igS WORMS. 



The sexes are separate, the reproductive organs are simple. 

 Except in the marine parasite Seison and two other forms, 

 the males are dwarfed and degenerate, destitute even of a 

 true food-canal. In many cases at least, sexual union 

 (effected by a penis) seems to be ineffective, and there is no 

 doubt that many if not most Rotifers are parthenogenetic. 

 The females lay three different kinds of eggs according to 

 their conditions and constitution — either small ova which 

 become males, or thin-shelled " summer ova," or thick-shelled 

 " resting or winter ova," the two last developing into females. 

 Many species, however, are viviparous. We include the 

 Rotifers beside the Annelids proper, because it seems possible 

 to regard them as derived from ancestors somewhat like 

 Annelid larvse. 



Rotifers living in fixed tubes or envelopes, — Melicerta, 

 Floscularia, Stephanoceros. 



Free Rotifers, — Notommata, Hydatina, Brachionus. 



Parasitic on the marine Crustacean J^ebalia, — Seison. 



Pedalion occupies a unique position ; it has hints of ap- 

 pendages and a peculiar jumping motion. 



Equally incerta scdis, but plausibly regarded as a specialised Trocho- 

 sphere, is the genus Dinophilus, with the nature of which advanced 

 students should make themselves acquainted. 



At this stage I may also mention that there are several sets of small 

 worm-like animals of which we know very little. It is quite possible 

 that some of them may become of great interest to the systematic 

 zoologist, but we do not yet understand what places in the system they 

 should occupy. Moreover, as they are small, unfamiliar, and unknown 

 to myself, I shall simply refer the curious to what more complete works 

 say about the Gasterotricha, Echinoderidse, Desmoscolecidae, and 

 Chsetoderidse. 



FIFTH SERIES OF "WORMS"— Four Classes. 

 Provisionally ranked together (by Lang) as Prosopygii. 



Class SiPUNcULOiDEA, e.g., Sipunculus. 



Marine worms usually living in the sand. The body is elongated, and 

 apparently unsegmented. The oral or anterior region can be invaginated 



