522 Miss Hilda M. Bishop on the 



respect. It is weakly developed in the common species, and 

 in its place we find a vascular network (cf. Pelseneer, 1895). 



In L. litorea, in the male (fig. 12) there are two glandular 

 areas — a large one on the ctenidial side of the rectum and a 

 smaller one on the other, i. e. the right side. 



In the female (fig. 13) there is a good deal of glandular 

 tissue on the ctenidial side, but none on the other, along 

 which the oviduct runs. 



In L. obtusata, in the male (fig. 14) there is a very small 

 amount of glandular tissue on the ctenidial side and a fairly 

 large amount on the other; in the female (fig. 15) there is no 

 definite gland at all. 



In L. rudis (fig. 16) the glandular areas in the male are 

 somewhat similar to those of L. obtusata, but in this species 

 in the female a small one is present on the ctenidial side of 

 the rectum (fig. 17). 



Thus the low-tide species, L. litorea, possesses the most 

 highly developed mucus-tissue in the branchial cavity. 



Its poorer development in the high-tide species, L. obtusata 

 and L. rudis, may perhaps be correlated with the habit 

 adopted by these species of complete retraction into the shell 

 during rest, by which means the branchial cavity is perfectly 

 protected, and also to their habit of remaining uncovered by 

 the tide, for mucus hardens on exposure to the air, and if 

 there were a great quantity of mucus present in the branchial 

 cavity, the latter might become clogged. 



Pelseneer (1895) agrees that the mucus-gland is weak in 

 Littorina, less so in L. litorea than in the other common 

 species. 



Bibliographical Summary. 



1880. Simroth, H. — "Das Fussnervensyslem von Paludina vivipara." 

 Zeitsch. f. wiss. Zool. xxxv. 



1882. Carriers, J. — " Die Fussdriisen der Prosobranchier mid das 

 Wassergefassystem der Lamellibranehien und Gastropoden." 

 Arch. f. rnikr. Anat. xxi. 



1883. Houssay, F. — " Opercule et glandes dupied des Gasteropodes." 

 Arch. Zool. exp. s6v. 2, tome ii. 



1887. Botjvier, E. L. — " Systeine nerveux des Gasteropodes Proso- 



branches." Ann. Sci. Nat. ser. 7, tome iii. 

 1890. Thiele, J. — " Ueber Sinnesorgane der Seitenlinie und das 



Nervensysteni von Mollusken." Zeitsch. f. wiss. Zool. Bd. xlix. 

 1890. Botjvier, E. L. — " Sur le Sytteme nerveux des Uyprees.'' Zool. 



Anz. xiii. 

 1890. Haller, B. — " Die Morphologie der Prosobranchier. — II. Cy- 



prcea testndinaria" Morph. Jahrb. xvi. 



