WOODWORTH: ILLINOIS TURBELLARIA. 3 
anterior and posterior ends. Gonopore (in preserved material) two sevenths of 
the total length from the posterior end. Length from 10 to 22 mm.; greatest 
breadth from 2 to 3 mm. 
Dendrocelum lactewm, which is one of the commonest and most widely dis- 
tributed of European fresh-water Planarians, has often been the object of study, 
and its structure is better known, perhaps, than that of any other Triclad. 
Chief among the papers dealing with this species is that of Iijima (1884) ; our 
American form agrees so closely with his account that a discussion of the finer 
anatomy of the species will not be entered into here, except in so far as regards 
the anterior adhesive disk, and some points in connection with the male sexual 
organs ; for other details the reader is referred to Iijima, 
It is remarkable that Tijima should have overlooked the adhesive disk of 
D. lacteum. Though he states (p. 362) that he did not find the organ, two of 
his Figures (Taf. XXII. Figs. 9 and 10) are very suggestive, and coincide 
with similar preparations of my own. The organ was seen by von Baer (1827, 
p. 715), who describes it as a “kleine Pupille” ; Duges (1828, p. 150) also 
speaks of it as “un renflement qui peut aisóment se creuser en cupule, en ven- 
touse semblable à celle de la queue des sangsues et la face inféricure des 
Douves,” and he shows it in his Fig. 7, Pl. 4. Leydig (1864) figures it (Taf. 
I. Fig. 2), and in his account of its structure mentions the absence of rhabditi 
and cilia in the cells lining the depression. The organ is characteristic of the 
genus Dendrocelum, and occurs in every other species of the genus whose struc- 
ture has been investigated, The failure of Tijima to find the organ is explained 
by Weltner (1887, p. 800) as being due to the great variability of the organ 
itself in the same individual, depending upon different phases of contraction 
and expansion, it being at times difficult to recognize it, The variability in 
the shape of the disk was also referred to by Leydig (loc. cit.), and Girard (1893, 
Pl. 4) figures many phases of its activity. I also can testify to the great 
mobility of the adhesive organ, and to its varying prominence in the same and 
in different individuals. 
As a rule, it is more prominent in the largest, oldest individuals ; in those 
10 mm. and under in length, I have often had considerable difficulty in 
recognizing it even in sections, there being nothing to replace it but a shallow 
groove. Figures 13-15 are from sections of an individual of the largest size, and 
represent the organ in an exceptionally prominent condition. The organ can- 
not be compared to the sucker of cotyligerous Turbellaria, or to the muscular 
sucking disks of other Platyhelminths, nor to that of the leech ; it lacks the 
special musculature often so elaborately developed in these. It is simply a 
depression at the anterior end of the animal into which open the numerous 
mucous glands which in most Triclads are found in this region. It is compara- 
ble, rather, to the frontal organ of the Accela, and more particularly to the 
organ existing at the anterior end of Mesostoma lingua, which has been de- 
scribed and figured by Graff (1882, p. 288). In this species the pit is formed 
by the inversion of that part of the anterior end where the two great tracts of 
rhabditi (“Stábchenstrassen ”) open to the exterior, and according to Graff is 
