ON THE DIPLOOHORDA. 363 



Actinotrocha will have been noticed throughout tliis paper, 

 and that upon Actinotrocha. A detailed comparison of the 

 two species could be given, but tbe same purpose will be served 

 more succinctly by the subjoined list of characters of the 

 Archichorda and its two sub-groups. 



Archichorda. 



Body composed of three archimeric segments, protomere, 

 mesomeres, and metamere. Ectoderm simple, in great part 

 ciliated and glandular, secretes mucoid exoskeleton (tube, 

 coenoecium). Nervous system still in connection with the ecto- 

 derm, consisting of central dorsal ganglion, pre-oral ring, post- 

 oral ring, dorsal and ventral cords, and, in addition, a more or 

 less diffuse nervous plexus. Mesoderm in four ccelomic pouches, 

 the protocoele and metacoele showing secondary indications of 

 a paired condition. Protocoele opens to exterior, usually by 

 two proboscis-pores ; the mesocoeles and collar-pores and the 

 metacoeles have either paired nephridia, functioning as genital 

 ducts, or closed genital ducts. Muscular system prominent in 

 protocoele (the " animal " organ), and in some a circular and 

 longitudinal layer in the metacoeles. A mesodermic skeleton 

 of chondroid tissue — a vascular system of hsemocoele spaces, 

 consisting mainly of subneural sinus (heart) ^ near the dorsal 

 ganglion, dorsal and ventral vessels, and a sinus round the 

 gut. A simple digestive tube, with paired lateral (or early 

 fused into one) notochords, never free from the gut, and one or 

 more pairs of pharyngeal clefts. A subneural gland, opening 

 primarily into stomodseurn, gonads confined to metacoeles. 

 Metamere bears a ventral organ of attachment, ventral sucker. 

 Habitat burrowing or sedentary. 



1. Hemichorda. — Body, especially the metameres, elon- 

 gated, and the latter showing in gonads and gill-slits traces of 

 true metameric segmentation. Well-developed muscular 

 system in metameres. Ventral sucker present only in young. 

 Notochords fixed and protruding forwards into protomere. 



' Only in the Eu- chorda, when the branchial gill-slits appear, is the typical 

 ventral heart of the Vertebrata found. 



