ALASKA NEMERTEANS 57 



Tetrastemma Ehrenberg. 

 Symbolae Physicae, Berlin, 1831. 



This genus includes a group of very small, slender worms, seldom 

 more than 20 to 30 mm. long, with slightly flattened body, and usually 

 with four well-developed ocelli, which form a quadrangle on the head. 

 In a few species (cf. T. aberrans) these ocelli are each replaced by a 

 group of two or three smaller ones, and in other species ocelli are 

 wanting entirely. The anatomical structures are very similar to those 

 of Amphiporus, and the distinctions between the two genera are not 

 clearly defined. The mouth and proboscis open together. The cere- 

 bral sense organs lie close in front of the brain. The proboscis sheath 

 extends to the posterior end of the body, and the proboscis is well de- 

 veloped, armed with central stylet and pouches with accessory stylets, 

 and usually provided with ten nerves. 



Only three species of the genus were met with on the expedition, 

 although it seems probable that a number of other forms of these minute 

 worms will be found later. 



19. TETRASTEMMA BICOLOR sp. nov. 

 pi. i, fig. 6. 



Body moderately slender, rounded both above and below ; much 

 larger and longer than most species of the genus, sometimes becoming 

 50 to 60 mm. in length in extension. 



Color. This species is bright brownish-red or orange the whole 

 length of the dorsal surface ; the whole ventral surface is pale gray or 

 whitish. The anterior border and lateral margins of the head, as well 

 as the lateral margins of the body for a short distance back of the head, 

 have the same whitish color as the ventral surface. A narrow, median, 

 white stripe, sharply marked off from the reddish color of the dorsal 

 surface, extends from the white, anterior border of the head nearly to 

 the posterior end of the body. Posteriorly the stripe becomes more 

 irregular and is usually lost near the posterior extremity. 



Ocelli. Four, rather large, rounded, arranged nearly in the form 

 of a square. 



Proboscis. Proboscis sheath and proboscis as in typical species of 

 the genus. Proboscis provided with a moderately heavy central stylet 

 about .075 mm. in length. Basis of central stylet somewhat conical 

 in shape, swollen behind, and about \y 2 times as long as the stylet it- 

 self. There are two pouches of accessory stylets, with usually three 

 or four stylets in each pouch. 



