REPRODUCTION .c 



45 



are alike, but in M. lumhrici those of one parent, the ' female ' 

 are rounded, and those of the other, the ' male ', pear-shaped 

 (Fig. 19). Each zygote is known as a sporont ; it now secretes a 

 boat-shaped, horny case, and is known as a pseudonavicella. It 

 was given this odd name because of its resemblance to the diatom 

 (a small plant) called Navicella ; this word means a little boat, 

 and refers to the shape of the organism. Within the case the 

 pseudonavicella divides by repeated fission into eight sickle- 

 shaped sporozoites. There are thus two generations of spores in 

 the life-history of Monocystis.'^ The cysts fall into the cavity of 

 the body (the coelom, see p. 160) and accumulate in the hinder 

 segments, but no further development takes place until the 

 pseudonavicellae get free from the worm. This, presumably, 

 sometimes occurs through the last few segments being broken 

 off by autotomy (the fracture of a body or limb by its own con- 

 tractions), or the parasite may have to await the death of the 

 worm. In either case the cysts might later be swallowed by 

 another worm with the earth from which it obtains its food, 

 but proof of this is lacking. The spore-cyst is dissolved in the 

 intestine of the worm, and the sporozoites come out and bore 

 their way through the wall of the gut and other tissues till they 

 reach the vesiculae seminales. Possibly, occasionally, the sporo- 

 zoites pass directly from one worm to another during coition, but 

 as young worms are free from parasites this is unlikely. In the 

 vesiculae seminales each enters a sperm-mother-cell, where it 

 grows by absorbing the protoplasm which is meant to serve for 

 the nourishment of the spermatozoa (see p. 173). The latter are 

 formed, but wither, their tails only remaining attached to the 

 young Monocystis, which looks as though it had a coat of cilia. 

 Finally they disappear, w^hile the Monocystis continues to grow. 

 Thus the sporozoites become trophozoites by development. 



It has recently been claimed that some species of Monocystis 

 ingest cells of their hosts and form food vacuoles as do other 

 Protozoa. If this observation is confirmed they would be less 

 modified for a parasitic mode of life than at first appears. 



^ A spore is a small reproductive body formed by multiple or repeated fission. 

 It may or may not be a gamete. If it be enclosed in a case it is known as a 

 chlamydospore (e.g. pseudonavicellae), if it be naked, as a gymnospore (e.g. spores 

 of the Amceba shown in Fig. 12). Amoeboid spores are known as ama-buiae or 

 pseudopodiospores, flagellate spores as fiagellulae or flagellispores. 



