ix EXTEROPXEUSTA CCELOMIC SACS 575 



throughout the larger part of the collar, viz. in all that part which lies behind the 

 circular vascular fold. In front of this fold, however, it is as highly developed as 

 in the same area in group 1. The muscles arise dorsally at each side of the pro- 

 boscidal skeleton, and form loops encircling the buccal cavity. 



3. The anterior wall of the collar ccelom. A strong bundle of muscle fibres 

 arises to right and left of the neck of the proboscis ; these radiate from the anterior 

 wall of the collar ccelom to the edge of the collar. Both ventrally and dorsally 

 the marginal fibres of these two radiating bundles usually pass beyond the median 

 line, cross one another, and intermingle at these points. 



Besides the musculature hitherto described of the parietal visceral and anterior* 

 walls of the collar ccelom, there are further isolated radial muscle fibres, which] 

 connect the outer with the inner wall, and also with the anterior wall. These\ 

 fibres together form a system of crossing fibres, some running slantingly from the] 

 outer wall inward and forward, and others inward and backward. 



The cavity of the collar is filled with connective tissue, which penetrates every- 

 where between the muscles, leaving only certain spaces free. Such a free space is 

 as a rule found to the right and left in the posterior part of the collar. The collar 

 ccelom here is continued on each side into a canal lined with a ciliated cylin- 

 drical epithelium ; each canal opens through a pore (collar-pore), not on the 

 external surface of the body, but on the anterior wall of the first gill-pouch, near 

 the branchial pore. The canal projects from this pore freely forward into the 

 collar ccelom, and, on its outer surface, that turned to this ccelom, is covered with 

 plate epithelium. 



As to the function of these two collar pores, we can say no more than what 

 was said above about that of the proboscis pore. 



In Balanoglossus Kupfferi a cushion-like thickening of the epithelium is found 

 on each side of the body on the septum dividing the collar from the trunk. This 

 thickening occurs both on the anterior surface, that turned to the collar ccelom, 

 and on the posterior surface, that turned to the trunk ccelom. These thickenings 

 probably function as lymph glands. 



C. The Coelomic Sacs and the Musculature of the Trunk. 



The Perihsemal and Peripharyngeal Cavities of the Collar Region. 



The trunk ccelom is uninterrupted throughout its whole length. 

 Its composition out of two lateral ccelomic sacs can still be recognised 

 in the adult animal, the ventral mesentery being entirely, the dorsal 

 partially, retained. 



The ccelomic sacs of the trunk send outgrowths anteriorly into 

 the cavity of the collar, which push before them the wall of that 

 cavity ; these are the perihsemal and peripharyng-eal cavities 

 (Fig/460). 



The perihsemal cavities are two dorsal prolongations of the trunk 

 ccelom, which traverse the collar region and the neck of the 

 proboscis as far as the proboscidal skeleton. They run below the 

 collar cord and above the buccal cavity. In the median line the two 

 cavities are separated by a structureless partition, a limiting mem- 

 brane, in which the dorsal vessel runs. The perihaemal spaces are 

 almost entirely filled by longitudinal muscle fibres formed by their 



