324 



ZOOLOGY 



Balano- 



glossus 



ment leading to a vertebrate type, they belie all such 

 expectations and change into a creature of relatively 

 simple structure and limited activities. They become 

 specialized in a new direction, and although they are 

 efficient and anything but degenerate in their own 

 particular line, it is impossible for us, who represent 

 the culmination of the other alternative, to regard them 

 without a certain sense of disappointment, almost of 

 reproach. It is also apparently true that following 

 their special line, they have abandoned all possibility 

 of extensive and varied evolution in the future. 



4. The Balanoglossus is a wormlike animal found in 



sand or mud or under rocks in the 

 sea, not far from the shore. This 

 general type includes a number of 

 genera and species, differing in 

 size, color, and various anatomi- 

 cal details. Some are orange, 

 others greenish or purplish. The 

 name Balanoglossus or "acorn 

 tongue" is derived from the more 

 or less acorn-shaped proboscis or 

 head-like structure at the anterior 

 end, which is used in burrowing. 

 Posterior to this is the collar, at 

 the anterior end of which is the 

 mouth, leading into a pharynx 

 with gill slits. At the anterior 

 end of the digestive tract, pro- 

 jecting into the proboscis, is a 

 small structure regarded as a 

 Drawing by w. P. nay notochord. Hence the animal 



FIG. 117- Balanoglossus: p, must bg assoc } at ed with the 

 proboscis ; m, mouth ; c, collar ; 111111 



g, gill slits; a, anus. Prochordata, although wholly 



