34 WALTER K. FISHER, 



The Mantle. 



In all the Acmaeidce the mantle has become a most important 

 auxiliary organ of respiration, and in such forms as Scurria and 

 Lottia may be said to have reached its acme in this respect. 



For the sake of discussion the mantle may be divided into an 

 outer or glandular zone and an inner or branchial zone. As shown 

 in Fig. 19 the inner zone is taken up by the so-called branchial 

 cordon which extends completely around the animal, with the exception 

 of a small space in front. This cordon consists of lappets, roughly 

 semicircular in outline which hang from the lower side of the mantle 

 and afford an increased surface for the aeration of blood. Each 

 lamella consists of a plate of muscle and connective tissue, between 

 which and the cubical epithelium are very many blood spaces as 

 shown in Fig. M. These lappets appear late in the development of 

 the animal. Small individuals do not possess them, the mantle ap- 

 pearing in all respects like that of an Acmaea. In an example with 

 a shell 17.5 mm long, the branchial cordon is in the process of devel- 

 opment. The lappets appear as buds and although the cordon is as 

 complete as in the adult the lappets in the anterior part of the 

 pallium are slightly larger and more numerous than those posteriorly, 

 showing perhaps the locality of first appearance. Occasional examples 

 occur as large as 22 mm, in which there is no trace of the cordon, 

 but 20 mm is the usual length at which it appears. I have one 

 specimen as small as 19 mm in which the cordon is like that of the 

 adult. By the time the creature has reached the length of 23 to 24 mm 

 the lamellae are always fully developed. Consequently when under 

 20 mm in length, Lottia gigantea does not possess its generic 

 character ! 



The circulation of the mantle is considered under the circulatory 

 system, and the complex innervation under the nervous system. 



In the outer zone of the mantle, between the circumpallial vein 

 and the free border is the glandular portion. At the margin empty 

 three separate glands which by reason of their relative position have 

 here been designated merely as dorsal, middle and ventral pallial 

 glands (Fig. L). Running around the edge of the mantle is a shallow 

 groove. Into this the median gland opens, while the dorsal and 

 ventral empty respectively above and below it, as shown in the figure. 

 The dorsal gland (Dors. Gl) consists of several layers of pear-shaped 

 cells with large nuclei, and long necks which pass between the epi- 



