Page:An Introduction to the Study of Fishes.djvu/92

64 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.


 * [J. Müller, Ueber den Bau und die Lebenserscheinungen des Branchiostoma lubricum, in Abhandl. Ak. Wiss. Berlin, 1844.]

The skeleton of the Cyclostomata (or Marsipobranchii) (Lampreys and Sea-hags) shows a considerable advance of development. It consists of a notochord, the anterior pointed end of which is wedged into the base of a cranial capsule, partly membranous partly cartilaginous. This skull, therefore, is not movable upon the spinal column. No vertebral segmentation can be observed in the notochord, but neural arches are represented by a series of cartilages on each side of the spinal chord. In Petromyzon (Fig. 30) the basis cranii emits two prolongations on each side: an inferior, extending for some distance along the lower side of the spinal column, and a lateral, which is ramified into a skeleton supporting the branchial apparatus. A stylohyal process and a subocular arch with a palato-pterygoid portion may be distinguished. The roof of the cranial capsule is membranous in Myxine and in the larvæ of Petromyzon, but more or less cartilaginous in the adult Petromyzon and in Bdellostoma. A cartilaginous capsule on each side of the hinder part of the skull contains the auditory organ, whilst the olfactory capsule occupies the anterior upper part of the roof. A broad cartilaginous lamina, starting from the cranium and overlying part of the snout, has been determined as representing the ethmo-vomerine elements, whilst the oral organs are supported by large, very peculiar cartilages (labials), greatly differing in general configuration and arrangement in the various Cyclostomes. There