982 MR. F. A. POTTS ON 
may be of some service to arrange the different types in a 
tabular form :— 
A single lobe without sete ....,.......---..-.-- +22. Ranzania, 16th and all succeeding 
segments. 
(A single seta ........... Most species of Phyllochetopterus 
A single conical lobe } in the posterior region. 
with setz. 
ie sete ............. Chetopterus and Phyllocheto- 
pterus aciculigerus, claparedet 
in the posterior region. 
Ranzania, 13th and 14th segments. 
; (Phyllochetopterus, (A conical or aliform erooved struc: 
A bilobed foliaceous | in the median region. | ture (with several capillary 
structure with seve-~ Telepsavus, 4 Prete ie bt 
ral capillary sete. | all segments behind | C’@topterus, Ist segment 0 
median region. 
| Mesochetopterus, median region. 
The two notopodia of the seg- 
ment fused toe form a fan-like 
structure. 
Chetopterus, segments 3-5 of me- 
dian region. 
(the anterior region. 
The structural types can thus be arranged in a neat series, and 
there is some evidence to show the direction in which evolution 
has taken place. Both the conical and the foliaceous types are 
adapted, the one for progression in the tube, the other for the 
respiratory function. But since the median region has been 
clearly developed from the posterior, and the conical type of 
posterior notopodium is far more widespread than the foliaceous 
type (Telepsavus alone), it seems reasonable to regard the conical 
notopodium as the move primitive. In this case, Telepsavus is x 
specialised form, in which all the posterior segments have de- 
veloped foliaceous notopodia and the original type has been lost. 
But Ranzania, though the parapodia are unilobed throughout the 
body, is not to be regarded as the primitive form from which 
the Cheetopterids diverged. The absence of sete from most of the 
posterior notopodia and of long tentacles may surely be regarded 
as secondary. The 13th and 14th segments are so distinct 
from the rest as to almost merit inclusion in a separate median 
region. 
The Head in the Chetopteride. 
In the structure of the head there is also a certain amount of 
variation in the family. The prostominm is in most cases provided 
with eyes, but it is small in all forms and almost surrounded 
by the peristomiwm, which forms a collar. In Chetopterus and 
Mesocheetopterus this collar is so complete as almost to hide the 
prostomium, while in Phyllochetopterus it is incomplete dorsally 
and shallow, allowing the prostomium to be seen easily. The 
peristomium gives rise to a pair of tentacles which vary greatly | 
in length, and in Phyllochetopterus there is also a second pair of 
