282 PHYLUM ANNELIDA 



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 " coelom," 

 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 the 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, Flosciilaria, 

 Stephanoceros. 



Free Rotifers — Notommata, Hydatina, Brachionus. 



Parasitic on the marine crustacean Nehalia — Seison. 



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

 a peculiar jumping motion. 



A. Class SiPUNCULiD^, e.g. Sipiinciilus , and 

 B. Class Priapulid^, e.g. Priapidus 



These two classes were formerly united with the Echiuridce as 

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

 Echiuridce are apparently modified Cha^topods, while the position of 

 the Sipunculidae and Priapulidae is quite uncertain. 



Both include marine worms, living in the sand or mud upon which 

 they feed, having unsegmented bodies with a capacious body cavity, 

 and an anterior protrusible proboscis or introvert, which is moved by 

 special retractor muscles, and bears the mouth at its tip. In most other 

 respects the two classes differ markedly from one another. 



In the Sipuncuhds, the large introvert terminates in a hollow 

 tentacular fringe, within the cavity of which closed blood vessels run. 

 The gut is much coiled, and the anus is dorsal and anterior. A nervous 



