372 



Cn.ITOPODA. 



prscoral or cephalic) lobe are widely distributed. Eye-spots may also 

 be present upon the posterior end of the body (Fabricici), or may be 

 regularly repeated upon the sides of each segment (PolyopWmLmus). 

 In species of Sabella, pigment-spots with refractive bodies are found 

 even upon the branchial filaments. The large cephalic eyes of the 

 genus Akiope* are the most highly developed, being provided with 

 a large lens and a complicated retina. The presence of auditory 

 organs seems less frequent. They appear as paired otolithic vesicles 

 upon the oesophageal ring of Arenicola, Fabricia, some Sabellidce and 

 young Terebettidce, etc. Besides the tentacles, cirri and elytra, other 



portions of the surface of the body may be sensi- 

 tive to tactile sensations. On such parts there 

 are either stiff hairs and tactile setse, or, as in 

 Sphcerodorum, special tactile warts with nerve 

 terminations. 



Reproduction. In the smaller Chcetopoda 

 asexual generation by fission and gemmation 

 may occur. Either (fissiparous reproduction) 

 a large number of segments of the parent be- 

 come separate and give rise to the body of the 

 new worm, as for example in Syllis prolifera, 

 where a series of the posterior segments, which 

 are filled with ova, become separated by a simple 

 transverse fission, after the formation of a head 

 provided with eyes ; or (gemmiparous repro- 

 duction) a single segment only, usually the last, 

 becomes the starting-point for the formation of 

 a new individual. In this way Autolytus pro- 

 lifer, one of the Syllidce', asexually reproduces 

 itself, giving rise to a male and female sexual 

 form, known respectively as Polybostrichii-s 

 Mutter i^ (male) and Sacconereis helgolandica 

 (female). This is a case of alternation of gene- 

 rations, for the asexual form, Autolytus, gives 

 rise by budding in the long axis to the sexual 

 forms (fig. 302). In this case a whole series of segments are developed 



* Greeff, " Ueberdas Auge derAlciopiden, etc.," Marburg, 1876 ;and " Unter- 

 suchungen iiber die Alciopidcn," Nov. Act. dcr A". Leop. Altad.. etc., Tom 

 XXXIX., Nro. 2. 



f Compare besides the works of 0. Fr. Mliller, Quatrefages, Leuckart, and 

 Krohn, especially A. Agassi/,, " On alternate Generation of Annelids and tho 

 embryology of Autolytus cornutus," Boston Journ. Nat. Hist., vol. iii., 18C3. 



PIG. S02. Autolytu cor- 

 nutut, with the male 

 animal Polybostrichus 

 (after A. Agassiz). F, 

 Tentacles ; CT, tenta- 

 cular cirri : f, tenta- 

 cles ; ct, tentacular 

 cirri of the male. 



