Family =Cebidae=. Mycetes--howling monkey. Ateles--spider monkey. Family =Cercopithecidae=. Cynocephalus--baboon. Macacus. Colobus. Family =Simiidae=. Hylobates--gibbon. Simia--orang. Gorilla. Anthropopithecus--chimpanzee. Family =Hominidae=. Homo--man. FOOTNOTES: [17] This classification of birds is essentially that of Gadow and Selenka in Bronn's _Classen und Ordnungen des Thierreichs_, Band VI., Abth. IV., Vögel. Leipzig, 1891. [18] The classification adopted is almost entirely that given in Flower and Lydekker's _Mammals Living and Extinct_. London, 1891. CHAPTER III. SKELETON OF HEMICHORDATA, UROCHORDATA, AND CEPHALOCHORDATA. SUBPHYLUM A. HEMICHORDATA. THE subphylum includes three genera, _Balanoglossus_[19], _Cephalodiscus_ and _Rhabdopleura_; and perhaps a fourth, _Phoronis_. The skeletal structures found in _Balanoglossus_[20] are all endoskeletal. They include: (1) The =notochord=. This arises as a diverticulum from the alimentary canal which grows forwards into the proboscis and extends beyond the front end of the central nervous system. It is hypoblastic in origin and arises in the same way as does the notochord of _Amphioxus_. Its cells become highly vacuolated and take on the typical notochordal structure[21]. The cavity of the primitive diverticulum becomes obliterated in front, but behind it opens throughout life into the alimentary canal. (2) The =axial skeletal rods=. These are a pair of chitinous rods which lie ventral to the notochord and in the collar region unite to form a single mass. (3) The =branchial skeleton=. The gill bars separating the gill slits from one another are strengthened by chitinous rods in a way closely similar to that in _Amphioxus_. But between one primary forked rod and the next there are two secondary unforked rods--not one, as in _Amphioxus_. (4) The =chondroid tissue=. This is of mesoblastic origin and may be regarded as an imperfect sheath for the notochord. In _Cephalodiscus_ and _Rhabdopleura_ as in _Balanoglossus_ the notochord forms a small diverticulum growing forwards from the alimentary canal into the proboscis stalk. Recent researches on _Phoronis_[22] show the existence in the collar region of the larva (_Actinotrocha_) of a paired organ, which is regarded by its discoverer as representing a double notochord. SUBPHYLUM B. UROCHORDATA (TUNICATA). Skeletal structures of epiblastic and hypoblastic origin occur in the Urochordata. Most Tunicates are invested by a thick gelatinous test which often contains calcareous spicules, and serves as a supporting organ for the soft body. The cells of this test are mesodermal in origin.