362 A. T. MASTERMAN. 



of Cep halo disc us the ventral sucker was also adapted for a 

 fixing organ, but in addition the pre-oral lobe became modified 

 for a like function, and the animal, protecting itself in a 

 spacious house or coenojcium, is most probably enabled to 

 travel about, somewhat after the manner of a leech, although 

 the budding function of the ventral sucker (or pedicle) points 

 to the fact that it is largely sedentary and fixed by this organ. 

 It is possibly the very intermediate character of this animal's 

 habits, partly sedentary, partly locomotory, that has given it its 

 peculiar structure. In the reduplication of the gut and trunk 

 and the pre-oral position of the branching tentacles are to be 

 seen very marked sedentary characters, whilst the persistence 

 of the epistome and the notochords are to be accounted for by 

 a still functional locomotory capacity. 



In the case of Balanoglossus, although the presence of 

 the ventral sucker in the young individual, and of a like organ 

 of attachment on the proboscis (Bateson), indicates a transitory 

 adoption of the distomial mode of progression, a burrowing 

 habit and locomotion in a longitudinal direction seems to have 

 been early adopted, with a loss of the ventral sucker and of 

 tentacles. At the same time, the burrowing habit is corre- 

 lated with the migration forwards of the notochord, with its 

 connected structures ; whilst the elongation of the trunk-region 

 and commencing metamerism are at least made possible by a 

 locomotory habit. 



It is possible that the Echinodermata may be descended 

 from a form in many respects not unlike Actinotrocha, in 

 which fixation took place, not by the ventral sucker, but by the 

 pre-oral hood. 



Cephalodiscus, isolated in its deep-sea coenoecium, has 

 therefore been removed from the active arena of life, where 

 fresh organs are evolved and primitive organs are modified, 

 whilst its own peculiar method of progression has saved it 

 from the fate following upon a completely sedentary habit^ a 

 fate involving the loss of many important organs, as is shown 

 by the anatomy of its sedentary ally, Phoronis. 



The resemblances of Cephalodiscus to Phoronis and tQ 



