212 Mr. J. Alder on Montacuta ferruginosa. 



But to return to Montacuta ferruginosa. The mantle, which 

 is open throughout the entire front and base of the shell, is 

 closed posteriorly, forming a small excretory orifice, not produced 

 into a siphon. The foot, as might be concluded from the much- 

 elongated anterior portion of the shell, is very large and mus- 

 cular ; there is a slight angle about half-way down in front, be- 

 yond which it is rather narrower and tapers to a blunt point : 

 the base is slightly undulating and grooved through its entire 

 length, though it does not appear to spread out into a fiat disc 

 like that of Lepton squamosum : the hinder portion is abruptly 

 truncated. 



After having kept my specimen for some days in sea-water, I 

 found one morning that the bottom of the glass was covered with 

 a minute white dust, w^hich I immediately concluded 'would be 

 the spawn, and on placing a small portion under the microscope 

 I found that such was the case. I consequently had it removed 

 into a separate glass with a fresh supply of water, in order to 

 observe its development. Though nearly round at first, the ova 

 soon assumed a subtriangular shape, and about the third day, 

 strong cilia were observed on one of the sides, and they began to 

 rotate very quickly. One after another assumed the rotatory 

 state, till nearly the whole were in motion. After rotating for 

 about a day, they apparently burst the envelope, and swam freely 

 about in the water in all directions, by means of their vibratile 

 cilia, and at the same time assumed more or less of a bell-shape ; 

 a slender style or thread projecting from the centre of the ciliated 

 base. This organ, which has been observed in the embryos of 

 other species, has been described as a kind of byssus, by which 

 the little creature can fix itself securely to other bodies. This, 

 however, I did not observe to be the case in the present instance. 

 It soon appeared to be absorbed ; the animal became gradually 

 elongated, and the cilia were withdrawn into the shell, which 

 then began to appear, but at what time it was actually formed I 

 could not make out, as, from its extreme transparency and simi- 

 larity of colour to the rest of the animal, it was very difficult of 

 detection. The cilia could be seen vibrating within the shell for 

 some time after the animal became quiescent ; a few isolated cilia 

 at one of the extremities, not observed before, being the only 

 ones that remained to perform their functions externally. These 

 produced a partial current without propelling the animal through 

 the water, as at this stage it gave up its natatory habits and took 

 to a quiet life. The internal portion, the parts of which could 

 not be very distinctly made out, appeared to be undergoing a 

 process of development. The mass was continually changing its 

 form, the separate parts being extended alternately in difi^erent 

 directions, and a portion, probably the incipient foot, was occa- 

 sionally pushed beyond the margin of the shell. At this point 



