20 



shorter than processus uncifer {^pu^ ; the proximal part of its outer margin is a Httle concave 

 and considerably longer than the distal part ; processus uncifer is somewhat slender. Processus 

 basalis {pb^ is of moderate length, conical, acute. Pars media is moderately slender. Processus 

 ventralis {pv^j is rather long, at the base extremely broad and tapers rapidly so that it in 

 more of half of the length is of moderate thickness; at the end it is expanded and cut off 

 with the terminal margin obliquely concave and the distal interior corner produced as a triangular, 

 acute process; in front along the terminal margin is found 8 triangular teeth. Lobus armatus 

 (/a.) projects beyond the processus ventralis; it is somewhat thick at the base, decreases towards 

 the obtuse end and is straight or only a little curved; its whole innermargin has a 

 row with about 13 hooks, the small terminal one included. Lobus connectens (/f.) is 

 somewhat small, oblong triangular, directed outwards with about 8 hooks on the distal half of 

 its anterior side. Lobus terminalis (//.) is moderately short, not decreasing in breadth to the 

 obliquely rounded end and curved outwards; its terminal part has a good number of hooks 

 especially on the margin and the front side. Lobus inermis (//.) is considerably longer than 

 Jobus terminalis, directed forwards and somewhat inwards, tapers from rather near the base to 

 the narrow subacute end, and it has no hooks. — (The thelycum differs much from that in 

 5". fidgens or S. Challengeri^ but having only a single full-grown female it was found very 

 difficult to draw a good figure, and without a figure a description of the protuberances and 

 impressions has scarcely any value). 



Length of the adult female 46 mm., of a large male 37 mm. 



b. jl/ash'goptes-Stages. - — These larvae are scarcely discernible from those of 5. corni- 

 culuni Kr., which has been established on such larvae measuring according to Kroyer scarcely 

 13 mm. Kroyer and Bate published descriptions and figures of such forms. In 1896 I described 

 a young stage measuring 6.4 mm. in length; in 1903 I pointed out that 5. longirostris Bate 

 is a very young Mastigopus of the same species. The Afasizgopus-stages of S. cornicubivi and 

 and 5. seminudus are distinguished from those of all other species of Group I by the exopod 

 of the uropods, as the ciliated part of the outer margin occupies from a little more than half 

 to about three-fifths of its length ; in the youngest specimens the end of the naked portion has 

 a well developed spine, and this spine is gradually reduced in size according to age, and dis- 

 appears in large specimens. The largest Mastigopiis in the "Siboga" material is 12 mm. long 

 and has the rostral process very thin and moderately short; one of the smallest specimens 

 measures 4.3 mm. not including the very long rostral process which reaches the middle of the 

 eyes; both specimens are from Stat. 144. The descriptions by Kroyer, B.a.te, and myself, 

 and the figures published by Kroyp:r, Bate, and Chun of various stages of the Mastigopus 

 of 5. cornicultim (and the very young stage named S. longirostris Bate) may be sufficient 

 for the present. 



c. Acanthosoma (or J/j/jzj--) S t a g e (PI. II, figs. \c — if). — Together with a single 

 Mastigopus 3 specimens of Acanthosoma were found in the Plankton material, and for various 

 reasons, viz. length of rostrum, shape of the eyes and of the exopod on the uropods, they 

 must belong to this species. The rostrum (fig. \c) is about half as long again as the carapace 

 in the median line, with many fine spines. The carapace (figs. \c — i^;') has four pairs of long 



