602 



INVERTEBRATE MORPHOLOGY. 



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 flows 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. 374. — Balanoglossus Kowalewskii (after Mdjot from Spkngel). 

 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 Pterobranchia, being developed. Below it rests 

 upon a thin basement-membrane. 



The coelom is clearly marked out, and consists of three 

 portions completely separated from one another and corre- 

 sponding to the three body regions. The proboscis-coelom 

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

 cavity lined with delicate cells and traversed by circular and 

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

 into two horns between which lies a mass of tissue consisting 



