KOFOID AND SWEZY: UNARMORED DINOPLAGELLATA 495 



finally gave the clue to the interpretation here presented. Paralleling the furrow of the trans- 

 verse flagellum throughout most, if not the wliole, of its course, both anterior and posterior to it, 

 are shallow depressions — the paracingular grooves. The anterior, or preeingular, groove is the 

 more plainly visible, more deeply impressed, and is separated by a somewhat wider interval 

 from the furrow than the j^osteingular groove, except where the latter skirts the right side of 

 the sulcus. 



The most remarkable feature in the structure of this bizarre animal is its ventral tentacle 

 or prod, a mobile and rhythmically contractile structure, located towards the posterior end of 

 the body in the midventral plane in the tentacular recess of the expanded sulcus. It is an 

 elongated structure, consisting of a base, shaft, head, and stylet, extending ventroposteriorly in 

 the median plane. It is held habitually in this plane at an angle of 10°-25° below the horizontal. 

 It is, however, capable of a considerable latitude of movement (fig. TT, 1, 2) and exhibits much 

 flexibility in action. It rises from a low, spreading, convex, granular base, which is 2.5 times 

 the thickness of its shaft in diameter, and extends for 0.5 its length in a rounded mass into the 

 elsewhere hyaline substance of the body. The exposed conical portion forms an asymmetrical 

 cone of about 40°, which forms about 0.33 of the length of the partially extended tentacle, or 

 almost wholly, disappears in the fully extended one (pi. 12, fig. 130). When the prod is fully 

 contracted this base swells up into a rounded dome which now includes most of the substance 

 of the cylindrical .shaft. Tliis shaft is the connecting link between the head and the base and 

 emerges at the expense of the substance of the latter. When fully extended it is cylindrical 

 throughout, about. 0.6 the diameter of the head in diameter and 0.8 of the length of the body in 

 length. The head is subsymmetrically biconical, 0.15 of the length of the body in diameter, and 

 its proximal and distal conical surfaces subtend angles of about 60° and 70° respectively. 

 Distally it passes abruptly into a rodlike stylet, 1 .25 its diameter in length. This is a cylindrical 

 hyaline rod, not a mobile flagellum, although it has some elasticity, bending wlien in contact 

 with the substrate, but having no independent motion of its own. 



Prom the right side of the base of this prod there passes distally on its surface in a partially 

 spiral course a shallow, granular line, possibly a groove. This terminates on the outer face of 

 the head. The only plausible explanation of this is that it is the morphological extension of the 

 sulcus. Owing to the folded contour of the sulcus at the base of the tentacle, nothing more 

 than the union of this groove at its proximal end with the sulcus could be detected. It is to be 

 noted that this point of juncture is immediately adjacent to the notch of the distal end of the 

 girdle and not far from the possible location of the flagellar pore. 



The internal structure of this remarkable organ consists merely of a light area, a vacuole in 

 the head, and two sets of antagonistic striations, outer circular and inner longitudinal, marked 

 by ligliter lines and attendant granulations in the case of the circular fibers. Their behavior 

 in the contracted and extended condition of the tentacle indicates their function as protractor 

 and retractor muscles. 



The second outstanding feature of this species is the eyespot or ocellus. It consists of three 

 parts, the lens, melanophore or melanosome, and core, the latter two constituting the pigment 

 mass. It is located far anteriorly, immediately po.sterior to the proximal end of the girdle in 

 the projecting lobe formed by the anterior angle of the hypocone to tlie left of the intercingular 

 sulcus. It lies beneath a rounded eminence more evident in an obli(|uely lateral view of the 

 right side of the body. The lens is directed anteroventrally and is elongate hemispherical in 

 form with its base buried in tlie pigment mass. It consists of thi-ee concentric nuisses of hyaline 

 material, tb.e middle one of which is internally trilobed. and tlie outermost shows minor con- 

 centric lines. It is not externally lobed or divided. On cytolysis it elongates and shows a semi- 

 segmented form with a clear membrane and then rounds up in a splieroidal lump (text fig. 

 TT, 4^7). 



