SIPUNCULIDS AND PRIAPULIDS. 247 
Appendix (3) to Annelid Series 
A. Class SIPUNCULIDA, e.g. Sipunculus, and 
’ B. Class PRiaPuLIDA, e.g. Priapulus 
These two classes were formerly ‘united with the Echiuride as 
Gephyrea, but it is improbable that the three are nearly related. The 
Echiuridee are apparently modified Cheetopods, while the position of - 
the Sipunculide and Priapulide is quite uncertain. : 
Both include marine worms, living in the sand or mud upon which 
they feed, having unsegmented bodies with a capacious body cavity, 
and an anterior protrusible proboscis or introvert, which is moved ‘by 
special retractor muscles, and bears the mouth at its tip. In most other 
respects the two classes differ markedly from one another, 
In the Sipunculids, the large introvert terminates in a hollow 
tentacular fringe, within the cavity of which closed blood vessels run. 
The gut is much coiled, and the anus is dorsal and anterior. A nervous 
system with a distinct brain, a gullet-ring, and a ventral cord is present, 
but the ventral cord is unsegmented. Peculiar ciliated vesicles or 
“urns” arise in some Sipunculids as buds from the blood vessels, 
and many swim freely in the body cavity. By collecting and agglutinat- 
ing particles they help to purify the coelomic fluid. Large nephridia or 
brown tubes, usually two in number, occur in the anterior region, and 
function also as genital ducts. The sexes are separate except in 
Phascolosoma minutum, and the reproductive cells develop on the lining 
of the body cavity. In the development, which includes a meta- 
morphosis, several peculiarities are observable, tending to show that 
the animals are not primitive. The larva of Sdpunculus is sometimes 
compared to a trochosphere, but differs from a typical trochosphere, 
notably in the total absence of segmentation, of ‘‘head kidneys,” of a 
pre-oral band of cilia, as well as in the position of mouth and anus, and 
the slight development of the pre-oral lobe. 
The class includes eleven genera, which are widely distributed; many 
of the species are large and conspicuous. It should be noticed that 
while Sipunculids are typically without trace of sete, some genera, 
e.g. Phascolosoma, have distinct hooks on the introvert. 
The Priapulidee include two genera—Priapulus and Halicryptus, both 
almost entirely confined to the northern hemisphere. They have no 
tentacles, no vascular system, no brown tubes, and no brain. The gut 
is straight, or has a single loop; the anus is posterior. A gullet-ring 
and ventral nerve-cord are present as in Szpaculius, but retain their 
primitive connection with the epidermis. There are complex genital 
ducts opening by a pore on each side of the anus, which in the young 
are connected with an excretory system of the Platyhelminth type, 
while in the adult they are overgrown and concealed by the repro- 
ductive cells, The development is unknown. In Prdagulus there is.a 
peculiar respiratory (?) appendage at the posterior end of the body. 
