ABNORMALITIES Ui 



These variations are sporadic and may be the result of some sHght injury; in one 

 species, however, sHght palpal abnormalities occur so frequently as to be characteristic 

 of the species. In Decolopoda antarctica, Bouvier (fig. i, a-e), the number of palpal 

 segments varies from 8 to lo (Table I). Until a large number of specimens are available 

 for study it is impossible to say whether eight or nine is the normal number of segments. 

 Three of the seven specimens examined have both palps eight-jointed as in Fig. i a; 

 one specimen has both palps nine-jointed. The numbers of palpal segments in each of 

 the seven specimens may be represented as follows: 8, 8; 8, 8; 8, 8; 9, 9; 8, 9; 

 8 + (fig. I J), 8 ; 8 + (fig. I 0' 9 + (fig- I ^)- The vestigial ninth or tenth segment 

 (Figs. I d and i e) may resemble a minute claw. Caiman (1920, p. 244) puts forward 

 the view that "the assumption of a claw-like form by the terminal segment may, 

 perhaps, be regarded as a case of homoeosis" since all appendages posterior to the palp 

 may end in claws. Occasionally the terminal segment is somewhat abnormal, as in 

 Fig. I b, suggesting regeneration of the apex after mutilation. Judging from the 

 abundance of encrusting organisms found on Decolopoda antarctica, it must be a very 

 sluggish species (see p. 132), and thus it may be unusually liable to have the palp nibbled 

 by an Isopod or some such animal. 



2. Oviger. In one instance— Pallenopsis sp.?, St. 256— the tip of the oviger has 

 bifurcated (Fig. 74 c) ; in fact segment 9 has almost the appearance of trifurcating. 



Quite a different type of abnormality occurred in a male of Pycnogomm magellanicim} 

 (WS 228), where the left oviger was entirely suppressed, that on the right side being 

 present. 



3. Ocular tubercle. A specimen of Ausfrodecus glaciale, Hodgson, had had the greater 

 part of the ocular tubercle removed as the resuh of some accident. The wound had 

 healed over without any apparent attempt at regeneration. The long ocular tubercle is 

 so conspicuous a feature of this species that the low blunt stump attracted attention 

 at once. Perhaps, had the animal undergone another moult, the deficiency might have 

 been made good. But there is no experimental evidence on this point. 



4. Abdomen. It is interesting to find, in a species showing frequent variation in the 

 distal segmentation of the palp, the following peculiarity. In the specimen of Decolopoda 

 antarctica from St. 164 there is, to the right of the abdomen, a short, ? movable, clavate 

 process projecting from the posterior end of the trunk (Fig. 74 b). A small circular area 

 at the posterior end is thin and membranous, but it is rather difficult to decide whether 

 it is perforate. Unfortunately the process was detached from the specimen after the 

 camera lucida drawing had been made; only a very small indication of the point of 

 attachment can now be observed. This process may be interpreted as an abortive second 

 abdomen. 



5. Auxiliary claw. The third right leg of Pentanymphon antarcticum} , Hodgson, 

 from St. 170, has two auxiliary claws on the anterior side, and only one, as is usual, 

 on the posterior side of the main claw. There is no reversal of symmetry here and no 



