220 VEIiTICBRATE AKLMALS. 



jointed t.) the bod}', and they are always turned away from that 

 side of the body (the " neural " side) upon wliich the great masses 

 of the nervous system are placed. The limbs may be altogether 

 wanting, or ]iartiaily undeveloped, but there are never more than 

 two p((>i-s, and they always have an internal skeleton for the attach- 

 ment of tlie muscles of the limb. 



A distinct blnod-vascular or " lurmal " system is present in all 

 Vertebrates, and in all except one — the Lancelet — there is a single 

 contractile cavity (jr lieart, furnished with valvular opening.s. 



Lastly, tlie masticatory oi-gaiis (jf all Vei'teljrates are modifications 

 of parts of tlie walls of the head, and are never uioditied limbs or 

 hard structures developed in the mucous membrane of the digestive 

 tube, as they are in the Invertebrates. 



The above are the leading characters which distinguish the Ver- 

 tehrata as a whole ; and before going on to consider the different 

 classes, it may be as well to give a .short and general sketch cjf the 

 anatomy of the Vertebrates, commencing with their bony frame- 

 work or skeleton. 



The skeleton of the Vvrtihrata may be regarded as consisting of 

 the bnnes which gii to form the trunk and head on the one hand, 

 and of thcise which form the sujiports for the limbs on the other 

 hand. Tlie biines of the trunk and head may be regarded as essen- 

 tially com|>iiseil of a series of l.Miiiy rings or segments, arranged 

 longitudinally. Anteriorly, these segments are much expaniled and 

 also nmch moditied tn form the Imny case which enclo.ses the brain 

 and which is termed the i-ruiiiinn i>r skull. Behind the head, the 

 segments enclose a mu( h smaller civlty in which is contained the 

 spinal cnrd, ami they are arrangeil one behind the other, forming 

 the " vertebral column." Tlie segments which form the vertebral 

 cnhnnn are called "vertebnv," and they have the following general 

 structure. Each vertebra (fig. 158, A) consists of a central portion 

 kniiwn as the "lu'dy," or "centrum" (c), placed immediately below 

 t!ie spinal curd, and giving origin to certain " ])rocesse.s." The ends 

 (■f the bullies of the vertcbi-;e .'ire all united together in diti'erent 

 ways, SCI as to givr the culumn great tlexibility. From the back of 

 the Ijiiily (if the ^"ertebra jiroceed two buny arches which unite 

 behind anil thus fmin with the centi'um a \nmy canal in which the 

 sjiinal cord is containrd. For this re.asim these arches {n) are called 

 the "neural" arches. Fi'nm the ]ioint where the neural arches 

 unite — tliat is tu say, fmni tlie back of the ncui'al canal — pmceeds a 

 lung jiriK-ess, sometimes clrft at its extrcinity, tcriiii'd the " .spinii\is 

 ]iiiicess ' (.<). S|)riiigiiJg alsii froiu each neural arch is a. second 

 short.rr jinn'i'ss ('/) teriiird the " articular ju-iicess," since by means 

 of the ic, a.'i wrll as ]\\ tlir bodies, the \'ertebra' are jiinted or " ar- 



