1878.] 369 [Morse. 
In most cases the Brachiopods are fixed, and it is well known that 
with the radical change in the habits or conditions of any group, a 
corresponding change is noticed in their structure. Thus among the 
Crustaceans the feature of attachment is accompanied with many 
changes in the structure of the animal which mask their proper affin- 
ities, as in the Cirripeds, and notably in the Lerneans. Similarly in 
certain low worms, internal parisitism is generally accompanied with 
a loss of gills, setze and cilia.t 
On the same grounds, we should naturally expect to see great and 
striking differences in those worms which are attached. Conse- 
quently those Annelids which are fixed, while in most respects 
resembling the free Annelid, yet differ from them in being eden- 
tulous, the mouth or head, in some cases, supporting a crown of cirri, 
oftentimes springing from two cephalic lobes, which may be 
developed into a closely twisted spiral. There is also a marked 
degree of cephalization, the anterior rings forming a thorax, and 
supporting branchiz, while the caudal region is often apodous, and 
without sete, and in some cases not even annulated, and often sep- 
arated from the thorax by a deep constriction. 7 
So in the Brachiopods, while in every feature of their internal 
structure betraying their annelidan affinities, and while the errantian 
forms with their long vermiform and annulated peduncle, their loco- 
motion by means of setz, and their power of fabricating a sand tube, 
unite them clearly with the fixed and highly cephalized Chetopods; 
yet in those groups that are attached, a remarkable concentration is 
seen, and many features are presented which have heretofore ob- 
scured the affinities of the group. 
To sum up the whole then. — Ancient Chetopod worms culminated 
in two parallel lines, on the one hand, in the Brachiopods, and on 
the other, in the fixed and highly cephalized Chetopods. ‘The diver- 
gence of the Brachiopods, having been attained in more ancient times, 
a few degraded features are yet retained, whose relationships we 
find in the lower Vermes; while from their later divergence the fixed 
and cephalized Annelids are more closely allied to present free Che- 
topods. 
And so we must regard the Brachiopods as ancient cephalized 
Cheetopods, while Serpula, Amphitrite, Sabella, Protula and others, 
may be regarded as modern (later) cephalized Cheetopods. 
1 Rolleston. Forms of Animal Life. 
PROCEEDINGS B. S. N. H. — VOL. XV. 24. OCTOBER, 1873. 
