ALEXANDROMENIA AGASRIZI. 133 



The pedal cords after traversing the body gradually approach each other 

 in the posterior end of the animal and termijiate in two ganglionic masses on 

 each side of the mid line a short distance in front of the two groups of spines that 

 project into the ventral furrow. There are strong indications that these poste- 

 rior ganglia are united by a conmiissure. Owing to the difficulties of observation 

 no other pedal commissures have been discovered and for the same reason latero 

 pedal connectives have not been found with certainty though at various points 

 there are indications that such exist. 



The posterior sense organ is located at the extreme hinder end of the animal 

 in the mid line. At this point the hypodermal cells, unchanged in appearance, 

 approach near to the outer surface of the body and there become continuous 

 with a sensory epithelium composed of slender fairly dense cells in which the 

 elongated nuclei hold an almost median position (Plate 32, fig. 11). Over the 

 exact centre of this area the cuticle is exceedingly thin but gradually increases 

 in thickness as the outer limits of the organ are approached, and contains con- 

 siderable numbers of small spines that in both specimens overarch the sensory 

 cup. Numerous muscles and connective-tissue fibres attach to its under surface 

 and in the meshwork thus formed blood corpuscles and nerve cells occur in 

 moderate quantity, the latter probably connecting with branches from a strong 

 nerve that may be followed into close proximity to the posterior pallial com- 

 missure. 



Alexandromenia agassizi, sp nov. 



Six specimens, one badly mutilat(xl, of this species were dredged in 4r)0 

 fathoms (Sta. 2992) near the Revillagigcdo Islands off the coast of Mexico. 

 All save one, clinging to a fragment of some land plant (Plate 2, fig. 5), were 

 unattached and nothing is known of their mode of life. The smallest specimen 

 is yellowish white; the remainder, of larger size, are yellowish brown. 



The largest individual measures 25 mm. in length and 5 mm. in average 

 diameter in the middle of the body; and of the five remaining sjiecimens three 

 are of about this same size. The smallest is in an uncontracted state and is 

 22 mm. in length and 3 mm. in average diameter. As may be seen in Plate 2, 

 fig. 5, the head is not distinct, usually bluntly pointed, and readily distinguish- 

 able from the posterior end where the borders of the cloacal opening are 

 widely expanded, in one specimen especially (Plate 5, fig. 5), exposing the gill 

 plates, about 40 in number. A dorsal sense organ is visible in sections but not 

 in surface view. 



