PROCEEDINGS OF THE NATIONAL MUSEUM. 



93 



NASAL DISK OF CONDYLUKA. 



(Enlarge.!.) 



It would be interesting to know more of the habits of Condyliira in order to learn 

 in what r<-spects they diifer from those of Talpa, and by this means to ascertain the 

 immediate causes of this remarkable sensorial adaptation. Since Talpa may be 

 considered the more primitive form of the two, it becomes a matter of considerable 

 interest to trace the development of these finger-shaped jirocesses in Condiilura. 

 One would naturallj^ expect that in their iirst stages of development they would 

 siai])ly resemble the rounded elevations of the papi Hated tattile surface of the 

 snout of such a form as Talpa europa^a; the parts of the snout most frequently brou"ht 

 in contact with foreign bodies ultimately de- 

 veloping the papilhe to a far greater extent 

 than tiio remaining portions of the surface. 



By a gradual elongation of such elevations, 

 to be accounted for on the jirinciple of adap- 

 tation to environment, they would acquire 

 the finger-like form present in Condyliira. 

 Their arrangement on the edge of a subreni- 

 form disk receives its explanation in the fact 

 that the nerve endings are in this manner 

 placed in a position most advantageous for 

 the exercise of their special function. 



When we trace the development of the 

 snout in Condylura, we find that the assumed 

 method of growth is very nearly an expression 

 of what actually takes place. In the course 

 of the development there arises an interest- 

 ing comi>lication of the simpler process, the evident meaning of Avhich is the 

 shortening, in point of time, of the growth of the papilhe — i. e., it is an economic 

 adaptation. This modification I shall now describe. 



At birth the star-nosed mole is nearly destitute of visible hair, and the tactile bris- 

 tles of the facial region have not made their appearance at the surface. The snout of 

 the young Condylura lacks all the distinctive characteristics of the adult, and the 

 entire body resembles that of Talpa much more than it does its parent. On a close 

 examination of the distal end of the snout of such a new-born animal (fig. 37), one can 

 distinguish a tract of skin which covers four-fifths of the circumference of the organ 

 (the part not specially marked off is tlie median ventral fifth). This dermal tract 

 extends for 3 mm. toward the liase of the snout and is marked oft' 

 from the remaining surface by a series of furrows running parallel 

 to the long axis of the body. A series of parallel ridges is thus 

 formed, each ridge being bounded on either side by a furrow. 

 At their anterior and posterior ends these ridges i)ass gradually 

 into the neighboring smooth surface. By a gradual ingrowth of 

 the bottoms of the furrows each groove is deepened and each 

 ridge suffers a correspondingly increased definition of form, while 

 at the same time the posterior end of each groove grows toward 

 its neighbor on either side. When the grooves have all united, 

 there is formed by their union a common groove which nearly 

 encircles the snout and separates the tactile from the remaining surface of that 

 organ. 



Commencing at the posterior margin of the tactile surface and advancing toward 

 the t\\} of the snout, the grooves deepen and grow toward each other in their bot- 

 tom portions until they finally coalesce underneath the ridges. The result of this 

 process is the production of free, finger-shaped processes comjiosed exclusively of 

 ectoderm, attached to the ante^yorend of the snout in the manner already described 

 for the adult. 



These processes of the ectoderm become the tactile rays of the adult. The nasal 

 area from which the tentacular processes are formed is not thereby denuded of 



CJ1. — m~ 



SNOUT OF YOUNG 

 CONDYLUEA. 



