2 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



neither neck nor tail. Head and trunk (also neck and tail when present) con- 

 stitute the axial part of an animal while the limbs are designated as the appendicu- 

 lar part. The head is the anterior end; the opposite region of the body is the 

 posterior end; and the terms "anterior" and "posterior" are also employed to 

 indicate the relative positions of other structures with reference to head or tail, 

 as, for instance, one would say that the fore limbs are anterior to the hind limbs. 

 The back or upper surface is the dorsal side; the lower or under surface, the 

 ventral side; and the regions between these are referred to as lateral. The middle 

 of the dorsal side is the median dorsal line, and that of the ventral side, the 

 median ventral line. A plane passing through these two lines from anterior to 

 posterior end is the median sagittal plane, and divides the animal into right and 

 left halves. If these two halves are identical or nearly so, the animal is said to 

 be bilaterally symmetrical. Is the frog bilaterally symmetrical as far as you can 

 observe from its external anatomy? Is man? There are several other types 

 of symmetry among animals, one of which will be met with later in the course. 

 An imaginary axis in the sagittal plane is the antero-posterior axis or sagittal 

 axis. Any axis in the median plane from the dorsal to the ventral side is a 

 dorsoventral axis; from the median plane to the sides, a medio-lateral axis, etc. 



These terms are applicable to the vast majority of animals, and the structures 

 of animals are arranged with reference to such planes and axes oj symmetry. 



Note differences in color between the dorsal and ventral surfaces, and observe 

 whether the color pattern is the same on all individuals. Read Holmes (p. 3 7) , on 

 the power of frogs to alter their color. 



The head ends hi a triangular snout which incloses the relatively large mouth 

 cavity, and which bears on its anterior extremity two small openings, the nostrils 

 or external nares. These, unlike our own, can be opened and closed by lowering 

 and elevating certain bones of the upper jaw (Holmes, p. 171). Posterior to the 

 nares are the large prominent eyes, in which may be distinguished the golden 

 iris, surrounding a central opening, the pupil. The eye is provided with the 

 following eyelids, as should be determined by lifting each with a forceps: an 

 upper eyelid, a well-developed fold of skin which covers the upper portion of the 

 eyeball; a lower eyelid, semicircular in shape, representing scarcely more than 

 the free edge of the skin; and the nictitating membrane, a thin, transparent, very 

 extensible membrane, which is really an outgrowth of the lower lid. A vestige 

 of the nictitating membrane is present in our own eyes as a small crescent- 

 shaped fold near the inner corners. 



Obtain a living frog, gently touch an eyeball and observe the action of the 

 eyelids. Which eyelids are movable? Stimulate the eyeball more strongly 

 and observe that the whole eye can be dropped down into the mouth cavity. 

 The socket in the skull which holds the eye is designated as the orbit. 



Returning now to the dead specimen, note a circular area of tense skin just 

 posterior to the eye. This is the tympanic membrane or drum membrane of tin* 



