AND HABITS OF THE AMMOCHARIDS. 233 
segment and those which follow it, but the next three segments 
are without septa and enclose a single chamber. The succeeding 
three “abdominal” segments increase in length, the seventh 
being sometimes four or five times as long as the buccal and 
“thoracic” segments taken together. From the eighth segment 
their length is rapidly reduced, the penultimate segment 
being very minute. The “thoracic” region and each of the 
“abdominal” segments is internally cut off from its neighbour 
by a septum; and with the exception of the anal, the penulti- 
mate, and antepenultimate segments, the anterior extremity of 
each is indicated externally by a bundle of dorsal or dorso- 
lateral sete. These sete are slightly pinnate at the extremity. 
Each bundle of sete is followed by a torus, which, in the 
abdominal region, is so long that the tori (t¢., Pl. 25. fig. 20) 
on the two sides almost unite at their extremities, and, at this 
part, form approximately a complete girdle; though in the last 
segments the tori will not extend over more than about one- 
sixth of the circumference. The tori of the first four segments 
are red owing to the presence of blood. From each torus 
proceed a considerable number of minute bifid uncini (PI. 28. 
fig. 4), which are disposed in very regular, close-set, longitudinal 
rows (w., Pl. 23. fig. 6); and it has been estimated by de Saint- 
Joseph (29. p. 401) that there will be about 7600 uncini in a 
single turus in the third “abdominal” segment. The total 
equipment of an entire animal is estimated at from 150,000 to 
450,000 hooks. 
This immense provision of uncini is necessitated by the bur- 
rowing habits of the animal, and renders it most difficult to expel 
the living worm, uninjured, from its tube. The anterior part of 
each uncinus is preceded by an enlargement, which tends to 
retain it in the epidermis in which its shaft is buried, though 
this provision is not always sufficient, as uncini may occasionally 
be found torn from the tori and embedded between the layers 
of the inner lining of the tube. Sickly worms, too, occasionally 
expel their uncini from the mouth of the tube. The shaft of 
the uncinus is flat and straight, and terminated proximally 
(Pl. 23. fig. 4, c’) by a thread-like elongation, which curves 
sharply backwards and penetrates the basal membrane. The 
uncini are actuated by appropriate muscles, and are used 
for seizing-purposes at will. There are no ventral uncini 
in the “thoracic ” segments, neither are there any dorsal sete 
