602 



INVEBTEBRA TE MORPHOLOG Y. 



ones are in the younger stages of development, a tonguelike 

 valve later growing down from the dorsal border of the pore 

 and giving it its U-shaped form. Water ilows in at the mouth 

 and passes out through the branchial slits, which thus pos- 

 sess a respiratory function. A few of the anterior slits open 



FIG. 274. Balanoglossus Kowalewsku (after MINOT from SPENGEL). 

 br branchial slit. c = collar. pr proboscis. 



externally into the atrium, being covered over by the back- 

 wardly-projecting atrial folds of the collar, but the majority 

 are quite uncovered and are plainly visible from the exterior. 



The ectoderm contains numerous mucous glands and is 

 ciliated throughout, no external cuticle or " house," such as 

 occurs in the Pterobraiichia, being developed. Below it rests 

 upon a thin basement-membrane. 



The ccelom is clearly marked out, and consists of three 

 portions completely separated from, one another aud corre- 

 sponding to the three body regions. The proboscis-co3lom 

 (Fig. 275, A, pc) is, in its anterior portion, a simple unpaired 

 cavity lined with delicate cells and traversed by circular and 

 longitudinal (Im) muscle-fibres. Posteriorly it is prolonged 

 into two horns between which lies a mass of tissue consisting 



