Morse.] Bon [March 19, 
the borders of the extended membranes. (See Plate I.) Asin worms, 
they are arranged upon the sides and front, in two series, above 
and below. (See Fig. 7, and compare with Fig. 1) and are also 
secreted by regular follicles, each seta protruding from the follicle 
singly, or, in some cases, two or threé setz issuing from the same 
orifice. In the fixed Brachiopoda, they have only a limited power 
of motion. In these forms they are very short in the adult, but 
very long in the young, even much larger than the animal, as I 
have repeatedly observed in young Terebratulina, Rhynchonella, and 
Discina; in the latter genus, even exceeding the length of the 
animal three or four times. In the errantian Brachiopoda, as in Lin- 
gula pyramidata, the setigerous follicles are entangled in a mesh of 
muscular fibres, and locomotion is effected chiefly by them; the 
setee swinging freely back and forth, the dorsal plate oscillating from 
side to side, as first noticed long ago in another species of Lingula, 
by Carl Semper. 
In Discina, the sete are very long, and crowded together. 
According to Fritz Miiller, in the early stages of a species of Discina, 
from the east coast of Brazil, the animal not only has the power 
of swimming, but uses the larger pair of sete thrust out behind, to 
push itself along. He says these bristles have great freedom of 
motion, sometimes thrust out horizontally, and again crossed to oppos- 
ite sides.} 
In the Annelid Arenicola, the first few anterior segments are seti- 
gerous only. They bury themselves in the sand, forming a sand 
tube, loose and not adhering, a tube which leaves room for the ram- 
ified branchia to display, and the sete, by arching over the bran- 
chia, protect them, and prevent the sand in which they are buried, 
from falling in upon the gills. 
Lingula pyramidata also protects the gills in the same way, as 
I have repeatedly observed in specimens kept in confinement. 
When buried in the sand, the dorsal and ventral shells considerably 
separated, the setee are brought together in such a position that their 
extremities meet, and the sand is thus kept back while the water 
freely enters. 
In worms the sete are often of various kinds in the same individ- 
ual. A similar diversity is seen in the bristles of Discina. The 
sete of the embryo worm are peculiar in being very long, strongly 
1 Reichert and Du Bois-Reymond’s Archiv, 1860, p. 72. 
