ACANTHOCEPHALI 



251 



they accumulate to form the structure called by Leuckakt 

 the embryonic nucleus or core. Moreover, layers of two 

 kinds can still be distinguished in the syncytium : an outer 

 tough one and a less firm inner one, which encloses the em- 

 bryonic nucleus. Leuckart had already shown that later the 

 greater part of the worm arises from this central portion 

 of the embryo. Furthermore, he compared it to a rudi- 

 mentary intestine, and showed how the solid body which he 

 found lying between the cephalic disc and the embryonic 

 nucleus (Fig. 113 B) could be interpreted as a rudimentary 

 pharynx. This conception seemed satisfactory in view of 

 the relationship of the intestineless Acanthocephali to other 

 worms (Nematoda). 



In the condition above described the embryos, enclosed in 

 their firm envelopes, pass out by means of the complicated 

 mechanism of the sexual conductive apparatus. They now 

 find themselves in the intestinal canal of the host— a fish 

 in the case of Echinorhynchus angustatus and proteus, the 

 hog in the case of Echinorhynchus gigas — and then reach 

 the outside world with the faeces of the animal. The em- 

 bryos of the latter species are swallowed by the larvae of 

 Cetonia aurata along with their food, whereas those of the 

 two woi-ms first mentioned are swallowed in the same way 

 by Asellus aquaticus and Gammarus pnJex. The embryonic 

 envelopes soften in the stomach of the new host, and the em- 

 bryo becomes free ; it immediately penetrates into the intes- 

 tinal wall, and comes to rest either here {Echinorhynchus gigas 

 and angustatus), or in the body cavity of the host. The 

 larva of Echinorhynchus angustatus also reaches the body 

 cavity later, but in a more passive manner as the i-esult of 

 its active growth and the rupturing of the intestinal mus- 

 culature. Here (in Gammarus pulex) are also found the 

 young stages of Echinorhynchus polymorphus, which as an 

 adult worm inhabits the intestine of ducks and other aquatic 

 birds (GtEEEFf). 



The further development of the larva is connected with 

 a metamorphosis of the external shape of the body, due to 

 the formative processes which take place within. In Echi- 

 norhijnchus gigas the middle part of the body swells greatly 



