WOODWOKTH : ILLINOIS TUKBELLAKIA. 3 



anterior and posterior ends. Gouopore (in preserved material) two sevenths of 

 the total length from the posterior end. Length from 10 to 22 mm. ; greatest 

 hreadth from 2 to 3 mm. 



Dendroccelum lacteum, 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 lijima (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 lijima. 



It is remarkable that lijima 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 aisement se creuser en cupule, en ven- 

 touse semblable a celle de la queue des sangsues et la face inferieure des 

 Douves,"and he shows it in his Fig. 7, PI. 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 Dendroccelum, and occurs in every other species of the genus whose struc- 

 ture has been investigated. The failure of lijima 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 difterent phases of contraction 

 and expansion, it being at times diflicult to recognize it. The variability in 

 the shape of the disk was also referred to by Leydig (loc. cit.'), and Girard (1893, 

 PI. 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 diificulty 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 Graft' (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 (" Stabchenstrassen ") open to the exterior, and according to Graff is 



