ALASKA NEMERTEANS 35 



their places. The ventral branches join to form the main median 

 cascum, from which short branches pass obliquely forward and dor- 

 sally at intervals. In cross section two pairs of branches are usuallj^ seen 

 besides the main caecum. One pair of these lies above and one be- 

 low the lateral nerves. They are disposed in such a way that one pair 

 ends at about the point where the second pair anteriorly originates. 

 The branches are only irregularly arranged in pairs. The esophagus 

 opens into the dorsal wall of the main caecum. 



Nephridia. — The nephridia occupy the anterior ^ of the esoph- 

 agal region. They are large, with numerous branches lying above the 

 lateral nerves, and frequently passing internally to the nerves and be- 

 neath the esophagus. They reach forward nearly to the brain. At 

 about Yz of their distance posteriorly the main nephridial tubes, lying 

 above the lateral nerves increase greatly in size and a pair of remark- 

 ably large efferent ducts pass externally to the lateral nerves to open on 

 the lateral aspects of the body slightly below the lateral margins. The 

 main duct reaching posteriorly from this point is larger than that in 

 front. 



Blood vessels. — There is a pair of large blood lacunae in the head as 

 usual ; they join anteriorly by a bi^oad anastomosis. The three longitu- 

 dinal vessels are well developed to the end of the body where they an- 

 astomose above the anus. Sometimes the dorsal vessel lies beside or 

 even above the proboscis sheath throughout a portion of its course, 

 instead of below the sheath as usual. 



Nervous system and sense organs. — The cerebral sense organs lie 

 well in front of the brain, and external to the blood lacunae. They 

 open into a slight furrow on the lateral aspects of the head a little an- 

 terior to their own position. The lateral nerves form a commissure 

 above the anus as usual. 



Reproductive organs. — The sexual products were nearly mature in 

 June and July. They are formed in numerous pouches which sur- 

 round the intestinal canal on all sides. In a male as many as twenty 

 sexual pouches were seen in a single section. They open directly to 

 the exterior, whatever be their position. 



Habitat. — This is a restless Nemertean, and on cloudy days was fre- 

 quently met with crawling about over the stones on the beach between 

 tides — which peculiarity has suggested its specific name. It was found 

 abundantly at nearly all the collecting stations from Victoria, B. C, to 

 Unalaska Island, and is one of the commonest Nemerteans of the coast. 

 It occurs from low tide well up toward high water mark in every 

 variety of location — under stones, among seaweeds, barnacles, mus- 



