572 COMPARATIVE ANATOMY CHAP. 



trunk, the two lateral sacs come in contact with one another above 

 the intestine to form a bilaminar dorsal mesentery, and below the 

 intestine to form a bilaminar ventral mesentery. 



In the adult animal these mesenteries are nowhere retained in 

 their full extent. 



Each coelomic sac has an anterior and a posterior wall. The 

 posterior wall of the collar sac becomes applied to the anterior wall 

 of the trunk sac and thus forms a bilaminar transverse and vertical 

 septum, separating the ecelom of the collar from that of the 

 trunk. 



The walls of the ccelomic sacs, in the larva, are epithelial. 

 Throughout the greater part of these sacs, however, the epithelial 

 cells become transformed into muscle fibres to form the musculature of 

 the body and of the intestine. And this takes place to such an 

 extent that over large areas no endothelial lining to the body cavity 

 is any longer demonstrable. 



Connective tissue is also produced by the walls of the coelomic 

 sacs. 



The musculature of the Enteropneusta consists exclusively of 

 smooth fibres. 



Lymph cells (probably amoeboid) float in the fluid of the body 

 cavities : these are presumably produced by the peritoneal endo- 

 thelium. 



A. The Coelom of the Proboscis. 



The proboscis coelom is, as above mentioned, unpaired. The 

 parietal wall lies under the proboscidal epithelium, the visceral wall 

 envelops not only the proboscidal diverticulum of the buccal cavity, 

 but a complex of other organs as well, which lie posteriorly in the 

 base of the proboscis ; these basal organs, to a certain extent, bulge 

 out the coelomic wall, like the finger of a glove, from behind forward, 

 into the proboscis cavity. 



This cavity has three outgrowths directed backward towards the 

 neck, one ventral and two, a right and a left, dorsal. The left out- 

 growth is produced backward into a canal lined with ciliated epithe- 

 lium, which opens outwards through the proboscis pore. This pore 

 lies dorsally to, and on the left side of, the neck, at a greater or 

 less distance from the median line. 



In a few forms (constantly in Balanoglossus Kupfferi and B. cana- 

 densis, and occasionally in Ptyclwdem minuta and B. KowalevsJcii) a 

 second proboscis pore occurs, through which the right dorsal outgrowth 

 of the proboscis coelom opens outward. This secondary proboscis 

 pore rises much later ontogenetically than the primary. 



It has been conjectured that water is taken in through these pores 

 for the purpose of swelling the proboscis. There is no justification 

 for ascribing to them any excretory function. 



The visceral wall of the proboscidal coelomic sac and, in general. 



