54 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



anterior to the stylet region, and 

 remain almost perfectly constant in 

 numl>er and position (Text-tig. 10). 

 In the stylet region, however, they 

 fuse into an irregular nerve plexus. 

 Sometimes' this nerve plexus is dou- 

 ble, and its inner portion supplies 

 the stylet apparatus, while its outer 

 portion continues backward beneath 

 the epithelium of the middle probos- 

 cis chamber. In the region of the 

 proboscis behind the middle chamber 

 the nerves are in some species again 

 arranged into their detinite number 

 ^^ of nerve cords, and these again be- 

 come situated in the longitudinal 

 muscles, but their size is very insig- 



FiG. 20. — Amphiporus cruentatus. .,, i -^i li >^ .c 



Outline of anterior portion of body, niticant as compared With that of 

 showing position of ocelli anil reia- ^he Corresponding nerves of the an- 



tion of cephalic (cv) to lateral {Iv) 

 blood vessels. 



terior chamber. 



Sense Organs. 



In addition to the sensory epithelium 

 found in the integument in all regions 

 of the body all nemerteans possess cer- 

 tain highly specialized sense organs. 

 Such sense organs include ocelli, oblique 

 and horizontal cephalic grooves or fur- 

 rows, cerebral, lateral and fi'ontal sense 

 organs, while the members of a single 

 genus (Carinoma) have a row of sen- 

 sory pits on the dorsal surface of the 

 head, and those of another genus (Oto- 

 TYPHLONEMERTEs) possess Otoliths. 



Ocelli. — Eyes are wanting in all the 

 Paleonemertea except Cephalotiirix 

 and HuBRECHTiA, l)ut are generally 

 present in the Hoplonemertea and in 

 the majority of species, including nearly 

 all genera, of the Heteronemertea. In 



Fig. 21. — Tetrasfemina sif/nifer. 

 Outline of anterior portion of 

 body, showing position of ocelli 

 and shape of cephalic marking'. 



