GEPHYREANS. 
445 
phageal collar, and a ventral chord, while the most important part of the vascular 
system is a dorsal vessel which lies above the alimentary canal. This class contains 
two orders, named the Achseta and Chsetifera. In the former the mouth is placed 
at the apex of the proboscis, which is retracted by a special muscle as in the 
Nemertine worms, and the alimentary canal opens posteriorly in the front half 
of the body. Moreover, as the name of the order indicates, the integument is 
without bristles. In the illustration on p. 444, B represents Phascolosoma, one of 
the genera of the order. Here the narrowed part of the body is the extended 
proboscis, which is furnished at the tip with a cluster of tentacles. A 
second family of this order is the Priapulidoe, containing the genus Priapulus, 
of which a representation is given in C of the figure. In this form the body is 
short, stout, cylindrical, and furnished at the tail-end with a tuft of oval papillae. 
The proboscis, which in the figure is represented as protruded, is short, stumpy, 
and covered with toothed ridges. The animal is found in deep water in the 
seas of Northern Europe, living in burrows on the sandy bottom. 
The Chsetifera, wdiich in some respects approach the Annelids, differ from the 
Achseta in having the mouth situated at the base of the proboscis, and the vent 
at the hinder end of the body, as also possessing a pair of large hooks 
upon the front half of the lower surface. The best known is the genus Bonellia, 
represented at A in the illustration on p. 444. The proboscis is of great size, 
being often many times the length of the body, and is forked at the end. The 
males differ from the females, being minute,—not more than about one-sixth of an 
an inch long,—covered with cilia, and living within the kidneys of the females. 
The Wheel-Animalcules, —Class Rotifera. 
The Rotifera, or wheel-animalcules, are small aquatic animals, varying from 
an eighth to the five-hundredth part of an inch in length, and derive their name 
from the circumstance that the circlets of hairs situated on the head give rise, 
when waving in the water, to the appearance of revolving wheels. The head 
end of the body is usually broader than the opposite extremity, and terminates in 
the wheel, or trochal disc, the edges of which are variously lobed, and clothed 
with the vibratile cilia, or threads. The body, which is indistinctly segmented, is 
either naked or enclosed in a hard transparent case, or lorica, open at both ends, 
which may be variously sculptured, and armed in front and behind with spiny 
processes, as shown in the annexed engraving. The posterior end of the body, 
termed the foot, ends usually in a pair of movable processes, by means of which 
the rotifers anchor themselves to foreign bodies of various kinds. The mouth, 
situated in the middle or at the side of the wheel-disc, is a funnel-shaped cavity, 
leading into a muscular gullet (a), provided with a peculiar armature of 
teeth, which serve to masticate particles of food that are swept into the mouth 
by the movements of the cilia on the wheel-disc. The nervous system consists of 
a single large ganglion, situated on one side beneath the disc, and sending forth 
nerves to the surrounding parts, and sometimes being furnished with one or more 
eye-spots. In all cases the males are smaller than the females, and further differ 
in having the alimentary canal aborted and reduced to a solid chord. Wheel- 
