Phylum Echinodermata and Related Groups 557 



The proboscis is a conical structure connected to the collar by a 

 slender stalk. It is used by the animal for burrowing through the soft 

 bottom mud. On the trunk region just behind the collar are numerous 

 gill pores ; also on the trunk are two well-defined ridges, the mid-dorsal 

 and the midventral. Genital ridges are located just behind the gill pores. 



Fig. 185. — SaccoglossT.is. A, View of entire animal; B, longitudinal section 

 through anterior portion; C, tornaria larva. 



The permanently opened mouth is located on the ventral anterior 

 part of the collar and opens into the buccal cavity inside the collar. This 

 is followed by the pharynx and the straight intestine which opens through 

 a posterior, terminal anus. Opening into the pharynx are the gill slits. 

 From the buccal cavity, an anterior diverticulum projects into the pro- 

 boscis; this is the structure assumed to be homologous with the noto- 

 chord. Mud and water enter the mouth; the food is sifted out and 

 enters the intestine while the water passes out through the gill slits in 

 the pharynx. 



The circulatory system is invertebrate in construction. A main 

 dorsal vessel carries the colorless blood anteriorly to the heart located 

 in the proboscis. This heart is dorsal to the anterior diverticulum from 

 the buccal cavity and consists of a pericardial sac with a ventral muscu- 

 lar wall and a central blood sinus between the sac and the roof of the 

 diverticulum. From the heart, blood is pumped forward to an organ 

 called a glomerulus. Here are many capillaries which form a pair of 

 vessels which pass ventrally and posteriorly to form the ventral blood 

 vessel. Blood flows posteriorly in this vessel. 



