174 Dr. E. L. Trouessart — Synoptical Revision 



The absence of tracheae and the form and arrangement of 

 the palpi, the last joint of which is pointed and styliform 

 (and not palpiform) , suffice to distinguish this family from 

 that of the Trombidiidse, with which it has been proposed to 

 unite the Halacaridae. From its characters the latter family 

 may be placed between the Gamasidae and the Sarcoptidae. 

 At present we know about 7 genera and 35 species. _ 



The Halacaridse live in the sea and in the brackish water 

 of estuaries and salt-marshes. They walk and climb, rather 

 than swim, upon the bottom, the rocks, Algae, and fixed or 

 slow-moving animals of which they are commensals. Their 

 food seems to be varied ; it consists in great part of Diatomeae 

 and of organic matters in course of decomposition derived 

 from the fragments rejected by the animals of larger size 

 upon which we find them, and which belong to all classes — 

 Crustacea, Mollusca (oysters, mussels, &c), Echinoderms, 

 Acalephae, Hydroids, Corals, Bryozoans, Sponges, &c. The 

 young of many species may be regarded as parasites, feeding 

 almost exclusively upon organic matters, especially the ova 

 of Copepoda, and attaching themselves to other animals to 

 profit by what falls from their table. The adults on the 

 contrary lead a vagabond life, and seek in preference the 

 Diatoms which they find in abundance attached to the fronds 

 of Algae. 



We find the Halacaridae from the zone of stranded Algae 

 to a depth of 30-50 fathoms ; but it is in the Laminarian 

 zone, or more exactly in the zone of the Corallines and Red 

 Algce, that they are most abundant, at depths of 5-10 fathoms. 

 Very few are found upon the brown Algae (Fucaceae) ; on 

 the other hand they are numerous and varied specifically 

 upon the red and calcareous Algae (Florideae and Corallinae). 

 Upon the great rocky bottoms destitute of vegetation we 

 meet with types (Scaptognathus, Coloboceras) which are want- 

 ing everywhere else. Their geographical distribution, 

 although in general pretty extensive, presents some remark- 

 able peculiarities ; thus, the genus Agaue, an essentially 

 southern type, widely diffused in the Mediterranean and 

 extending to the southern hemisphere (Tierra del Fuego), does 

 not appear to advance in the ocean to the north of the mouth 

 of the Loire, and is wanting in the North Sea and the 

 Baltic. 



The distinction of the species in this group presents great 

 difficulties in consequence of the great uniformity of the type, 

 of variations of colour due to the kind of food, and of this 

 peculiar fact, which it is important to mention, that the 

 nymplis, before their last moult, already present a rudiment of 



