IX EXTEROPKEUSTA—GCELOMIG SACS 573 



the walls of the posterior outgrowths, retain an epithelial character, 

 while the parietal wall develops muscle and connective tissue. This 

 parietal wall consists of the following parts : — 



1. Close under the basal or limiting membrane of the body 

 epithelium, there is an outermost layer of cireular muscle fibres. 



2. This latter is followed by a massive layer of longitudinal 

 muscles, filling up the greater part of the proboscis. The very 

 complicated course of the longitudinal muscle fibres cannot here be 

 described in detail. They are stretched like tlie strings of an 

 instrument between two points of the proboscidal wall, one behind the 

 other, so that they cross one another in every direction. 



3. Dorsoventral muscle fibres form a dorsoventral muscle 

 septum exactly in the median plane of the proboscis. This is, 

 however, not developed through the whole length of the cavity, 

 but reaches only as far forward as the proboscidal diverticulum of 

 the intestine or its vermiform process. This muscle septum thus has a 

 free anterior edge. The fibres of the septum, which descend from the 

 median line, when they reach the basal organs, diverge to right and 

 left, clasping these organs between them, then again uniting beneath 

 them, form the ventral portion of the septum. This ventral portion 

 is distinctly a double muscle lamella. The two constituent lamellae 

 are separated by a structureless limiting lamella, which is a continua- 

 tion of the limiting membrane of the ventral jDroboscidal epithelium. 

 The circular muscle layer of the proboscis passes through the limiting 

 membrane of the ventral septum in bundles. 



The ventral septum is interrupted at its most posterior part, so 

 that it has a free posterior edge as well as a free anterior edge. 



That portion of the proboscidal cavity which is free from muscle 

 fibres, is to a great extent filled with connective tissue, in which 

 irregular spaces are found as remains of the cavity. A space free 

 from connective tissue and varying in size is retained round the basal 



B. The Coeiomic Sacs and the Musculature of the Collar. 



The collar region of the body contains not only its own two 

 ccfilomic sacs, but outgrowths or processes of the trunk ccelom as 

 well; these latter have been called peripharyngeal or periheemal 

 cavities, and will be described with the trunk coslom. The two 

 lateral ccelomic sacs of the collar are, in adults, nowhere completely 

 separated from one another by mesenteries. The median ventral 

 mesentery is retained for a short distance in the posterior region of the 

 collar. The dorsal mesentery extends further forward, but never as 

 far as to the anterior end of the collar. In Balanoglossus Kupfferi, both 

 the mesenteries are altogether wanting. 



The divisions of the collar coelom are complicated by the appear- 

 ance of folds in the inner or visceral wall. The two lamellae of 



