III. ANNELIDA 



269 



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boscis, which may be retracted by muscles and exserted by contraction of 

 the muscular body wall. This proboscis bores into the intestinal wall and is 

 held in place by numerous retrorse hooks (fig. 250). The 

 entire absence of an alimentary canal marlcs them off from 

 Nematodes and Gordiacea, as also the peculiar structure 

 of the reproductive organs and a closed vascular system in 

 the body wall which extends into two sacs, the lemnisci, 

 lying beside the proboscis sheath. The unpaired ganglion 

 lies on the proboscis sheath between the lemnisci. An 

 intermediate host occurs in development, the larva living 

 in an arthropod. Thus the larva of Echinorhynchus gigas* 

 of the pig lives in the larva of the 'June bug' (Melohnlha), 

 that of E. prokus of European fresh-water fishes in Crusta- 

 cea. E. hominis is extremely rare in man. 



The Acanthocephali are dioecious. The ovaries of the 

 female early break up into groups of eggs which float in 

 the body cavity. The ripe eggs have a peculiar method 

 of escape from the body. There is a muscular uterus 

 which connects by two narrow canals with the vagina and 

 thus with the outer world. The uterus picks up immature 

 and fertilized eggs indiscriminately by its wide mouth, but 

 only those which are elongate, have a shell and contain 

 embryos can pass the canals; the immature eggs are led 

 through a ventral opening back to the ccelom. In E. 

 gigas protonephridia open beside the genital opening. 



Class III. Annelida. 



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-^ 



The metamerism, which occurs in a slight degree 

 in the Cha^tognathl, reaches its highest development 

 in the Annelids, where it appears both in the outer 

 ringing of the body and in the arrangement of the 

 most important systems — excretory organs, nervous 

 system, blood-vessels — internal segmentation. To 

 this is added an extraordinary increase in number 

 of body segments {somites, metameres), which can 

 far exceed a hundred. The epithelial longitudinal 

 muscles are reinforced by an outer layer of mesen- 

 chymatous circular fibres. We can thus define the 

 Annelids as worms with ccelom and with external 

 and internal segmentation. In the development 

 there frequently occurs a type of larva, the trocho- 

 phore (p. 238). 



The above account applies most closely to the 

 Chaetopoda and ArchianeUida. In other forms one 

 may be lacking — in the Gephyr^a segmentation of 

 Hirudinei most of the ccelom and the trochophore. 



-A 



Fig. 250. — Male 

 Echinorhynchus angu- 

 status (from Hatsch- 

 ek). b, penis sac; 

 de, seminal vesicle; 

 dr, giands; g, gan- 

 gtion; /, femnisci; lig, 

 iigament; m^m2, re- 

 tractors of proboscis 

 and its sheatli; p, 

 penis; r, proboscis; 

 rs, proboscis sheatli ; 

 t, testes; vd, vas def- 

 erens. 



or more features 



the body; in the 



Yet these are so 



