TYPE PROTOCHOBDATA. 601 



occurs in the Pterobranchia, and nothing is yet known con- 

 cerning the embryonic development of either of the genera 

 contained in the order. 



3. Order Enteropneusta. 



The order Enteropneusta contains a small number of 

 closely-related forms which, until recently, have been grouped 

 together in a single genus, Balanoglossus, and which, notwith- 

 standing that they present in some respects a decided advance 

 in structure over the Pterobranchia, yet may properly be as- 

 sociated in a class with that order. 



All the species live buried in sand, and the body is elon- 

 gated and vermiform, the digestive tract being practically 

 straight and the anus terminal. The anterior portion of the 

 body (Pig. 274, pr) has the form of a cylindrical proboscis, 

 corresponding to the epistome of the Pterobranchia and 

 united to the second region by a narrow neck, upon the dor- 

 sal surface of which is a pore (occasionally two) which places 

 the proboscis-coelom in communication with the exterior, 

 while upon its ventral surface, just where it joins the second 

 region, is the mouth. 



The second region is comparatively short and forms the 

 collar (c), its posterior border being prolonged backwards for 

 a short distance in the form of a fold on each side, as in 

 Cephalodisctis. These folds enclose between their inner walls 

 and the sides of the anterior part of the third portion of the 

 body a space which communicates behind freely with the 

 exterior and is known as the atrium. The posterior or trunk 

 region of the body is much longer than either the proboscis 

 or collar regions, and contains the greater portion of the 

 digestive tract and the reproductive organs. Anteriorly it is 

 somewhat flattened and presents on the dorsal surface a 

 ridge, on either side of which are to be found a varying num- 

 ber of U-shaped pores (br) with ciliated margins. These 

 pores lead into short tubes opening internally into the cavity 

 of the digestive tract and are the branchial slits. They seem 

 to increase in number during the life of the animal, the pos- 

 terior ones being merely circular openings, as the anterior 



