PISCES. 1ST 



The cranium is a somewhat rectangular box of cartilage enclos- 

 ing the brain, and incomplete dorsally, where there is a large 

 gap or fontanelle closed by membrane. Posteriorly the cranium 

 articulates immovably with the vertebral column by two rounded 

 projections (condyles), between which is a large aperture (foramen 

 magnum), where the spinal cord and brain are united. There are 

 nerve-exits in front and at the sides, while the cranial floor is 

 continued forwards as a median nasal septum bearing in front a 

 slender pointed rod. 



The olfactory capsules are large thin-walled structures sepa- 

 rated from one another by a median septum, open below, and 

 fused with the front of the cranium. The auditory capsules are 

 much firmer. They enclose the organs of hearing, and are fused 

 with the sides of the cranium in its hinder region, each of them 

 appearing as a squarish projection. 



Visceral Skeleton. In the embryo dogfish seven thickenings, 

 visceral arches, appear on each side of the neck, and between them 

 six openings, visceral clefts, placing the cavity of the pharynx in 

 communication with the exterior. The arrangement may be 

 indicated as follows for the left side, using strokes for the arches 

 and numbers for the clefts ; the arrow points to the front. 



\'\'VV\\'\ 



Beginning in front, the arches are termed mandibular, hyoid, 

 1st, 2nd, 3rd, 4:ih, and 5th branchials. The first cleft is the 

 hyomandibular, and becomes the spiracle; the rest are named 

 like the arches which bound them behind, and become the gill- 

 clefts. 



Curved supporting rods of cartilage are developed in the vis- 

 ceral arches, and become converted into the visceral skeleton. 



The mandibular bars are converted into two cartilages which 

 support the lower jaw. From the upper end of each a forward 

 outgrowth is developed, which becomes separated off and supports 

 the corresponding half of the upper jaw. 



The hyoid bars are segmented into upper pieces, the hyo- 

 mandibular cartilages, which suspend the jaw-cartilages from 

 the auditory region of the skull, and lower pieces (cerato-hyals) 

 which unite with a median ventral basi-hyal cartilage. A skull 

 like this, in which the jaws are suspended by means of hyo- 



