28 MORPHOLOGY OP 



surrounds the gut anteriorly. The principal skin sinuses are a 

 pair of large ones which extend on each side of the dorso- 

 lateral regions of the proboscis (fig. 51). 



The Collar Pores. — On the outer wall of each atrial cavity 

 appears a thickening at about eight gill-slits. This thicken- 

 ing acquires a perforation which leads from the collar body 

 cavity to the atrial cavity. These perforations acquire a 

 curious folded lumen and become ciliated constituting the 

 collar pores. Their opening into the atrial cavity is con- 

 tinuous with that of the first gill-slit. From analogy it may 

 be expected that these pores are of an excretory character. 

 With regard, however, to the direction of the flow through 

 them, the evidence is as unreliable as that as to the currents 

 in the proboscis pore. Spengel states that water is taken into 

 the body cavity at these points, while 1 was unable to find that 

 coloured particles ever entered it. Similarly, however, such 

 particles placed artificially in the collar body cavity were 

 washed out at these points. 



The Middle and Posterior Body Cavities. — These 

 cavities become in adult life more or less filled with connective 

 tissue, &c. The cavity of the middle pair becomes practically 

 obliterated owing to the great development of loose tissue in 

 it. But in the posterior cavity this proliferation is never so 

 great. The middle pair of body cavities is far more choked 

 up in B. Kowalevskii than in B. minutus, but in B. 

 Brooksii the amount of connective tissue is even greater. 

 This fact is interesting in the present state of views as to the 

 morphological meaning of " ccelom," as presenting an example 

 of a body cavity arising in a most typical " mesodermic " 

 manner, assuming ontogenetically precisely such an appear- 

 ance as is presented by the " mesenchyme " of Platyhel- 

 minths, &c. 



A statement as to the origin of the generative organs 

 is reserved for the present, as some doubt exists as to the layer 

 from which they are derived. The general appearance is, how- 

 ever, suggestive that they are of epiblastic origin. 



Before beginning a detailed account of the later development 



