NOTES ON COMMON SPECIES OF TROCHUS. 463 



high tide form contrasts with the reduction of the gland 

 observed by Pelseueer (6) in the high tide Littorinas. 

 There can be no doubt, however, that the roof of the 

 branchial cavity in the latter, with the prolongation of 

 the ctenidial leaflets across it, is much more specialised 

 for respiratory purposes. The mucus gland of T. crass us 

 pi'eserves the condition found in many other species of the 

 genus, and, in spite of its approach to a high-tide habitat, 

 there is greater need for damp protecting slime here than in 

 Littorina. T. crass us keeps its foot in contact with the 

 rock throughout a period of exposure, while a high-tide 

 Littorina withdraws deep into its shell, the edge of which 

 remains adherent to the rock by means of a dried film of 

 mucus. In other words, the branchial cavity of T. crassus 

 cannot be nearly so completely protected (by retraction) 

 from drying during a period of exposure as can that of 

 Littorina, and the greater development of mucus gland may 

 be a compensation for this. T. crassus is found in shadier 

 and less exposed corners than the high-tide Littorinas often 

 manage to occupy, and not usually so far up the shore. 



(b) Gill. — The gill is built on the same lines in the two 

 species under discussion, but shows several specialisations on 

 the condition found in more primitive Rhipidoglossa. The 

 loss of the right gill and the migration of the anus and 

 excretory openings towards that side has been discussed by 

 Ainsworth Davis (1) as a device for more complete separa- 

 tion of incurrent and excurrent streams through the branchial 

 cavity. The surviving left ctenidium in Troclius remains 

 biseriate, as in Haliotis and Pleurotomaria, but the details of 

 its disposition are very different, as is well known. In those 

 more primitive types a median axis Ijetween the two series of 

 leaflets contains the longitudinal afferent and efferent blood 

 channels' of the ctenidium, and that axis is attached to the 

 wall of the branchial cavity on the side of the efferent 

 channels only. Right at the back of the cavity the afferent 

 axis is also attached to the roof on either side (fig. 8), and 

 the basi-branchial sinus, which feeds the afferent ctenidial 



