960 MR. F. A. POTTS ON 
In the notopodium of Chetopterus there are two long straight 
rows of capillary sete, embedded for the greater part of their 
length in the parapodium, but projecting from the surface for 
short but equal distances. They are all lanceolate in type, but 
the dorsalmost sete differ from the rest in being slender and 
scarcely dilated. This tendency to differential development. is 
greatly exaggerated in the 4th segment. The dorsal sete 
preserve their slender lanceolate character, but a number of 
the ventral sete become short, strong, truncated at their ex- 
tremities, and black in colour. Those situated most ventrally 
present these characters in the highest degree (text-fig. 4 A). 
In Mesochetopierus the sete (Pl. ILI. fig. 9) are more delicate 
than those of Chetopterus, aud have a different arrangement, owing 
to the shortness of the parapodia. They are mostly of a distinct 
lanceolate type and equal in length, but a few of the dorsalmost 
are much longer and more slender, projecting in a ragged tuft 
from the extremity of the neuropodium. ‘They occur in a slightly 
curved row. In the fourth parapodium there is a single line 
making a very pronounced curve. Ventrally there are about 
12 modified setz. 
Median Region.—Though consisting of three segments only, it 
is much longer than the anterior region. In Chetopterus the 
median region is characterised by the transparent nature of the 
body-wall and the reduced width of the segments; here every 
segment is of a uniform width greater than that of the anterior 
region, the dorsal surface is flattened and glandular, and enclosed 
on each side by a continuous border formed by the thin upturned 
edges of the segments (PI. I. fig. 2, fr.), and the ventral muscles 
are much larger and stouter than in Chetopterus. 
When the animal is alive and inside its dwelling the lateral 
borders are approximated so as to form an imperfect tube 
dorsally. Their appearance in text-fig. 1 does not do justice to 
their extent in the living animal. 
The parapodia in Mesochetopterus have not undergone the 
great and diverse modification occurring in Chetopterus (compare 
text-fig. 1, A, B). They resemble very closely those of the abdo- 
minal region and attain a similar development in all three 
segments. The notopodia are short and conical, little larger 
than in the abdomen; they possess an internal skeleton of two 
or three stout capillary sete. They are on the distal surface 
and approximated to the middle line. On the inner surface 
they possess a ciliated groove which meets the median groove. 
T think that there is little doubt that they act as accessory organs 
for the collection of microscopic food, interrupting the dorsal 
channel, and separating food-particles from the current by the 
action of the cilia contained in these grooves. The neuropodia 
are slightly different in the three segments. The ventral surface 
of the anterior region (PI. IIT. fig. 5) is entirely oceupied, as in 
Chetopterus, by a “plastron” (pl.) with a slightly wrinkled 
appearance to the naked eye and composed of high epithelial 
