58 



COE 



The blood is dark red, and the blood vessels may be traced in the 

 living worm the whole length of the body. 



Habitat. — The species was found only at Kadiak, where it was 

 dredged in about three fathoms. It slightly resembles some varieties 

 of T. vermiculus Quatrefages, but the longitudinal bands of dark 

 pigment between the two ocelli of the same side are lacking, and the 

 median white line is sharply demarkated, so that the two species are 

 undoubtedly specifically distinct. 



20. 



TETRASTEMMA ABERRANS sp. nov. 



This is a minute Nemertean, the specimens found not exceeding 1 2 



mm. in length in greatest extension. Body moderately slender, slightly 



flattened. A pair of slight vertical slits on sides of head. 



Color. — Pale yellow throughout, both above and below. 



Ocelli. — Of moderate size, or rather small, arranged in four groups 



which form a rectangle, as do the single eyes of typical species of the 



genus. Each of the four groups is composed of three to five ocelli of 



variable size. The two anterior groups lie well toward the tip of the 



snout, while the two posterior groups lie above or slightly in front of 



the brain (fig. 15). The appearance of the eyes 



is such as to give at once an impression that the 



multiple nature of each group has arisen from the 



fragmentation of single ocelli. This I consider to 



be the case. At least one other species of Tetra- 



stemma ( T. crticiatuni Burger ) is known in 



which the four ocelli are double, and I have 



often noticed other species of the genus — notably 



the fresh water T. rubruvi (Leidy) — which 



showed almost conclusively that one or more of 



the six or seven eyes present had been derived 



Fig. 15. Tetrastem- fj-Qj-^ 3 splitting of the primary ocelli. In one 



ma aberrans. Dorsal j^g^^^^g ^j^g fragmentation had been carried so far 



view of anterior por- 



lion of body, showing that no fewer than 20 to 30 pigment spots were 



outline of brain and ar- present. 



rangement of the four Proboscis sheath a?id proboscis as in other 

 groups of ocelli. X 25. species of the genus. Mouth and proboscis open 

 together ; proboscis sheath reaches the posterior end of the body. Pro- 

 boscis provided with a rather slender central stylet and basis ; the two 

 accessory stylet pouches each with two or three stylets of typical form. 

 The proboscis possesses twelve nerves, and in this respect again shows 

 a departure from the typical Tetrastemma, where there are usually 

 but ten proboscidial nerves. 



