248 



METAZOAN PHYLA 



288. Reproduction. — Reproduction has been referred to in the case 

 of the earthworm and of the leech. Sexual reproduction in other forms 

 is similar. There is, however, a type of asexual reproduction by means 

 of transverse fission which sometimes occurs in annelids, the result of 

 which is to produce a colony of several individuals moving about together 

 (Fig. 149). In Aeolosoma this is the ordinary mode of multiplication, 

 and sexual reproduction seems to be rare. 



In some of the polychaets the sex cells are produced from the epithe- 

 lium lining the coelomic cavities in the posterior metameres of the body- 



In such cases, as these cells mature, 

 this part of the body becomes changed 

 (Fig. 150). Sometimes the para- 

 podia increase in size and exhibit 

 foliaceous outgrowths, and new 

 setae are developed, larger than the 

 ordinary ones and flattened, both of 

 these changes being in the nature of 

 adaptations for swimming. Then 

 the sexual part of the body separates 

 by fission, forming a swimming sexual 

 zooid with a new head, tentacles, 

 and eyespots. The asexual zooid 

 which remains regenerates all the lost 

 metameres and is ready to repro- 

 duce again. In one family the sexual 

 zooid does not at once separate; both 

 it and the asexual one multiply by 

 transverse fission and this continues 

 until a chain of as many as 16 

 individuals is produced, the anterior 

 ends of each of which are asexual 

 and the posterior sexual. All of the 

 individuals in a chain are of the same sex. In still another type, which 

 lives in a sponge in the eastern seas at a depth of over 100 fathoms, 

 a branched colony of worms is produced by lateral budding. 



The larva of most of the marine annelids is a trochophore (Fig. 143 A 

 and B). This larva does not occur in fresh- water forms and of course is 

 never seen in the terrestrial types. 



289. Occurrence and Economic Importance. — The distribution of 

 various annelids, especially the marine forms, is very general, though 

 characteristic species are found in each region. Economically the earth- 

 worm has been shown to be generally a very beneficial type. The 

 leeches, which live a parasitic life, should be considered injurious, enemies 

 of both man and the domestic animals which serve him. The medicinal 



Fig. 149. — A chain of individuals 

 formed in an annelid, Myrianida sp., by 

 budding. {From Hertwig and Kingsley, 

 "Manual of Zoology," after Milne-Edwards, 

 by the courtesy of Henry Holt & Company.) 

 The budded individuals develop in order, 

 the one at the end of the chain being the 

 oldest; compare with asexual reproduction 

 in the tapeworm colony. 



