942 Dn. n. J. uansbn 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 intei-mediate itucje betiveen the small S. ancylops, Kr. 

 (with its abnormal eyes, its rudimentary last pair of trunk-legs, 

 and its doreal spines on some of the abdominal segments, Ac), 

 and large, mature specimens ofH. atlanticus, M.-Ediv. (S. frisii, Kr.), 

 ivas found. "We 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 yS. 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 Mastigopus-Bifigei of S. atlanticus, M.-Edw., 

 and that the idea of Mastigopus must be extended to embrace such 

 stages as only diifer from the Masiigopus of Claus, Bate, and 

 Ortmann in having the fourth and fifth pairs of trunk-legs some- 

 what longer, while their eyes in shape and colour have still 

 preserved the essential characters of the Masiigopiis. And with 

 that I had gained a result rendering it easy to study the alterations in 

 shape and armature of all the various parts of the animal during its 

 development, and a starting-point for the consideration of other 

 species. Soon afterwards 1 made out that S. rinJdi, Kr., is the 

 Mustigopxis of S. arcticus, Kr., &c. And now let us look at the 

 characters of the larva3 in contradistinction to those of the adult 

 animals. 



When a species is mature the male sex always possess a large 

 petasma and — so far as we kuow — a peculiar development of the 

 exterior ilagellum of the anteunulfB. For the females I have not 

 found any character of discriminative value. But while the well- 

 developed petasma is necessary to decide the real maturity of 

 the male, and the female must have readied 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 a 

 specimen wiihoxit a petasma has arrived at the adult stage — viz. that 

 its different parts, such as shield with rostrum, eyes, external maxil- 

 lipeds and legs, uropods, telson, ^'c. have almost or totally assumed 

 the shapie to lie found in the mature and sometimes unknown form — or 

 ivhether it must be considered as a larva. As declared in the 

 introductory remarks, about two-thirds of the established species 

 are but larvse ; in reality they present several characters immediately 

 stamping them as such, and, furthermore, they show peculiarities 

 which indicate very difiereiit 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 tlie 

 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 Mastigopus-iormB, it is most frequently 

 long or even very long. In the larvse the fourth and fifth pair of 



