26 MORPHOLOGY OF 



Nearly all the proboscis cavity is eventually filled up with 

 loose tissue. This is composed of a number of concentric rings 

 of longitudinal fibres and connective tissue. These rings 

 attain the maximum number of eight. In fig. 51 the general 

 appearance of an older proboscis cavity is shown. The con- 

 centric arrangement is, however, not yet attained in the stage 

 there figured. 



A free space is always present between these rings of tissue 

 and the central structures in preserved specimens. 



The proboscis gland becomes a large mass of tissue com- 

 posed of anastomosing blood-vessels covered with conical cells 

 fixed on the vessels by their apices. Many of these cells 

 contain remarkable yellow granules, which are also to be 

 found outside the cells, sometimes presenting a conglomerate 

 arrangement. They would seem to be formed in the cells and 

 thrown out. They are also to be found in the sac of the 

 proboscis gland. This sac is blind posteriorly, but anteriorly 

 the loose tissue which it contains passes into unbroken con- 

 nection with the remarkable cellular layers covering the blood- 

 vessels. Hence the sac is in communication with the central 

 body cavity through the tissue spaces of the gland. The 

 function of this gland is quite unknown. Spengel suggests 

 that it is an "internal gill." It does not seem probable to me 

 that an animal with some sixty pairs of true branchial clefts 

 would also possess another large and complicated organ of 

 entirely different structure also for respiratory purposes. The 

 presence of the brown granules suggests that it may be excre- 

 tory. If this were so, the excreta might be expected to pass 

 out by the proboscis pore which opens into the cavity in which 

 the gland lies. No direct evidence was obtained as to the 

 normal direction of the flow through this pore. Spengel and 

 other observers state with regard to B. minutus that water 

 is taken into the body cavity at the proboscis pore, but my 

 own observations do not confirm this statement. On the 

 contrary, particles of Indian ink or carmine held in suspension 

 in the water in which the animals have lived for days, cannot 

 be found to enter the proboscis cavity, while similar particles, 



