1 88 Elementary Zoology. 



IV. Annulosa. 



These "worms" are usually segmented animals; but a 

 number of forms belonging to the group, known as the 

 Gephyrea, and doubtfully included here, show but little traces 

 of segmentation. The earthworms, marine worms, and leeches 

 are, however, all of them, plainly segmented, the internal 

 organs being largely arranged in correspondence with the 

 external segmentation. The Annulosa have a well-developed 

 ccelom metamerically divided, and a complete and closed 

 vascular system. The excretory organs are typically a series 

 of pairs of tubes, also metamerically arranged. The nervous 

 system consists of a supra-cesophageal pair of ganglia, and of 

 a ventral cord connected with this by a circum-cesophageal 

 commissure, which is ganglionated excepting in the Gephyrea. 

 There are usually setse — bristles — imbedded in the skin, which 

 are used for progression. 



V. Brachiopoda. 



These animals are marine, with the appearance of bivalves. 

 They are, however, more allied to the Annelids in structure. 

 The valves of the shell are dorsal and ventral, not lateral as in 

 the Pelecypoda. The ccelom is spacious, and one or two pairs 

 of quite typical nephridia exist. Setse are sometimes present. 



VI. Bryozoa, or Polyzoa. 



These are small and, with one exception, invariably 

 colonial animals, which secrete a thick horny or calcareous 

 skeleton; the numerous "cells" of the colony form incrusta- 

 tions upon plants, stones, etc., or form erect masses of a solid 

 or branched character. The mouth has a circle of tentacles, 

 as in the Brachiopods ; there is a spacious ccelom, from which, 

 in a few cases, nephridia have been observed to lead to the 

 exterior; the gut is U-shaped, and between mouth and anus 

 lies a ganglion. They always multiply by buds, as well as by 

 the sexual process. 



