CRUSTACEA MALACOSTRACA. III. 



Tanaidacea; several species of each order live constantly in both areas, as is the case with a few species 

 of Echinodermata. The hypothesis on pelagic occurrence proposed by Stephensen as an explanation 

 must be discarded as untenable. And the zoogeographical "law" that the submarine ridge is an 

 absolute line of separation between two faunas must be reduced, recognized only as a generally very 

 good, most useful and interesting rule, but with a number of exceptions. 



Sub-Order Asellota. 



In 1893 I proposed the theory that three joints in the sympod of all biramous 

 appendages in the Crustacea ought to be considered the primary condition, and 

 showed that this number of joints was still extant not only in various appendages in Phyllopoda and 

 Copepoda, but besides in the antennae of the Mysidte and the Asellota, in the maxillulae of nearly all 

 and in maxillse of all orders of Malacostraca, in the thoracic limbs of the Leptostraca. But the first 

 of the three joints in question, the prcecoxa, has hitherto not been pointed out in the maxillipeds and 

 thoracic legs af any species of Malacostraca, excepting the Nebaliidse. In examining closely the maxil- 

 lipeds with the corresponding sternite of two large forms of Asellota, viz. Ianira pulchra n. sp. and 

 Munnopsurus giganteus G. O. S., I discovered the hitherto missing praecoxa (PI. I, fig. 4 e, «., and PI. 

 XII, fig. 5 a, pcx.) as a somewhat small but sharply limited and well chitinized plate between the 

 sternite and the joint generally described as the first. In Aselius and in Munnopsis typica the prsecoxa 

 does not exist. But as it is found in the two genera Ianira and Munnopsurus, so far distant within 

 the very rich family Parasellidie, it exists in all probability also in some other forms. In vain I have 

 looked for the prtecoxa in representatives of the other sub-orders (excepting Phreatoicidea), but its exi- 

 stence in the Asellota mentioned is, in my opinion, a morphological feature of some importance. 



G. O. Sars has pointed out that in Aselius the basal part of the maxillipeds is in the female 

 with marsupium produced in a lappet directed backwards, and on his figure of the maxilliped of 

 Munna Boeckii a much smaller lappet is seen. This peculiar structure, which is highly developed in 

 the majority of Flabellifera, in the Bopyridse, etc., seems to be rare within the Asellota. 



A feature observed in several genera or groups of genera of the Asellota is partial or complete 

 fusion of segments, and such fusion has, excepting in the abdomen, been overlooked in most cases by 

 nearly all authors, who have figured articulations where only a feeble suture or even a transverse 

 impression without suture exists. Some instances may be mentioned here; in the descriptions of 

 genera or groups they shall be treated more fully. 



Beddard figured his Munnopsis australis and Tattersall his Munnopsoides Beddardi as having 

 the three posterior thoracic segments and the abdomen separated by three transverse articulations. 

 These do not exist; the three segments and abdomen are immovably fused, constituting a single piece, 

 and the lines separating them are only impressions, and especially the two anterior of these impres- 

 sions are, besides, extremely curved, so that f. inst. the impression between the fourth and fifth segments 

 has the part at the dorsal median line far in front of its lateral portions (PI. XIV, figs. 2 a and 3 a). 

 In Munnopsis typica we find a somewhat similar structure, as the three thoracic segments and the 



The Ingolf-Expedition. III. 5. 2 



