GENERAL STUDY OF TYPICAL CHORDATES 25 



3. Trunk. The trunk is incased in a strong, thick shell consisting of the 

 following parts: the dorsal arched portion, the carapace; the ventral, flat portion, 

 the plastron; and the lateral bridges connecting the carapace and plastron. The 

 shell is composed of bony and horny plates which will be studied in detail later. 

 The trunk bears two pairs of limbs, stout and strong, and composed of the same 

 parts as in the animals already studied. There are five digits to each limb, 

 each bearing a claw, except the fifth digits of the hind limbs. The digits are 

 more or less united by webs. The parts of the limbs have undergone a torsion 

 and alteration from the primitive position similar to that observed in the case 

 of the lizard. The student may determine for himself the details of these changes. 



4. Anus. At the base of the ventral side of the tail is the rounded anus or 

 cloacal aperture. 



L. EXTERNAL ANATOMY OF THE PIGEON 



The pigeon is a typical representative of the class Aves, or birds. Examine 

 specimens with and without feathers. 



1. Body form and skin. The proportions of the body, as seen in the plucked 

 specimen, bear a general resemblance to those of the turtle. The head is well 

 developed, the neck long and flexible; the trunk is shorter and considerably 

 plumper than normal, and the tail reduced to a stump, the uropygium. The 

 ancestors of birds had long tails like lizards, bearing feathers along their entire 

 length. 



The body is clothed with a covering of feathers, which conceal its shape. 

 These feathers are called contour feathers. The contour feathers are of two 

 kinds, the large, stout feathers of the wings and tail, called quills, and the smaller 

 feathers, the coverts, which cover the bases of the wings and tail, and the general 

 surface of the body. On the plucked bird from which the contour feathers have 

 been removed note the presence of hairlike processes, the hair feathers or 

 filoplumes. In the plucked bird, also, note the delicacy of the skin and the 

 presence in it of numerous deep pits, the feather follicles, into which the contour 

 feathers were set. 



2. Head. The head terminates in the elongated beak, which consists of 

 the upper and lower jaws incased in horny sheaths. Teeth are absent on all 

 modern birds, although extinct birds possessed them. The base of the upper beak 

 bears a cushion-like protuberance, the cere, a structure occurring only in certain 

 families of birds. Under the anterior margins of the ceres are the slitlike external 

 nares. The remarkably large eyes are provided with upper and lower lids and 

 with a nictitating membrane which may be drawn across the eye from the anterior 

 corner. The ear is behind and below the eye and is observable only on the 

 plucked specimen. There is visible only a deep and narrow passage, the external 

 auditory meatus. The middle ear, which is flush with the skin in the frog and 

 most reptiles or only slightly depressed, in birds has sunk below the surface to 



