The anntomy of Lottia gigantea Gray. 33 



along the whole of the right side of the pericardium. The afferent 

 ctenidial vessel leaves a short sinus above the left kidney and is much 

 less roomy than the efferent vessel. Two stout muscles pass along 

 its dorsal and ventral sides, and a nerve along its outer, buried in 

 the muscular wall. This vein drains blood from the left nephridium 

 and the fore part of the vein-net of the right. 



The vein leading from the ctenidium opens into a sinus of the 

 mantle hood, whence the blood reaches the auricle through numerous 

 openings. This vessel also possesses two muscles and a double nerve. 

 The leaves of the gill are arranged in a double, or dorsal and ventral, 

 series and are attached at their bases to a median membrane, through 

 which the blood can pass from one vessel to the other. About the 

 edges of each gill-leaf is a rather prominent space through which the 

 main stream of blood passes, and from which it seeps, by lesser 

 spaces toward the base of the lamella. The histology has been de- 

 scribed by Haller (1. c). 



There is little foundation for Thiele's dictum ^) that the single 

 ctenidium of the monobranchiate Docoglossa is a secondary structure 

 and that the primary gill has degenerated. As a matter of fact 

 nothing is known of the development of these forms, while everything 

 about the structure points to the fact that the existing ctenidium is 

 not secondary. The nervous supply is very abundant and springs 

 from the selfsame ganglion as does that of the corresponding gill of 

 Haliotis^ which this investigator is so anxious to prove primitive and 

 in a direct line of descent. Furthermore the attachment of the gill 

 by its base in the Acmaeidœ is much more similar to the condition in 

 the chitons, than is that of Haliotis, where the gill is fastened along 

 the side of the efferent vein. The passage of the blood in Lottia from 

 the ctenidium into a palliai sinus, and thence into the heart is more 

 primitive than the direct connection found in Haliotis, and again 

 agrees more closely with the condition of affairs found in chitons. 

 Here both the vessel distributing blood to the gills, and that bearing 

 it to the heart are no more than palliai spaces, into which the two 

 main vessels of each ctenidium communicate. Consequently it would 

 seem ill-advised to insist very strongly on the secondary nature of 

 the Acraaeid gill, until something definite is known of the development 

 of these forms. The structure and relations of the organ can just as 

 readily lead to the opposite conclusion. 



1) Thiele, 1. c. p. 332 and 356. 



Zool. Jahrb. XX. Abth, f. Morph. 



