PYCNOGONIDA; ENTOMOSTRACA 959 



limbs is occupied by a transparent liquid, in which are seen floating 

 a number of minute transparent corpuscles of irregular size ; and 

 this fluid, which represents the blood, is kept in continual motion, 

 not only by the general movements of the animal, but also by the 

 actions of the digestive apparatus ; since, whenever the caecum of 

 any one of the legs undergoes dilatation, a part of the circum- 

 ambient liquid will be pressed out from the cavity of that limb, 

 either into the thorax or into some other limb whose stomach is 

 contracting. The fluid must obtain its aeration through the general 

 surface of the body, as there are no special organs of respiration. 

 The nervous system consists of a single ganglion in the head (formed 

 by the coalescence of a pair), and of another in the thorax (formed 

 by the coalescence of four pairs),* with which the cephalic ganglion 

 is connected in the usual mode, namely, by two nervous cords which 

 diverge from each other to embrace the oesophagus. In the study 

 of the very curious phenomena exhibited by the digestive apparatus, 

 as well as of the various points of internal conformation which have 

 been described, the achromatic condenser will be found useful, even 

 with the 1-inch, f-inch, or ^-inch objectives ; for the imperfect 

 transparence of the bodies of these animals renders it of importance 

 to drive a large quantity of light through them, and to give to this 

 light such a quantity as shall sharply define the internal organs. 1 



Entomostraca. This group of crustaceans, many of the existing 

 members of which are of such minute size as to be only just visible to 

 the naked eye, is distinguished by the fact that they never have more 

 than three pairs of their appendages converted into mouth-organs, 

 nor possess any appendage on such segments as may lie behind the 

 generative orifices. The segments into which the body is divided 

 are frequently very numerous, and are for the most part similar to 

 each other ; but there is a marked difference in regard to the 

 appendages which they bear, and to the mode in which these 

 minister to the locomotion of the animals. For in what have been 

 called the Lophyropoda, or 'bristly-footed' tribe, a small number of 

 legs not exceeding five pairs have their function limited to locomotion, 

 the respiratory organs being attached to the parts in the neighbour- 

 hood of the mouth ; whilst in the Branchiopoda, or 'gill- footed ' tribe, 

 the members (known as ' fin-feet ') serve both for locomotion and for 

 respiration, and the number of these is commonly large, being in Apus 

 as many as sixty pairs. The character of their movements differs 

 accordingly ; for whilst all the members of the first-named tribe dart 

 through the water in a succession of jerks, so as to have acquired the com- 

 mon name of 'water-fleas,' those among the latter which possess a great 



1 Certain points of resemblance borne by Pycnogonida to spiders make the 

 careful study of their development a matter of special interest and importance, as 

 there is some reason to regard them rather as Arachnida adapted to a marine 

 habitat than as Crustacea. See Balfour's Comparative Embryologtj, pp. 448, 449, 

 and the authorities there referred to. The most recent additions to the literature 

 of the Pycnogonids are Dr. A. Dohrn's Die Pantopoden des Golfes von Neapel 

 &c., Leipzig, 1881 ; Dr. P. P. C. Hoek's ' Report on the Pycnogonida of the Challenger,' 

 1881, and his ' Nouvelle Etude sur les Pycnogonides,' in Archives de Zool. Exper. ix. 

 p. 445 ; and Professor G. O. Sars's report in the Zoology of the Norwegian North Sea 

 Expedition. 



