ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 157 



edge of the body behind the anus ; and in other species such processes 

 develope accessory folds, until in Carinaria we find fully-formed 

 branchia; (see Eydoux and Souleyet). The ciliated subspiral band(rf) 

 which will be found to have its homologue in Atlanta, is the only 

 structure which appears to be capable of assisting the respiratory 

 function ; but its small size must render it of very little importance. 

 Mayitle. — There is no distinct mantle in Firob'ides Desmarestii. 

 Carinaroides (Eydoux and Souleyet) evidently forms the transition 

 from the Heteropods without a mantle to those with one. It is in 

 this genus placed at the lower posterior angle of the body, and carries 

 a minute shell ; the branchia; are developed between it and the 

 anus. 



In Carinaria there is a proportionably small mantle and shell, but it 

 occupies a position more resembling that of ordinary Mollusks ; and in 

 Atlanta, as will be seen, the relative proportions of the mantle and 

 body are pretty nearly those found in ordinary Gasteropods. 



Contractile Sac or Urinary Organ (r, figs. 2, 3). — Between the rectum 

 and the heart, and therefore bathed by the returning venous blood, 

 there lies an elongated, flattened, delicate, and transparent sac, A\-hose 

 walls are usually very much wrinkled and sacculated. 



This sac opens by a rounded aperture in its upper part upon the 

 right side of the animal, and is, of course, continually filled by the 

 surrounding water. As the sac is incessantly contracting, however, 

 this water must be continually renewed, and hence the organ, simple 

 as its structure appears and small as its size may be, is probably a 

 very efficient depurating agent.^ 



Considerations to be stated hereafter, lead me to the belief 

 that it is in fact the urinary organ, at once kidney and urinary 

 bladder. 



Reproductive System. — Firoloides is dioecious. The male may be 

 distinguished at once from the female, by the peculiar penis (/) 

 attached to the right posterior inferior angle of the body (fig. 4). It 

 consists of two portions ; the larger is cylindrical, but enlarged at its 

 extremity into a globular head, from one side of which a small pointed 

 process projects. The globular body contains many large cells as a 

 sort of lining, and within these there is a cavity which communicates 

 with the exterior through the pointed process. A vast number of small 

 oval fatty-looking particles may be made by pressure to pass out from 

 the cavity. 



The smaller portion is like a trifid leaf ; it is placed at the base 



' In the "Explication des Planches," Eydoux and Souleyet call a similar organ in Firola 

 " organe de la depuration urinaire ?" 



