HEAD 



263 



side, 1 and in the Terebellidae these prostomial processes become 

 very numerous. 



Iii the Cryptocephala there is never more than a single pair 

 of tentacles, and these are gener- 

 ally reduced to a group of sen- 

 sory cells, though in Sabellaria 

 they retain a considerable size. 



In a few genera, such as 

 Aphrodite, Nepkthys, Capitella, 

 the first postoral segment is 

 distinguished from the succeed- 

 ing segments only by its position 

 with regard to the mouth (Fig. 

 132) and by its smaller size. 

 But in the remainder of the 

 Polychaeta, with here and there 

 an exception, the peristomium is 

 achaetous in the adult. 3 



Except in the Nereidiformia, 

 peristomial or tentacular cirri 

 are rare, being represented in 

 the Spioniformia by the very 

 long " tentacles." In the Nereidi- 

 formia one or more of the 

 following segments may be added 

 to the peristomium, and share 

 in the " cephalisation," which 

 is so characteristic a feature in 

 this group. In Amphinomids 



the first three or four chaetigerous segments are incomplete 

 ventrally, owing to the shifting of the mouth backwards ; these 

 segments form lateral lips, but they are not otherwise modified. 

 In Phyllodoce, however, there are four cirri on each side of the 

 mouth, and from the arrangement in the Alciopids we are 

 justified in concluding that the segment which carries the four 

 pairs of cirri is really made up of three segments (Fig. 134, 

 E). Among the Hesionids there are four such " cephalised " 

 achaetous segments with long cirri. 



1 It is possible that some of these may be peristomial. 

 2 Individual cases in which chaetae are present have been recorded. 



VI 



Fia. 135. Sabellaria alveolata L. Ventral 

 view of anterior region, x 10. a, 

 Notopodial cirrus ; b, notopodium ; c, 

 neuropodium ; ch, peristomial chaetae ; 

 d, neuropodial cirrus ; m, mouth ; P, 

 multifid palp (gill filaments) ; /"', ridges 

 after removal of gill filaments ; , 

 ventral (tubiparous) gland shield ; T, 

 tentacle ; I, hood formed by peri- 

 stomium ; II to VI, following seg- 

 ments. 



