BRITISH MARINE TURBELLARIA. 445 



are disposed in alternate longitudinal rows, which converge 

 anteriorly towards the opening of the ''frontal gland/' 

 Eyes are absent. The otolith is concavo-convex^ with a 

 central "nucleus." The two genital pores are posterior, the 

 female in front of the male. Opening to the exterior through 

 the former is the spermotheca. The curved penis receives 

 the contents of the seminal vesicle. 



Habitat.— In tide-pools, Millport (v. Graff) . Two speci- 

 mens among Zostera and Corallina, Plymouth (F. W. G.). 



6. CoNVOLUTA PARADOXA, Oersted (16), 1844. 



1777. Planaria convoluta, O. F. Miiller (4). 



1844. CoNVOLTJTA PARADOXA, Oersted (16). 



1845. Planaria macrocephala, Johnston (20). 

 1853. „ HAUSTRUM, Dalyell (29). 

 1855. CoNVOLUTA PARADOXA, Gosse (30). 

 1861. „ „ Claparede(35). 



1865. „ „ Johnston (38). 



1866. „ „ Lankester (39). 



Length 1—3-5 mm. Schmidt (28) found specimens up to 

 9 mm. in length in the Faroe Islands and other northern 

 localities ; v. Graff records equally large examples from Mill- 

 port. Specimens from more southerly places are usually much 

 smaller, and it might therefore appear that this species 

 attains a larger size in the northern than in the southern seas. 

 Claparede, however (35j, working on the north-west coast of 

 Skye, examined a large number of specimens, none of which 

 exceeded 2 mm. in length; while recently (55) v. Graff has 

 found " giant specimens ''' at Roscoff. It seems more likely, 

 as Claparede suggested, that this species attains a considerable 

 size before the reproductive organs begin to develop. Thus 

 he found individuals I'S — 2 mm. long without a trace of 

 gonads, and I have myself observed the same thing. 



The form of the body changes with different states of con- 

 traction and expansion. When freely swimming the form is 



