CHAPTER V. 

 The Worm-Like Protochordates, — Class Enteropneusta. 



Balanoglossus. 



The last, and at the same time the lowest, group of the Proto- 

 chordates is typically represented by the marine Balanoglossus. 

 Living buried in the sand or mud of the seashore, these worm-like creatures exhale 

 a peculiar odour resembling that of the 

 chemical substance termed iodoform, and 

 secrete a copious supply of slime, to which 

 adhere particles of sand, thus forming 

 a protective tube for their bodies. At the 

 front extremity of the creature, writes 

 Mr. Willey, " there is a long and extremely 

 sensitive proboscis, which is capable of 

 great contraction and extension, and is in 

 the living animal of a brilliant yellow 

 or orange colour. Behind the proboscis 

 follows a well-marked collar-region, con- 

 sisting externally of a collar-like expansion 

 of the integument, with free anterior and 

 posterior margins overlapping the base 

 of the proboscis in front and the anterior 

 portion of the gill-slits behind. (The gill- 

 slits are seen in our illustration below the 

 collar.) In the ventral middle line, at the 



base of the proboscis, and concealed by the collar, is situated the mouth. 

 Following behind the collar is the region of the trunk or body proper, which, 

 in the adult of some species, reaches a relatively enormous length, even extending 

 to 2 or 3 feet. The ectodermal covering of the body consists in general of ciliated 

 cells, among which are scattered unicellular mucous glands; the cilia, however, 

 appear to be more prominent on the proboscis than elsewhere. In the region 

 of the trunk, which immediately follows upon the collar-region, there are a great 

 number of paired openings on the dorsal side of the body placing the anterior 

 portion of the digestive tract in communication with the outer world. These are 

 the gill-slits, and they are arranged strictly in consecutive pairs to the number 

 of upwards of fifty in the adult. In their structure, and more especially in the 

 possession of tongue-bars, they bear a remarkable resemblance to the gill-slits of 

 the lancelet. This is particularly striking in young individuals. As the adult 

 form is approached in the development, the bulk of the gill-slits sinks below the 



A YOUNG balanoglossus (much enlarged). 



