ECHINODERMA. 329 



(7) The CH^TOPOD ANNELIDS have segmented bodies, the segmentation 

 showing itself in ringing of the body wall and in the separation of the 

 coeloem into a series of pouches by transverse septa and the metameric 

 arrangements of blood-vessels, ganglia, and excretory organs. 



(8) The CH^ETOPODA are distinguished from other annelids by the 

 chaetaa (usually four bunches in a somite) arising in special follicles. The 

 chaetae are few in the hermaphroditic Oligochaetae, numerous and borne on 

 special parapodia in the Polychaetae. 



(9) The GEPHYR^A are closely related to the Chaetopoda. They are 

 saccular, with a crown of tentacles or well-developed preoral lobe. They 

 have largely or entirely lost the segmentation. Evidence of segmentation 

 appears in some cases in development and in the ventral nerve cord and 

 nephridia. 



(10) The HIRUDINEI are hermaphroditic Annelida which lack chaetae 

 but have sucking discs. Their flattened bodies, rudimentary ccelom, and 

 rich body parenchyma give them a certain similarity to the Plathelmin- 

 thes. 



(11) The Hirudinei have either a protrusible pharynx (Rhynchobdella) 

 or three toothed jaws (Gnathobdella). To the latter belongs the medici- 

 nal leech (Hirudo medicinalis). 



(12) The POLYZOA are like the Hydrozoa in being colonial and having a 

 circumoral crown of tentacles. They are distinguished by the complete 

 alimentary canal, the large coelom, and the ganglionic nervous system. 



(13) The PHORONIDEA are closely like the Polyzoa. 



(14) The BRACHIOPODA have a bivalve shell, the valves being dorsal and 

 ventral. 



(15) The body cavity is divided by two septa into three (paired) cham- 

 bers, of which one, rarely two, are provided with nephridia. 



(16) Most brachiopods are attached by means of a stalk. They are 

 divided into Ecardines, without a hinge and with anus, and Testicardines, 

 with a hinge and no anus. 



PHYLUM V. ECHINODERMA. 



The Echinoderma are separated from most other animals by 

 their radial symmetry, but recall in this respect the Ccelenterata, 

 a fact which led to their inclusion by Cuvier in the group 

 ' Radiata/ a view of their relationships which was set aside by 

 Leuckart on account of their different structure, especially the 

 presence of a coelom. In fact the radial symmetry of the echino- 

 clerms has a different value, for while in the Coelenterata the 

 number four or six (apparently derived from four) is fundamental, 

 Echinoderma are, with few exceptions, five-radiate. Further, the 

 radial symmetry of the Coelenterata is primitive, that of the 

 Echinoderma, as development shows, is derived from the bilateral 



