310 



COMPAEATIVE ZOOLOGY. 



the animal, wliose larval structure is that of a Vertebrate, 

 degenerates in its adult stage into an Invertebrate. 



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Division II. — Acrania, 

 Vertebrates without a skull. 



Class. — Pharyngobranchii 

 The Acrania are represented by 

 the singular animal Amphioxus or 

 Lancelet. It is about two inches long, 

 semi-transparent, without skull, limbs, 

 brain, heart, or red blood-corpuscles. 

 It has for a skeleton a notochord only. 

 It breathes by very numerous gill 

 arches, without fringes, and the water 

 is drawn in by cilia, which line the 

 gill slits. The embryo develops into 

 a gastrula closely resembling that of 

 the Invertebrates. The animal lives 

 in the sandy bottom of shallow parts 

 |i f.ll of the ocean, and has been found in 

 the Mediterranean Sea, in the Indian 

 5 = ". Ocean, and on the east coast of North 

 and South America, 



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Division III. — Craniota. 

 Vertebrates with a distinct skull. 



Class I. — Pisces. 



Fishes are the lowest of Verte- 

 brates. They fall far behind the rest 

 in strength, intelligence, and sensi- 

 bility. The ej'es, though large, are 

 almost immovable, bathed by no tears, 

 and protected by no lids. Dwelling 

 in the realm of silence, ears are little 



