968 MR. F. A. POTTS ON 
division of labour, a number of segments is included in the region, 
larger than in Jesochcetopterus. 
T have not found it possible, in the absence of experimental 
observations, to reach any such clear conclusions as to the 
functions of the different organs of Mesochetopterus. Food is 
partly collected by two long grooved tentacles, which are constantly 
projecting from the mouth of the tube and sweeping over the 
surface of the sand in search of small fry. Enders makes a 
similar observation on another Cheetopterid with long tentacles, 
to which he applies the name Spiochetopterus oculatus? He 
describes it as scraping with its tentacles the sides of the aquarium 
in which it was kept; the diatoms thus dislodged were swept 
up the ciliary grooves of the tentacles into the mouth. Nutritive 
particles are, no doubt, also collected by the action of the cilia of 
the buccal funnel as in Chetopterus, and, lastly, I think, the 
notopodia of the median region have a similar function to those 
of the 12th segment in the above-mentioned worm. A branch 
of the median ciliated groove runs along the inner surface of 
each of them, and in my view such minute organisms as are not 
strained from the circulating sea-water anteriorly are here 
arrested by the parapodia, mixed with mucus secreted by the 
glandular epithelium of the surface and swept into the median 
groove and along to the mouth. 
The ciliated groove in Mesochetopterus, as in Phyllechcetopterus, 
is quite continuous in its course from the head to the tail. The 
primary function of such a groove is to maintain a respiratory 
current through the tube, as is well seen in forms with a trans- 
parent dwelling like Phyllochetopterus prolifica. 
In Chetopterus, owing to the development of the respiratory 
fans, the ciliated groove does not extend farther back than the 
13th segment and has changed its function, being now employed 
in the collection of food. We may well suppose that in Meso- 
cheetopterus the action of the cilia causing a flow of water from 
head to tail is not continuous, but is reversed on occasion to carry 
food back to the mouth. 
Cheetopterus is without doubt the form most specialised in 
structure in the family, and this is shown not only by the modi- 
fications of the median region but also in the discontinuous 
ciliated groove and the shortness of the tentacles. It is impossible 
to agree with Enders, who supposes that the long tentacles of 
Spiochetopterus “have undergone a considerable specialisation ” 
and speaks of the shorter tentacles of Chetopterus as more 
primitive than those of Spiochetopterus. 
The group of Cheetopterids, which includes Ranzania, Phyllo- 
chetopterus, Telepsavus, and Spiochetopterus, ave distinguished by 
possessing long tentacles (the most important organs in procuring 
food) and a complete ciliated groove. A differentiation of the 
median from the posterior region is sometimes not found at all 
(Telepsavus, Ranzania). If it occurs, the number of segments 
in the median region is very variable (in species and even In 
