338 
E. W. MACBRIDE. 
coeloinic pockets, one is tempted to believe that lie means 
to give his adhesion to the famous “ Annelidan ” theoi-y of 
the origin of Vertebrates put forward by Dohrn, to which 
we owe such a quantity of excellent work. This seems the 
more likely, as in the twenty-two years which have elapsed 
since its final revision the Nemertinean theory has gained 
little or no support. That the Nemertinea are distantly 
related to the common stock of Echinodermata and Vertebrata 
is quite possible ; but that they stand anywhei*e near the 
direct line of descent is negatived by the consideration that 
the J^ilidium larva in its development stands near the Trocho- 
phore larva of Annelida, and is widely different from the 
Tornaria larva of Balanoglossus or the allied Dipleurula 
larva of Echinodermata. The Annelidan theory did justice 
to a number of remarkable I’esemblances between Annelida 
and Vertebrata. In proportion, however, as zoological 
research has advanced, it has transpired that most of these 
resemblances — cf. ciliated tubes leading from the body- 
cavity to the exterior, development of genital organs from 
the lining of the body-cavity, etc. — are common to a wide 
range of coelomate animals, and only one specific resem- 
blance is left, viz. the metameric repetition of organs, and 
above all the metameric segmentation of the muscles in the 
two groups of animals. On the other hand, if Vertebrata 
were derived from Annelida, we are forced to assume the 
production of a new mouth and the abandonment of the old 
one, and a total change in the manner of developing the 
nervous system, whilst gill-slits and notochord must have 
been developed entirely de novo. 
The great advantage of Bateson’s theory of the origin of 
Vertebrata is that we are not forced to make any such violent 
reconstruction as the formation of a new mouth, and of noto- 
chord and nerve-cord we already have the beginnings. All 
we require to postulate is the appearance of metamerism, and 
surely this is no unreasonable assumption considering how 
often the repetition of similar organs crops up in the most 
widely separated phyla of the animal kingdom. 
