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 Priapulidce, 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, which 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- 



