218 SEGMENTED WORMS OR ANNELIDA. 



and Floscularid) sheltered within an external tube. There is no 

 internal segmentation, but there are sometimes external rings, and the 

 attaching outgrowth or ' ' foot " is sometimes segmented. The anterior 

 end bears, on a retractile ridge, the ciliated ring or " trochal apparatus." 



The nervous system is a single dorsal ganglion with a few nerves. An 

 unpaired eye and some tufts of sensory hairs are usually present. 



The food canal extends- along the body in a well-developed ccelom, 

 and the fore-gut contains a mill, in which two complex hammers beat 

 upon an anvil. The canal ends posteriorly on the dorsal surface 

 between the body and the foot, and as the terminal portion also 

 receives the excretory canals and the oviduct, it is called a cloaca. 



There is no vascular system, but a nephridial tube of a primitive type 

 lies on each side of the body, and opens posteriorly into the cloaca. 



The sexes are separate ; the reproductive organs are simple. Except 

 in the marine parasite Seison, in Rhinops vitrea, and two or three other 

 forms, the males are dwarfed and degenerate, destitute even of a true 

 food canal, and often "little more than perambulating bags of 

 spermatozoa." 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. No males have as yet been found 

 in Philodina, Rotifer, Callidina, or Adineta. 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. The so-called winter eggs may occur at any season, and 

 seem usually to have been fertilised. Many species, however, are 

 viviparous. We include the Rotifers beside the Annelids proper, be- 

 cause it seems possible to regard them as derived from ancestors 

 somewhat like Annelid larvae. 



Rotifers living in fixed tubes or envelopes, — Melicerta, Floscularia, 

 Stephanoceros. 



Free Rotifers, — Notommata, Hydatina, Brachionus. 



Parasitic on the marine crustacean Nebalia, — Seison. 



Pedalion occupies a unique position ; it has hints of appendages and 

 a peculiar jumping motion. 



At this stage it may be mentioned 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, un- 

 familiar, and unknown to myself, I shall simply refer the curious to 

 what more complete works say about the Gasterotricha, Echinoderidse, 

 Demoscolecidse, and Chsetosomidfe. 



Appendix (3) to Annelid Series. 



A. Class Sipunculid^e, e.g. Sipunculns, and B. Class 

 PriapuliDjE, e.g. Priapulus. 



These two classes were formerly united with the Echiuridaa as 

 Gephyrea, but it is improbable that the three are nearly related. The 



