ECHINODERMA 291 



(6) The ACANTHOCEPHALI lack alimentary tract, have a spiny proboscis 

 and a very complicated reproductive apparatus. The adults are parasitic 

 in vertebrates, the young in arthropods. 



(7) The CH/ETOPOD ANXELIDS have segmented bodies, the segmentation 

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

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

 blood-vessels, ganglia, and excretory organs. 



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

 (usually four groups in a somite) arising in special follicles. The cruets are few 

 in the hermaphroditic Oiigochaeta?, numerous and borne on special parapodia 

 in the Polychaette. 



(9) The GEPHYR^A are related to the Chaetopoda. They are saccular, with 

 tentacles or well-developed preoral lobe. They have largely or entirely lost 

 the segmentation. Evidence of segmentation appears in some cases in develop- 

 ment and in the ventral nerve cord and nephridia. 



(10) The HIRUDIXEI are hermaphroditic Annelida which lack chata-, but 

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

 body parenchyma give them a certain similarity to the Plathelminthes. 



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

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

 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 ali- 

 mentary canal, the large ccelom, 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) chambers, 

 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 Echinoderms differ from most other animals by their radial 

 symmetry, but recall in this respect the Ccelenterata, a fact which led to 

 their inclusion in the 'Radiata' (p. 206), a view of their relationships which 

 was set aside on account of their different structure, especially the pres- 

 ence of a ccelom. In fact the radial symmetry of the echinoderms has 

 a different value, for while in the Ccelenterata the number four or six 

 is fundamental, Echinoderma are, with few exceptions, five-radiate. 

 Further, the radial symmetry of the Ccelenterata is primitive, the echino- 

 derms have descended from bilateral, possibly worm-like, ancestors, as 

 is shown by the bilateral larrae and many indications of bilaterality in 

 structure, especially in the more primitive forms (crinoids). This 

 primitive bilaterality is to be sharply distinguished from that resulting 

 from modification of radially symmetrical organs like the sexual and 

 ambulacral systems of highly differentiated echinoderms (bivium of sea 



