OCR Text |
Show 942 DR. H. J. HANSEN ON CRUSTACEANS [Dec. 1, yellowish eyes, which possessed a shape recalling somewhat the very curious, ovate, and obliquely implanted, but much larger eyes in S. ancylops, Kr. The result of further comparison was that every conceivable Intermediate stage between the small S. ancylops, (with its abnormal eyes, its rudimentary last pair of trunk-legs, and its dorsal spines on some of the abdominal segments, &c), and large, mature specimens of S. atlanticus, M.-Eclw. (S. frisii, Kr.), was found. W e possess S. ancylops from 17 localities, and in 10 of these it was taken in company with larger transition-stages to, or completely developed specimens of, S. atlanticus. That typical specimens of S. ancylops and transition-stages to the black-eyed form do not possess any petasma, scarcely needs mention, but neither was it found in the smallest of the black-eyed specimens. The result was that S. ancylops, Kr., must be considered as the Mastlgopus-stage of S. atlanticus, M.-Edw., and that the idea of Mastigopus must be extended to embrace such stages as only differ from the Mastigopus of Claus, Bate, and Ortmann in having the fourth and fifth pairs of trunk-legs somewhat longer, while their eyes in shape and colour have still preserved the essential characters of the Mastlgopms. And with that I had gained a result rendering it easy to study the alterations in shape and armature of all tbe various parts of the animal during its development, and a starting-point for the consideration of other species. Soon afterwards I made out that S. rinkll, Kr., is the Mastigopus of S. arctlcus, Kr., &c. And now let us look at tbe characters of the larvae in contradistinction to those of the adult animals. W h e n a species is mature the male sex always possess a large petasma and-so far as we know-a peculiar development of the exterior flagellum of the antennulae. For the females I have not found any character of discriminative value. But while the well-developed petasma is necessary to decide tbe real maturity of the male, and the female must have reached the same length as the adult male before it can be admitted as being mature, such comparison Is not necessary for the decision of the question whether specimen without a petasma has arrived at the adult stage-viz. that its different parts, such as shield ivlth rostrum, eyes, external and legs, uropods, telson, Sfc. have almost or totallg assumed the shape to be found in the mature and sometimes unknown form-or whether It must be considered as a larva. As declared in the introductory remarks, about two-thirds of the established species are but larvae; in reality they present several characters immediately stamping them as such, and, furthermore, they show peculiarities which indicate very different stages of metamorphosis. In a multitude of " species " dorsal spines on some or almost all the abdominal segments are present, and such spines only exist in the larval stages, but in many older larvae the abdomen is quite smooth. In the adults the rostrum is rather short or very short, but, especially in the younger Mastlgopus-iorms, it is most frequently long or even very long. In the larva? tbe fourth and fifth pair of |