296 Dr. T. Williams on the Mechanism of Aquatic 



terizes the adult fluid. The former is a real chylaqueous com- 

 pound, is moved by means of the general muscles of the body, 

 and undergoes the change of aeration without the intervention 

 of any special organs. 



Central parts of the Circulatory System. 



In this class the heart occurs under the character either of a 

 saccular vesiculiform viscus, or under that of the vasiform or 

 tubular. 



In the higher species, in which the organ is partly branchial 

 and partly systemic, it is the point of departure of an arterial 

 system of distinctly walled pulsatile tubes which in the lower 

 becomes abortive. It is placed in the axis of the body, directly 

 under the shell, at the anterior part of the back, and is often 

 attached to the internal surface of the dermal skeleton by mus- 

 cular fibres : it is the chief propelling power of the blood. In 

 the Siphonostoma and Lophyropoda it is a simple sac, either 

 spheroidal or elongated in figure : it has only two orifices, a 

 venous behind and an arterial in front. This organ in the Deca- 

 pods, occupying the middle of the cephalothorax and star-shaped, 

 passes off into arteries in front, behind and below, the returning 

 venous blood entei-ing through orifices at the upper and lateral 

 portions. In the Poecilopoda, Isopoda, Amphipoda and Laemodi- 

 poda it is tubular in form, and occupies the mid-region of the 

 dorsum, sends off arteries before, behind and laterally, and re- 

 ceives the venous blood through lateral venous orifices. It is 

 most highly developed in the Stomapoda. In the Phyllopoda it 

 approaches the Myriapodal chambered type. 



In the lower Crustacea the blood passes fi'om the heart di- 

 rectly into inter visceral lacunae : no defined vessels exist. In 

 the higher, in which the organ is unarticulated and more cen- 

 tralized, arterial trunks occiir ; after a short course they are lost 

 in the interstices of the tissues. 



The venous currents converge from the lower part of the body 

 into various intercommunicating sinuses, situated some upon 

 the median line and others at the base of the appendages. From 

 these sinuses the blood proceeds to the branchiae and thence into 

 the dorsal sinus, the walls of which are thin and non-contractile, 

 and within which the heart is entii'ely enclosed. 



This dorsal sinus is filled during the systole, and the artc- 

 rialized blood which it contains is absorbed during the diastole 

 through the venous orifices of the heart without any aid on the 

 part of the walls of the sinus *. 



* See Anatomy of the Invertebrata, by Siebold, translated by Burnett, 

 for very copious bibliographic references on the literature of this and all 

 other classes of Invertcbrated animals. 



