_arch_ } 61. Partie supérieure _Epibranchials_ } de l’arceau } branchiale } 62. Os pharyngian Pharyngo- Os pharyngeum _Upper pharyngeals_ supérieur branchial superius 63. _Gill-rakers_ 65. Rayons de la _Pectoral rays_ Brustflossen- pectorale Strahlen 67, 68. Vertèbres _Abdominal Bauchwirbel abdominales vertebræ_ 69. Vertèbres caudales _Caudal vertebræ_ Schwanzwirbel 70. Plaque triangulaire [Aggregated Verticale Platte _Hypural_(Huxley) et verticale interhæmals] 71. _Caudal rays_ Schwanzflossen Strahlen 72. Côte _Rib_ Rippen 73. Appendices or _Epipleural Muskel-Gräthen stylets spines_ 74. Interépineux _Interneural Ossa interspinalia spines_ s. obere Flossentræger 75. Épines et rayons _Dorsal rays and_ Rückenflossen-Strahlen dorsales _spines_ u. Stacheln 76. _First interneural_ 78. _Rudimentary_ _caudal rays_ 79. Apophyses épineuses _Interhæmal Untere Flossentræger inférieures spines_ 80. _Pubic_ Becken 81. _Ventral spine_ Bauchflossen-Stachel CHAPTER IV. MODIFICATIONS OF THE SKELETON. The lowermost sub-class of fishes, which comprises one form only, the Lancelet (_Branchiostoma_ [s. _Amphioxus_] _lanceolatum_), possesses the skeleton of the most primitive type. [Illustration: Fig. 28.--Branchiostoma lanceolatum. _a_, Mouth; _b_, Vent; _c_, abdominal porus.] [Illustration: Fig. 29.--Anterior end of body of Branchiostoma (magn.) _d_, Chorda dorsalis; _e_, Spinal chord; _f_, Cartilaginous rods; _g_, Eye; _h_, Branchial rods; _i_, Labial cartilage; _k_, Oral cirrhi.] The vertebral column is represented by a simple _chorda_ _dorsalis_ or _notochord_ only, which extends from one extremity of the fish to the other, and, so far from being expanded into a cranial cavity, it is pointed at its anterior end as well as at its posterior. It is enveloped in a simple membrane like the spinal chord and the abdominal organs, and there is no trace of vertebral segments or ribs; however, a series of short cartilaginous rods above the spine evidently represent apophyses. A maxillary or hyoid apparatus, or elements representing limbs, are entirely absent.