1907.] NATURAL SCIENCES OF nilLADELPHIA. 89 



of naked protoplasm; the large sense hairs, as Korschclt (18S2) has 

 observed, are capable of movement. This movement coasists in a 

 gentle waving to and fro of the tips, especially noticeable when these 

 cilia come into contact with foreign objects, and demonstrating clearly 

 their tactile function. On the dorsal surface of the head, just behind 

 the eyes, is another group of sense hairs, all of which are very small. 



Like both D. gyrociliatus and apatris, which it closely resembles in 

 other respects, D. conklini is colorless. Although lacking in color, 

 it is relatively opaque; this quality appearing to be due to the refractive 

 qualities of the protoplasm, rather than to anything contained within 

 the body of the animal. Wlien seen by reflected light its shining opaque 

 whiteness makes it a conspicuous object when viewed against the dark 

 background formed by the seaweeds on which it is commonly found. 

 This lack of transparency natm-ally renders more difficult the study of 

 the internal structure of the living animal. 



The size of this species is subject to great variation. Measurements 

 of the length of adult examples (in alcohol) of the first lot, taken in the 

 winter of 1901-2, vary from 0.57 mm. to 0.91 mm. ; the average of this 

 lot being about 0.73 mm. This variation in length is doubtless in part 

 due to the differing degree of contraction of the examples taken, but 

 it also, to a considerable extent, represents a real variation in size. 

 Fully extended living individuals would yield measurements possibly 

 one-tenth to one-fifth greater, but it is doubtful if any would exceed 

 1 mm. The examples of the second or summer lot, although less con- 

 tracted by the fixing fluid than those of the first, show a range of 0.309 

 mm. to 0.60 mm., the average length being only 0.46 mm. From the 

 third lot (winter 1903-4) only a few measurements were taken. The 

 maximum of these was 0.74 mm., w^hile the average was 0.616 mm. 

 From these measurements it seems to follow that summer individuals 

 are much smaller than those taken in cold weather. This may really 

 be the case, but unfortunately these observations are not extensive 

 enough to form the basis of any certain conclusions regarding this 

 matter. All the examples taken were, or appeared to be, sexually 

 mature. Considerable variation in size has also been noted by Kor- 

 schelt (1882) in D. apatris, and by Weldon (1886) in D. gigas. The 

 first mentioned author says, ' ' Seine Lange ist bei geschlechtsreife Thiere 

 ziemlich verschieden. " Weldon states that "The length of the body 

 varied greatly, the smallest specimens found being al^out 0.75 mm., 

 while the largest were nearly two millimeters in length. ' ' Whether all 

 these were sexually mature is not stated. 



Of the males of this species, only one was observed. This was still 



