462 H. J. FLEURE AND MUKIEF. M. GETTINGS. 



correlated with the greater activity of the former in the 

 shore-zone where a high spire woukl give too much purchase 

 to a side blow from a wave. 



Branchial Cavity. 



(a) Mucus Gland. — The mucus gland of T. obliquatus 

 is developed on the right side of the rectum in ridges and 

 furrows running towards the anus (fig. 5). This tissue is 

 evidently the equivalent of the mucus gland of the right side 

 in Haliotis and Pleurotomaria^ and its disposition reveals its 

 primary function of coating rough excreted fragments 

 (expelled from the right kidney or the anus) with slime, 

 so that they may be less likely to injure the gill-leaflets 

 and other delicate organs before being washed away. There 

 is also a special aggregation of this tissue around the aper- 

 ture of the right kidney (fig. 5). The mucus gland of the 

 left side is not nearly so well developed in Trochus 

 obliquatus as in the more primitive Rhipidoglossa. It 

 forms ridges which coat the transverse pallial vein just 

 in front of the external aperture of the left kidney, and 

 stretches forward along the rectum to some extent; it is 

 developed also along the afferent axis of the ctenidium, 

 where this axis unites with the roof of the branchial cavity, 

 and especially at the front end of the uniting fold. 'I'his 

 indicates that its special function is to protect the main blood 

 channels of this region and the gill leaflets hanging in the 

 branchial cavity from the damage due to grit or hard 

 fragments which may have wandered into tlio cavity or may 

 have been expelled from the kidneys or the anus. 



In T. crassus the mucus ghmd epithelium is found in the 

 corresponding places, but is also present on ridges in the 

 roof of the branchial cavity between ctenidium and rectum 

 to a far greater extent than in T. obliquatus [Gg. 4). This 

 comparatively greater development of the mucus gland in a 



