PHYLOGENY OF THE PELECYPODA. 335 



the foot as seen in Mytilus, Modiola and Nucnla. On the other hand, in Pecten, PI. 

 xxvin, fig. 1, Anomia, PI. xxix, fig. 2, and their allies which lie on the right valve, the 

 cleft in the foot is in the right side, this being the side apposed to the ground in crawl- 

 ing. Whether the position of the cleft has changed, or the apparently changed position 

 of the cleft is brought about by a twist in the foot, is not known. 



Young Pecten irradlans dlfters from the adult in many features of anatomy and habit. 

 I will first consider the condition in the young and will call attention to the diiferences 

 when discussing older stages. The mantle border of the young, PI. xxviii, fig. 1, is a 

 simple thickening of the edge as shown more highly magnified in PI. xxviii, fig. G. It 

 has not yet acquired the complex structure of the mantle l^order of the adult. The eyes 

 are arranged on the border, one in each plicated fold of the shell. As the plications in 

 the two valves alternate, so the eyes alternate on the two mantle borders, each retaining 

 a position in the furrow of its apposed valve. The tentacles at this stage alternate with 

 the eyes, as shown in the figures; they arc highly extensible, sensitive, prehensile, and 

 are covered with pajjillose projections. When crawling, the young scallops use the 

 tentacles actively and they constantly lay hold of the glass by their sensitive tips. 

 Whether they assist in crawling by their prehensile activity is not lv:nown;butit is possil)le. 

 They will wind the tips around a body put in their way, such as a needle or pencil point, 

 and I have seen very young Pectens suspended fi'om the glass sides of a beaker by the 

 clinging power of extended tentacles, the animal being otherwise completely inactive. 



At a little later date, a few Pectens were found considerably older than those thi:s far 

 considered. The eyes of this stage, PI. xxviir, fig. 2, are still single in each plication of 

 the shell, no more having yet appeared; the active tentacles, however, differ from the 

 earlier stage, fig. 1, in that there are now two alternating with each eye. The mantle 

 border has as yet no perpendicular wall or guard tentacles (described later) but presents 

 one striking feature. When at rest the two borders are proti'uded beyond the edge of 

 the shell in a short tubular form at the base of the dorsaP ear as shown in PI. xx\arr, 

 fig. 2. At this point excurrent action takes the direction indicated by the arrow and a 

 rapid but intermittent outflow may be traced Iiy means of the contained particles of ef- 

 fete matter. This extension of the mantle is at that time a pseudo-siphon; it is com- 

 posed of delicate tissue and is i-etracted and extended exactly as may be observed in the 

 anal siphon of young clams (vide section xiv), but it ditt'ers from a true siphon in that 

 the walls are not joined by the concrescence of their apposed borders. Ko siphonal ex- 

 tension of the mantle was seen in Pectens younger or older than that here described, but 

 in the adnlt it is important to notice that all excurrent action takes place at this same 

 area of the shell. Dr. Dall informs me that he has seen a similar condition of siphonal 

 extension of the mantle border in Pectens dredged from gi-eat depths. 



It is an interesting fact in young Ostrea and Pecten that excurrent action takes 



' lu considering tlic relative position of tlie axes of tlie and tlie gills lie on tlie ventral border. The ear possessing 

 body to the shell it must be borne in mind that as in Ostrea the byssal notch through which the foot extends is tliere- 

 (cWe p. 300) tlie axes of Pecten are difl'orent from those of fore the? ventral ear, not tlic anterior ear as comnionlv eon- 

 typical dimyarians (PI. xxv, tig. 8). The position of the sick'red; and Ihe ear on tlie opposite side is correlatively 

 palps and foot in Pecten indicates tlie uinlios as the anterior tlie dorsal not the posterior ear. 

 end of the shell, the free ends of the valves are posterior, 



