PHYLOGENY OF THE PELECYPODA. 335 



the foot ns seen in Mytilns, Motliola and Nucnla. On the other hand, in Pecten, PL 

 XXVIII, fig. 1, Anomia, PI. xxix, fig. 2, and tlieii' 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 ci-awl- 

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

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



Young Pecten irradians difiers from the adult in many features of anatomy and habit. 

 I will first consider the condition in the yonng and will call attention to the difl'erences 

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

 siin|)le thickening of the edge as shown more highly magnified in PI. xxviii, fig. 6. It 

 has not yet acqnired the complex structnre of the mantle border 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 are highly extensible, sensitive, prehensile, and 

 are covered with papillose 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 known ; but it is possible. 

 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 from 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 thus far 

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

 the shell, no more having yet aj)peared; the active tentacles, however, diff'er from the 

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

 border has as yet no pei-pendicular wall or guard tentacles (described later) but presents 

 one striking feature. When at rest the two borders are protruded beyond the edge of 

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

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

 rapid but intermittent outfiow may be traced by means of the contained particles of ef- 

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

 jjosed of delicate tissue and is reti-acted and extended exactly as may be observed in the 

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

 the walls are not joined by the concrescence of their apposed borders, l^o siphonal ex- 

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

 in the adult 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 great depths. 



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



' In consiileriiiij; the relative position of tlie axes of the aiul tlie gills lie ou the ventral border. The ear possessing 

 body to the shell it must be borne in mind that as in ( )streii the byssal notch throngh ^\•hich the foot extends is there- 

 {vide p. 309) the axes of Pecten are dift'erent from those of fore the ventral ear, not the anterior ear as commonly con- 

 typical dimyarians (PI. xxv, flg. 8). The position of tlie sidered; and the ear on the opposite side is correlatively 

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

 eiul of the shell, tlie free ends of the valves are posterior, 



