ALASKA NEMERTEANS 45 



the posterior end of each marginal cluster the ocelli are more closely 

 and more irregularly placed, and often occupy several rows. In ad- 

 dition to these marginal clusters a closely set group of about a half 

 dozen smaller ocelli is situated in the light area lateral to the posterior 

 end of each of the dark cephalic spots. These ocelli lie deeper in the 

 tissues of the head, and near the brain (pi. i, fig. 4). 



Proboscis. The proboscis is remarkably large, and its constituent 

 layers are very sharply defined (pi. xn, fig. 2). The proboscis 

 sheath has a correspondingly massive development, 

 and reaches to the very extremity of the body. The 

 stylet apparatus of the proboscis is very characteris- 

 tic of the species, because of the extreme minuteness 

 of the basis of the central stylet. The central stylet 

 itself is very long and slender, while the length of 

 its basis is but half as great. The basis is con- 

 stricted in the middle, and is ^ as wide as long FJG TI 

 (pi. vm, fig. 2). There are usually four pouches porus bimaculatus. 

 of accessory stylets. These pouches are not evenly Outline of head to 



distributed on the circumference, for two lie close snow position of 



.i j f ., 1-1 markings and ar- 



together on one side of the proboscis, the other two , 



L rangement of ocelli. 



on the opposite side. There are usually five to \/ 5. 

 seven slender stylets in each of the four pouches. 

 Most of the stylets are much smaller than the central stylet. Measure- 

 ments of the stylets of one individual about 100 mm. long are : central 

 stylet .12 mm. long, .015 mm. wide near base; basis of central stylet, 

 .06 mm. long, .04 mm. wide ; largest accessory stylet, less than .1 mm. 

 long. The proboscis is provided with 16 large nerves (pi. xn, fig. 2). 



The mouth and proboscis open together. There are three large 

 communicating blood lacunae in the head, one on the right, one on the 

 left, and one dorsal to the rhynchodaeum. 



Cerebral sense organs. Remarkable for their large size, being 

 fully as large as either of the brain lobes. They lie lateral to the 

 brain, slightly behind the commissures, and in the angle between the 

 dorsal and ventral lobes. A large process from the posterior end of 

 the dorsal lobe furnishes the sense organs of the same side with an 

 abundant innervation. Their posterior ends extend backwards beyond 

 the dorsal brain lobes, against the posterior faces of which they are 

 closely pressed. Behind the dorsal brain lobe the sense organs lie 

 directly dorsal to the lateral nerve-cords and are bathed on their 

 internal borders by large blood lacunae. A section through this point 

 is not very different from a corresponding section of a Heteronemer- 



