88 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Some of the Caducichordata remain solitary 

 throughout life, e.g. Ascidia, or Boltenia (which is 

 remarkable for its long stalk) ; others become fused 

 into a common colony, as Botryllus, Pyrosoma, colo- 

 nies of which may be more than a foot long, and Salpa, 

 the chains of which are sometimes several feet long. 



0. In the true Vertebrata the anterior end of 

 the central nervous system is enlarged into a brain, 

 which becomes surrounded and protected by a carti- 

 laginous capsule or skull ; supporting and protect- 

 ing arches, which finally become distinct vertebrae, 



are developed 

 around and 

 above the noto- 

 chord, which, in 

 the adults of the 



Pig. 44. Pyrosoma ; A, The atrial or excurrent higher forms, 



opening- is completely 



aborted. Optic, 



auditory, and olfactory organs are developed; there 

 is a centralised heart and a distinct liver appended to 

 the enteric tract. They are divisible into two groups, 

 distinguished by the fact that, in the higher, an an- 

 terior gill-arch becomes modified to form jaws at the 

 sides of the mouth. 



o. Cyclostomata, or Round-Mouths ; these are 

 the lampreys (Petromyzon), and hags (Myxine). There 

 is here no maiidibular arch, no appendages in the 

 form of limbs, and the olfactory organ is single and 

 median. The hags are parasitic in habit. 



ft. Gnathostomata. In this division all the 

 remaining Yertebrata are included ; in them an ante- 

 rior gill-arch becomes niandibular, two pairs of 

 lateral appendages are typically developed, and the 

 nasal sac is double. 



In all divisions of the animal kingdom we may 

 observe groups which seem to stand near the ancestral 



