SENSE ORGANS REPRODUCTION 



273 



between the prostomium and peristomium, and capable of ever- 



sion (Fig. 144). They are most characteristically developed in the 



Capitelliformia, where each organ abuts upon 



a special lobe of the brain. The function of 



these " ciliated organs," which bear a great 



resemblance to those of the Nemertines, is a 



matter of speculation. Similar organs, in the 



form of simple pits or grooves, occur in many 



of the Nereidiformia, Terebelliformia, and 



others. 1 



Otocysts are rare. Arenicola possesses a 

 pair at the base of the prostomium, each of 

 which in some species retains an opening to 

 the exterior. 2 They probably serve as "organs 

 of direction " rather than of " hearing." 

 Aricia and Polyophthalmus likewise have such 

 organs on the prostomium ; whilst Fabricia, 

 Myxicola, Terebella, and a few others possess 

 them in the peristomium, or in some other 

 segment of the body. 



Reproductive Phenomena. With a few 

 tioned below, the Polychaeta are unisexual, 

 are developed in all cases from the lining epithelium of the body- 

 cavity. The exact spot at which this occurs varies in different 

 cases ; it may be, though rarely, on the floor of the body-cavity ; 

 it is more usually on the wall of some blood-vessel, either the 

 ventral vessel or on branches of it ; or on the many blind blood- 

 vessels of Aphrodite. The number of such genital organs is 

 very great in most worms, but in those presenting two regions 

 of the body they are confined to the posterior segments (Sabelli- 

 formia, Terebelliformia, Capitelliformia). The number is very 

 limited in Arenicola and other worms presenting but few neph- 

 ridia : in the former genus there being six pairs, in Trophonia 

 only one pair. 



The following genera are hermaphrodite : Amphiglena, Sal- 

 macina, Protula, Spirorbis, belonging to the Sabelliformia, to 



1 They are specially large also in the Typhloscolecidae ; while Racovitza (Ann. 

 May. JV. H. (ser. 6), xv. 1895, p. 279) has recently suggested that the caruncle of 

 Ainphinomidae belongs to the category of nuchal organs, and compares it with the 

 ciliated lappets of Ptcrosyllis. 



2 Ehlers, Zeitschr.f. wiss. Zool. liii. 1892, p. 217. 



VOL. II T 



aulogaster Rathke, en- 

 larged. (FromCuning- 

 ham.) Anterior end. 

 a, Prostomium ; b, 

 everted buccal region ; 

 c, notopodial cirrus ; 

 X, ciliated organ 

 everted; I, II, III, 

 first three segments. 



exceptions men- 

 The sexual cells 



