61 
In this place I will briefly mention an organism, which I found on the glass after 
having prepared Spher. microcephala, and which, I suppose, is the pupa of this rather 
deviating species. It is somewhat depressed (pl. VIII, fig. 2k); its outline is ovate with a 
straight posterior margin, whereas its rounded front margin bears a somewhat protruding 
adhesive plate (the stripes of which are too strongly marked in the illustration); its dorsal 
surface is provided with some short hairs, the ventral surface is naked. It is 20 mm. long, has 
neither mouth nor other outer organs, nor do we find distinct indications of internal organs. 
In Mysidion commune the metamorphosis is more complicated than in the preceding 
forms, but, unfortunately, my material is not large enough to allow me to elucidate it in all 
details, besides, the forms in hand present several features which I do not understand. I 
have found altogether three stages of development, two instances of the earliest, one of the 
medium, and two of the last and largest stage. I will begin with this last stage, which 
indeed presents a kind of semi-pupa, or a young female in possession of features which it 
afterwards loses. The two specimens found are of about equal size, the one illustrated in 
pl. XII, fig. 1d is ‘31 mm. long. The body is elongate ovate, rather pointed at the 
front extremity, which has a mouth provided with a border and its surrounding hairs; on 
its sides are the maxillule, and on the ventral surface, a little behind the mouth, are the 
maxille and the maxillipeds, which, though well developed, in some small, unimportant 
points deviate from those of the adult female; on the dorsal side, at a rather good distance 
from the mouth, we find the one-jointed organs, which for a long time I considered to be 
the antennul (a), but which no doubt are better explained as being the antenne. On the 
ventral surface, at a considerable distance from the posterior extremity, we see an odd, 
strongly protruding, elongate and somewhat pointed process (x), and nearer the posterior 
margin, somewhat up on the back, the scarcely fully developed crescent (r) which surrounds 
the future genital aperture. In the middle of the back appears an odd, rather low, blunt 
excrescence, from which proceeds a most peculiar fixation-thread, consisting of two divisions. 
The first part (u) is somewhat shorter and thinner than the basal joint of the maxilliped, 
and its distal part is tubular; from the inside of this tube the second division comes out as 
a thread, which is thin in a considerable part of its length, then dilates rapidly and widely 
(v), forming a low collar at its widest expansion; it continues beyond the collar rather thick, 
in the middle somewhat thinner; this part is hollow, very light and is no doubt furnished 
with very thin walls, and its end is fastened to a plate of the marsupium of the host. Can 
this singular fixation-thread be considered as homologous with the frontal thread of other 
pupe? This would seem probable, though it is placed rather far backward; how it is 
produced is incomprehensible to me, but its distal end is very like the thread I have de- 
scribed in the male of Mysidion abyssorum, and its proximal part exhibits great likeness to 
the frontal thread in the male of Spher. danica. Somewhat in front of this thread, on each 
side, at a short distance from the outline of the back, we see a conical process (t), which 
for a long time was inexplicable to me, but which I suppose must be explained as the 
