Evolution of Animals 465 



anterior facet or articular area enlarged into a cup, and its 

 posterior facet hollowed into a socket, so that the vertebrae 

 now are firmly but most flexibly locked together. In the 

 last condition the entire notochord has been replaced mainly 

 by bone, there being only cartilaginous patches over the articu- 

 lar surfaces. 



In the present connection it may be noted — though possibly 

 the point may be of minor importance — that just as Hubrecht 

 has shown that filling up and progressive differentiation of 

 the proboscis sheath in nemerteans starts posteriorly, so in 

 the formation of completed dorsal arches in cyclostomes, and 

 later in the appearance of ventral arches in these, the process 

 starts posteriorly. 



In cyclostomes the cartilaginous skull is attached by muscles 

 to the continuous notochord. In apodous types owing to 

 high ossification of the skull — probably as suggested through 

 action and reaction in the burrowing habit — two occipital 

 condyles are formed that fit into sockets of the first vertebra. 

 This vertebra however is a mere ring. But in the urodeles 

 a part of the first vertebra grows forward as a hanging peg 

 for the skull, and is the odontoid process. 



The heart and blood-vascular system in cyclostomes have 

 already been compared with those of nemerteans, but can 

 now be compared with those of Apoda and Urodela. In cyclo- 

 stomes the heart is placed — in contrast to the condition in 

 other fishes — some distance back from the head, and is three- 

 chambered. The venous sinus passes from the dorsal to the 

 ventral side, there opens into a tliin-walled auricle, and from 

 it by a two-valved aperture into a thick-walled ventricle. 

 This is continued forward — at least in Bdellostoma — into an 

 elongated conus arteriosus, that is separated by two valves 

 from the ventricle. From the conus the ventral aorta is con- 

 tinued forward to the level of the gill pouches that are the 

 fourth pair from the front, and here it breaks up into four 

 pairs of efferent vessels distributing venous blood to the pouches. 



The heart in Apoda, alone amongst Batrachia, is placed 

 some distance back as in the last, and shows also the same 

 divisions and valvular parts in one row. The conus also is 

 elongated, straight, simple, and is continued into the ventral 

 aorta, from which four similar pairs of afferent branches spring. 



