult worm without a mouth and ali- 

 mentary canal, but the digestive tract is not even indi- 

 cated in the embryonic development. This tends to 

 prove that the ancestors of Echinorhynchus, near and 

 remote, have led an endoparasitic life, living upon a sup- 

 ply of already digested food which has been absorbed by 

 the walls of the body. 



Another change has taken place in these worms, 

 whereby the ventral side of the body cannot be distin- 

 guished from the dorsal side, a most unusual modifica- 

 tion of structure. 



The Annelida (consisting of the Chaetopoda, Oligo- 



