CLASS PISCES: ORDER PHARtNGOBRAKOHII. 199 



fresh water. Its mouth is simply a circular sucking-cup, 

 armed with numerous teeth. The tongue, working like a 

 piston, produces a vacuum by which the animal adheres to 

 any object. Assisted by the current, it thus drags away quite 

 large stones from the spot it chooses for depositing its spawn. 



ORDER PHARYNGOBRANCHII. 



Pharyngobranchii (gullet-gilled). The Lancelet, a sin- 

 gular little fish not over two ^ 

 inches long, found on differ- 

 ent coasts, is the only repre- 

 sentative of this order. It has Braru=Mosmnalarwemtus,L.nc.X.r. 

 no skull, jaws, brain, heart, 



true fins, nor eyes. The mouth is only a slit. It swims by 

 means of a membranous border above and beneath. Yet the 

 little creature breathes, eats, and its blood circulates by means 

 of rhythmical contractions of the vessels. It is considered the 

 lowest type of organization yet known in the Vertebrata. 



HOMOLOGIES AND ANALOGIES OF THE VERTEBRATES. 



A comparison of Figs. 36, 58, 70, and 116, illustrating the anatomy of 

 some of the various orders of Mammals, shows a remarkable similarity 

 of structure. Thus, while the figure and size of the head of the mam- 

 mals vary greatly, it is always composed of the same parts, marked by 

 the same processes and united in the same order. Any difference is 

 only a modification of form and structure, rather than of omission or 

 addition. These variations are least observable in the embryonic state, 

 and become more apparent in maturity, through the ossifying or non- 

 ossifying of the sutures, the greater or less development of the 

 processes for muscular attachment, and the adaptation of the cranium 

 to the size and shape of the brain. The so-called "facial angle," 

 formed by a line drawn from the forehead to the most prominent part 

 of the upper jaw, and meeting a line from the middle of the ear to 

 the base of the nose, shows the difference in the form of the head, and 

 is an index to the intelligence of the several orders of mammals. 



Homologies of Limbs in the sam.e Animal.— In the 



skeleton of the horse, a careful study of the angle formed by the 

 scapula and humerus with that formed by the ilium and femur, will 

 show how antagonism in direction gives stability of position. Were 



