PHYLOGENY OF THE PELECYPODA. 373 



The tubes are divided from the base beyond the shell margins and are not coalesced at 

 any point visible when fully extended. The relation between this condition and that 

 found in the adult is of much interest and was fully illustrated in the specimens which I 

 studied alive. The excurrent tube, PI. xxx, fig. 13, is com])osedof most delicate trans- 

 parent tissue. It is prolonged in a proboscis-like fashion and is frequently extended and 

 retracted with a jerky irregular motion. Watei' is expelled intermittently, and between 

 the expulsions the si[)hon tube is somewhat retracted. The tube has a small contracted 

 orifice and a tentacle was noticed at its base. More tentacles at this region doubtless ex- 

 isted, for many are found in the next stage, but they were not observed. The incurrent 

 branchial siphon tube, fig. 13, is nuich shorter than the excurrent tube. It has thicker 

 walls, bears a circlet of simple tentacles at its orifice and does not take pai't in the fre- 

 quent extensions and retractions characteristic of the excurrent tube. The siphon of an 

 older specimen measuring about 7 mm. in length is represented in PI. xxx, fig. 14. Short 

 termini of the excurrent and incurrent siphon tubes similar to those of fig. 13 exist at the 

 tip; Ijut between these and the shell, a massive coalesced growth of the tubes has de- 

 veloped by intei'stitial growth of the extended mantle border. Thus the portions of the 

 siphons which are found in fig. 13 are now pushed out to the terminus of what may be 

 called the massive siphon. The massive siphon is pigmented in dots, especially near the 

 terminus which approaches nearest to the light, and it is covered by a delicate epi- 

 dernud layer as in the adult. Tentacles fringe the free, distal border of the massive tube 

 around its entire circumference, and a line of black pig'ment closely underlies the tentacles. 

 The excurrent terminus, as in fig. 13, is composed of delicate tissue, is extensible and has a 

 small contracted orifice which is plain, bearing no tentacles. Under higher magnification 

 small dots regularly placed were observed on the walls of the excurrent terminus. The 

 incuri'ent siphon terminus, as in fig. 13, is shorter than the excurrent. It is composed of 

 thicker tissue and is constantly extended, not intermittently extended and retracted 

 as is the excuri'ent terminus. Tentacles surround the orifice of the incurrent siphon and 

 between the tentacles is black pigment bordered by orange; the tentacles also are them- 

 selves slightly pigmented. The pedal orifice of the mantle border in specimens of this 

 age bears short papillose tentacles. 



The development of the siphon from the condition just desci'iberl, PI. xxx, fig. 14, to 

 the adult condition is merely a series of mechanical changes. The massive siphon elon- 

 gates proportionately until, as Vei-rill says, in the adult it can be extended to the length 

 of a foot or more. The termini of the tubes described in the young exist in the adult, 

 but are so reduced in proportion to the entire siphon that they are hardly noticeable. 

 The incurrent terminus is so short that it appears, as I have seen it described, that two 

 rows of tentacles exist at this opening, whereas the fact is that one row surrounds the 

 tip of the massive siphon and the other the free border of the incurrent siphon terminus 

 as in fig. 14. The true condition of the parts in the adult is clearly figured by Morse, in 

 his " First Book of Zoology." 



A condition of the siphons, similar to that of PI. xxx, fig. 14, exists in adult Mya trun- 

 cf/^r/, as figui'cd by Forbes and Ilauley, and a condition similar to that of the earlier stage 

 of Mija airinii-ia, PI. xxx, fig. 13, is found in the adidts of several genera, some closely al- 

 lied to Mya and some not. Most important to us is the fact that this early condition of 



Mli.MOIUS HOSTUN SOC. NAT. HIST., VOL. IV. 50 



