92 
Proceedings of Royal Society of Edinburgh. [sess. 
forms as Peripatus and Chiton , of the supra-anal commissure upon 
which some writers have laid considerable stress. 
(4) In the Lineidse examined considerable variation occurs in 
the range and topography of the excretory system, as well as in 
the number of ducts. In none of the forms studied is to he found 
that incipient metamerism in the arrangement of the ducts which 
some observers claim to have demonstrated for other species. 
(5) The vascular system of the Lineidse shows but very little 
variation in the different species, except in so far as in the pie- 
cerebral region there may be either a well-marked head loop, or else 
a vascular network — -a fact already pointed out by Burger. It is 
worthy of note that there appears to be some correlation between 
the caudal extent of the excretory system and the point of exit of 
the dorsal blood-vessel from the proboscis sheath. This vessel in 
all the species here examined (with the exception of one case 
where the preservation was unsatisfactory for the determination of 
this point) leaves the sheath within a few micro-millimetres of the 
hinder termination of the excretory system, wherever that may be 
situated. 
(6) The frontal organ characteristic of most Lineidse is not 
always present. 
(7) The structure of the skin is highly characteristic for each 
species. 
