112 R. C. PUNNETT. 



hand and Cerebratnlus and Micrura on the other (the small 

 genera Borlasia and Langia being sufficiently well marked to 

 be left out of account here) . Though its presence shows its 

 possessor to be either a Cerebratnlus or a Micrura^ its absence 

 does not necessarily establish the specimen in question as a 

 Lineus, unless a large amount of material is forthcoming, 

 since it is a delicate structure which may easily be broken off. 

 In his monograph Burger (3) attempts to place these genera 

 on a firmer basis by taking into account certain anatomical 

 features, such as the sidefolds usually characteristic of Cere- 

 bratnlus, the presence or absence of a diagonal muscle layer, 

 and of neurochord cells. Based mainly on a study of the 

 Neapolitan forms, the attempt is for them fairly successful. 

 For exotic forms, however, it is less so. Thus 0. natans, 

 which closely resembles many of the Mediterranean forms in 

 its general form, its well-marked side-folds, and its swimming 

 habits, differs markedly from them in the absence of neuro- 

 chord cells and of a diagonal muscle layer. Again, C. brun- 

 neus agrees with the genus Micrura in the absence of side- 

 folds and a diagonal muscle layer, whilst its comparatively 

 large size and stoutuess of build incline one to associate it 

 rather with the genus Cerebratnlus. The small and slender 

 C. erythrus, again, with its absence of a diagonal muscle 

 layer, might be relegated to the genus Micrura, were it not 

 for the presence of small side-folds and of neurochord cells. 

 Other instances might be taken, but the above are enough to 

 show that Biirger's system is not altogether satisfactory. 

 However, before we can hope to improve upon it more ana- 

 tomical evidence must be forthcoming. And, indeed, on many 

 important points much yet remains to be made out in the 

 species inhabiting our own shores. Especially is this the case 

 with the excretory system, its range, topography, and the 

 position and number of its ducts. We are only acquainted 

 with these details in two of the thirtj? odd species of Cerebra- 

 tnlus, and in six or seven cases in nearly the same number of 

 species of Linens. The figures at the end of this paper will 

 give some idea of the diversities shown by this system, even 



