294 BULLETIN : MUSEUM OF COMPAEATIVE ZOOLOGY. 



Ocelli. — The four ocelli are remarkable in that each consists of an 

 irregular group of three to five isolated pigment masses of variable 

 size (PI. 22, fig. 150). These groups are arranged in the fonn of a 

 square as are usually the four simple eyes in related species. This 

 condition has evidently arisen from the fragmentation of originally 

 simple ocelli. Such a fi-agmentation often occurs in the fresh-water 

 Stichostemma, and to a certain extent in numerous other forms, 

 Tetrastemma cruciatmn for example, having four double ocelli. It 

 is probable that individuals of the present species will later be found 

 in which the fragmentation is not carried to such an extent as in the 

 specimens here described. 



Color pale yellow throughout. 



Proboscis has twelve nerves, although but ten are found in nearly 

 all other species of the genus. Basis of central stylet rather slen- 

 der, as is also the stylet itself. Accessory pouches as in related 

 species. 



Nephridia extend forward to l)rain, and are limited to a short dis- 

 tance in anterior portion of esophageal region ; with a single effer- 

 ent duct near the posterior end of each of the pair of main canals. 



Cephalic glands well developed. 



Intestinal caecum extends into anterior fourth of esophageal 

 region. Pylorus opens into intestine far behind the most anterior 

 sexual glands. 



Cerebral sense organs large, situated immediately in front of brain 

 and projecting somewhat laterally thereto. 



Habitat. — Glacier Bay and Prince William Sound, Alaska. 

 Among hydroids in 4 fms. and between tides (Coe, : 01). 



79. Tetrastemma albidum, s}). nov. 

 PI. 9, fig. 6li; PI. 17, figs. 104, 105; PI. 22, figs. 145-149. 



Body very slender, widest and somewhat flattened in intestinal 

 region ; head narrow, but not pointed anteriorly. Lateral, oblique 

 grooves on head as usual. 



Size. — Usually 10 to 15 mm. long and 0.5 to 1 mm. in width, 

 although some individuals were at least 25 mm. in length. 



Color. — Milk white, creamy, or flesh colored, somewhat influenced 

 by color of intestinal canal and of sexual products when mature. 



