of Tracheary Arachnidans. 51 



the great central ganglion expands on either side into two pouches, 

 from each of which a large csecal prolongation would seem to pass 

 backwards, running along the sides of the body to its posterior 

 extremity. Between these two caeca the canal is continued, ex- 

 periencing in its course two other dilatations, and finally ter- 

 minates by a straight rectum which opens at the posterior end 

 of the body between three long bristles. Just before its termi- 

 nation the rectum receives the two long csecal tubes which we 

 have already described in the adult, and which here also contain 

 an opake fluid, and may be traced forwards into the first pair of 

 legs. The alimentary canal through its whole length is sur- 

 rounded by a large quantity of the so-called adipose matter. 



The central nervous mass is here also very easily demonstrated. 

 It presents anteriorly two lobes, which are not however very pro- 

 minent, and the lateral prolongation so conspicuous in the adult 

 cannot here be detected. A filament would seem to be traceable 

 from each of the anterior lobes to the organs of the mouth, but 

 I have been unable to make any satisfactory observation with 

 reference to the connection of other filaments with the central 

 mass. 



I have in vain sought for traces of a respiratory system. 



Though but three pairs of legs are visible externally, yet the 

 fourth or posterior pair which becomes developed in the adult 

 exists in the larva in a rudimental condition. It may with some 

 care be detected, confined as yet beneath the integument, and 

 presenting the appearance of a pair of conical organs diverging 

 at first as they pass backwards and then turning forwards and 

 inwards. The muscular fasciculi may already be seen with their 

 fibres assuming a spiral arrangement in the interior of the rudi- 

 mental limb. The larvse are all much more active than the adults, 

 but not natatory. 



In the structure of the oral organs as well as in other points 

 of its external anatomy, Halarachne possesses very close affinity 

 with Gamasus. From the latter genus indeed, the principal 

 zoological differences will be found in the remarkably elongated 

 cylindroid body of Halarachne, which contrasts so strongly with 

 the flattened, more or less orbicular body of Gamasus. The habits 

 of the two genera, though in both cases parasitical, are in all other 

 respects totally dissimilar; Halarachne not only presenting us 

 with a marine habitat, but being absolutely entozoal. 



EXPLANATION OF PLATES II., III. 



Fig. 1. Halarachne Halichoeri magnified. 



Fig. 2. Ditto, opened from below to show certain details of internal ana- 

 tomy: a, mandibles; b, lower lip ; c, palp; d, anus; eee, tracheae; 



4* 



