1897-98.] Dr Masterman an Archimeric Segmentation. 289 
follow from before backwards along the primary axis as normally 
as in any chsetopod larva, and apart from the fact that the meta- 
meres are perhaps bent upon themselves, their inter-relationships 
are unaltered in the adult. The Brachiopoda are still placed by 
some writers with the Polyzoa and Phoronis , as the Molluscoida 
( cf Hatschek), and it is interesting that at least one specialist on 
the group, Yan Bemmelen* regards them as allied to the Chcetog- 
natha. Mr Shipley,! in criticising his arguments, points out that 
there are no lophophore, shell, or stalk, in the latter. On the 
theory here put forward we must assume that the Clicetognatha 
are an ancient group which have remained in a free swimming 
pelagic habitat, and the Brachiopoda , another archaic group, 
which has, for an enormous period, become adapted to a sedentary 
existence. 
The presence of a ventral pedicle of attachment, the lophophore 
and shells may be legitimately connected with the adaptation to 
a sedentary environment. We are justified in this assumption by 
a comparison of Balanoglossus and Cfephalodiscus , and their respec- 
tive habitats. 
The Cephalula larva, representing the neo-embryonic stage of 
Hyatt, reminds one, in many features, of the typical Ar chi-coelomata, 
and is evidently its phyletic equivalent. After this stage the 
development diverges from all other known types, so that we are 
justified in supposing that the Brachiopod group only meets the 
rest of the Coelomata at the archi-ccelomate stage. 
Echinodermata. 
The Echinodermata are usually regarded as owing their radial 
symmetry and some other features to a descent from sedentary 
ancestors. There appear in the greatly modified present-day forms 
to be representatives of the right and left metacoeles, the left (and 
right?) mesocoele, and even the protocoele (axial sinus, parietal 
canal). 
In most of the Echinodermata there are found the typical 
characters of the Ar chi-coelomata. The nervous system is typically 
* Jenaische Zeitschrift , 1883. Cf. Hertwig. 
f Loc. cit. 
