61 



In this place I will briefly mention an organism, which I found on the glass after 

 having prepared Splicer, microcephalia, and which, I suppose, is the pupa of this rather 

 deviating species. It is somewhat depressed (pi. VIII, fig. 2 k) ; 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 ivhich it 

 afterwards loses. The two specimens found are of about equal size, the one illustrated in 

 pi. XII, fig. 1 d 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 maxillulas, and on the ventral surface, a little behind the mouth, are the 

 maxilla? 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 antennulse (a), but which no doubt are better explained as being the antennae. 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 

 pupa;? 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 ahyssorum, and its proximal part exhibits great likeness to 

 the frontal tlu-ead in the male of Splicer, 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 



