4 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



not permanent or constant. I have elsewhere (1891, p. 23) expressed my 

 belief in the homology of the " Stabchenstrassen " of the Rhabdoccels with the 

 cephalic slime glands of Triclads, and a comparison of the sagittal section of 

 M. lingua (Graff, 1882, Taf. VI. Fig. 3) with Figure 15 of this paper is strikingly 

 suggestive in this regard. Kennel (1888, p. 455) has also shown that in many 

 fresh-water Triclads there appears in preserved material a more or less shallow 

 groove, or depression, on the ventral surface at the anterior end corresponding 

 to a region by which the living animals are able to attach themselves. It is 

 the region where the cephalic slime glands open to the exterior. He suggests 

 for the ridges at the sides of these ventral depressions the names " Haftwul- 

 sten " and "Haftlappen." Grube (1872) has also described similar pits or 

 grooves in several fresh-water Phmariaus. 



The organ in D. ladeum is not a true sucker, nor does the animal employ its 

 anterior end for the purposes of attachment to any greater degree than the 

 posterior or lateral margins of its body, along the ventral surface of which 

 numerous mucous glands have their openings. In truth, it is the margins and 

 posterior end that adhere more firmly to a support ; often when the animal is 

 forcibly removed from the side of the aquarium the parts of the margin or the 

 posterior end will adhere so firmly to the glass that the points of attachment 

 are drawn out into digitate processes. Figures 13 and 14 are from transverse 

 sections through the adhesive pit of B. lacteum, Figure 13 being through the 

 almost extreme anterior part, while Figure 14 is somewhat more posterior. Both 

 sections are from the same individual, — one in which the organ was unusually 

 well developed. Figure 15 represents a sagittal section through another indi- 

 vidual, and Figure 9 is a surface drawing of an individual killed in hot corro- 

 sive sublimate. It will be seen that the region involved in the organ embraces 

 all that portion of the hypodermis occupied by the openings of the mucous 

 glands. As already shown by Leydig (1864), the hypodermis which lines the 

 depression is devoid of cilia and rhabditi, the latter being replaced by the 

 mucous glands, and the transition from one to the other being gradual. The 

 exact histological character of the hypodermis lining the depression could not 

 be ascertained, the terminal ducts of the glands being so closely compacted as 

 to mask all details in this region. In Figures 14 and 15 are seen cross and 

 longitudinal sections of the retractor muscle of the organ. Nothing in the 

 shape of a protractor muscle could be discovered, this function possibly being 

 assumed by the circular muscles of the dermo-muscular sac. 



The variation in the number of the eye-spots is not a feature peculiar to the 

 American form of D. lacteum, as I stated in a previous note (1896", p. 1048), 

 for accessory eyes, " Nebenaugen," have been observed in this species by Car- 

 riere (1882, p. 164) and lijima (1884, p. 438), according to whom they are not 

 rare. According to ray observations there is in such cases usually one pair of 

 eye-spots, coiTesponding to the normal single pair, which is more prominent 

 than the accessory eye-spots. In what appears to be a closely allied form, Soro- 

 celis gutata Grube (1872), there exist two series of eye-spots arranged in the 

 form of two arcs ; the number of eyes in each arc is usually seven, but there 



