backward from the annular cartilage. These spines lie to 

 either side of the midline copular cartilage (Figure 5-26). 

 This lies below the anterior end of the piston cartilage, 

 which bears on its tip a medial apical cartilage and bilateral 

 supraapical cartilages. The latter support the anterior and 

 posterolateral tooth plates of the rasping apparatus. 



The branchial skeleton is complex in form. It is composed 

 of soft elastic cartilage, which in preserved specimens is 

 darker than the cartilage of the mouth area and neurocra- 

 nium. The arches are attached dorsally to a strip of soft 

 cartilage extending along the side of the notochord and 

 attached to the neurocranium behind the otic capsule. The 

 basket is also attached by a band to the subocular arch just 

 lateral to the base of the styliform process. The branchial 

 skeleton is peculiar in that the arches are not divided into 

 segments and in having epi and hypotrematic horizontal 

 connectives and a pericardial basket enclosing the heart. 



EMBRYOLOGiCAL DEVELOPMENT In the lamprey, the bran- 

 chial arches appear first and develop from front to back; 

 they are well along when the neurocranium begins to form 

 (Figure 5-27). The branchial bars are formed first in pro- 

 cartilage. Balfour in 1881 pointed out that these arches 

 develop more superficially than those of gnathostomes. They 

 lie lateral to the branchial blood vessels and nerves, not 

 medial to them. Later in development the dorsal ends of 

 the arches are extended forward and backward along the 

 notochord as the chordal rod. Ventraily a midline connec- 

 tion also develops. 



The parachordals are restricted to the otic and postotic 

 region and seem to arise as anterior continuations of the 

 chordal rods. The trabecles (including the polar cartilages) 

 are derived from the first and second somites. They arise 

 lateral to the notochord and anterior to the otic vesicle. Be- 

 low the trabecles arise the parabuccal rods, which later 

 connect to the posterior end of the trabecles and join across 

 the midline by a mesenchymatous strand below the noto- 

 chord and internal carotid arteries. These rods give rise to 

 the velar skeleton. In the area where a rod joins the trabecle 

 a basitrabecular process, or pedicle, extends outward from 

 the latter. 



The olfactory capsule is initiated by lateral cartilage 

 centers. The subocular arch forms from a mucocartilage 

 strand. Other bands of this material give rise to the styliform 

 cartilage, the extrahyal band, and the mouth cartilages in 



general. The annular cartilage and the anterior lateral car- 

 tilage are derived in part by breakdown of larval muscula- 

 ture accompanied by mesenchymatous cartilage formation. 



The later development of the head skeleton involves union 

 of the trabecles anteriorly, through an ectomesodermal con- 

 nective. An orbital cartilage appears which is anchored 

 anteriorly by a preoptic pillar. The orbital cartilage later 

 joins the otic capsule through an orbitoparietal comniissure 

 and the basis cranii through an antotic pillar. Still later, a 

 weak metoptic pillar appears. The otic capsule has become 

 cartilaginous and is perforated by the facial nerve. 



Development of the definitive skeleton does not always 

 follow directly from mucocartilaginous precursors. Some 

 mucocartilage disappears without contributing to any skele- 

 tal element. Parts or entire elements are produced more-or- 

 Icss directly from mesenchymatous condensations. 



Hagfish The neurocranium of Myxine (or Eptalretus) is very 

 incomplete (Figure 5-28). The otic capsule opens widely on 

 its medial aspect for the auditory nerve and the endolym- 

 phatic duct, and is attached by anterior and posterior 

 basicapsular commissures to a parachordal rod lying next to 

 the notochord posteriorly and to the side of the large hy- 

 pophyseal fenestra anteriorly. The rods are joined across 

 the midline so as to enclose the tip of the notochord. 



The parachordal rods are continued anteriorly as the 

 trabeculae, which are widely separated at first by the 

 hypophyseal fenestra but converge anteriorly to attach to the 

 nasal capsule, above, and the hypophyseal cartilage, below. 

 Ventrolateral to the trabecula is a subocular arch joining 

 the otic capsule posteriorly. This posterior connective lies 

 behind the facial nerve, the nerve passing ventromedial to 

 it (in Pelromyzon the nerve passes outside of the connective). 

 The subocular arch extends down as the visceral plate. This 

 has a large fenestra in it and lies lateral to the branches of 

 the fifth nerve and medial to the distal part of the seventh. 

 Both the external and internal divisions of the first bran- 

 chial arch attach to the subocular arch at a point just below 

 the otic capsule. 



The subocular arch extends forward, past a connection 

 with the posterior end of the trabecula, to form a midline 

 palatine commissure with the arch of the opposite side. The 

 palatine commissure lies below the olfactory capsule and the 

 posterior end of the subnasal cartilage. There is a single, 

 large, horny tooth, the ethmoid tooth, attached by a pad of 



lingual tooth mass 



supraopica 



skeleton 



I carti 



^piston cartilage 



Figure 5-26. Skeleton of rasping organ as seen in ventrolateral view. 



AGNATH FISHES 



131 



