64 FISHES. 



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 carti- 

 laginous rods above the spine evidently represent apophyses. 

 A maxillary or hyoid apparatus, or elements representing 

 limbs, are entirely absent. 



[J. Miiller, Ueber den Bau und die Lebensersclieinungen des BrancMo- 

 stoma luhricuw, in Abhandl. Ak. Wiss. Berlin, 1844.] 



The skeleton of the Cydostomata (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, there- 

 fore, 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 Pdromyzon (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 larvre of Pctromyzon, but more or less cartilaginous in 

 the adult Pctromyzon and in Bddlostoma. 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 etlmio-vomerine 

 elements, whilst the oral organs are supported by large, very 

 peculiar cartilages {lahials), greatly differing in general con- 

 figuration and arrangement in the various Cyclostomes. There 



