638 CRUSTACEA ! PYCNOGONID^ | ENTOMOSTRACA. 



extremity of the body. From the central stomach five pairs of 

 Csecal prolongations radiate ; one pair (d) entering the Feet-jaws, 

 the other four (e, e) penetrating the Legs, and passing along them 

 as far as the last joint but one ; and those extensions are covered 

 with a layer of brownish -yellow granules, which are probably to 

 be regarded as a diffused and rudimentary condition of the Liver. 

 The Stomach and its Caecal prolongations are continually executing 

 peristaltic movements of a very curious kind ; for they contract 

 and dilate with an irregular alternation, so that a flux and reflux 

 of their contents is constantly taking-place between the central 

 portion and its radiating extensions, and between one of these 

 extensions and another. The Perivisceral space between the 

 widely-extended Stomach and the walls of the body and 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. Of the Reproduction of these animals, very little is yet 

 known.* — 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, 2-3rds inch, 

 or 4 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 quality as 

 shall define the internal organs as sharply as possible. 



497. Entomostraca. — This group of Crustaceans, nearly all the 

 existing members of which are of such minute size as to be only 

 just visible to the naked eye, is distinguished by the enclosure of 

 the entire body within a Horny or Shelly casing ; which sometimes 

 closely resembles a Bivalve shell in form and in the mode of junc- 

 tion of its parts, whilst in other instances it is formed of only a 

 single piece, like the hard envelope of certain Rotifera (§ 363, in.). 



* A curious account is given by'Mr. Hodge in "Ann. of Nat. Hist.," 

 Ser. 3, Vol. ix., p. 33, of the development of a species of Pycnogon, which 

 in its larval state is parasitic on the Polypary of Coryne. 



